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UnderstandingPathophysiology-Mosby2016bySueEHuether.pdf
Understanding Pathophysiology
SIXTH EDITION
Sue E. Huether, MS, PhD Professor Emeritus College of Nursing University of Utah Salt Lake City, Utah
Kathryn L. McCance, MS, PhD Professor Emeritus College of Nursing University of Utah Salt Lake City, Utah SECTION EDITORS
Valentina L. Brashers, MD Professor of Nursing and Woodard Clinical Scholar Attending Physician in Internal Medicine University of Virginia Health System Charlottesville, Virginia
Neal S. Rote, PhD Academic Vice-Chair and Director of Research
Department of Obstetrics and Gynecology University Hospitals Case Medical Center William H. Weir, MD, Professor of Reproductive Biology and Pathology Case Western Reserve University School of Medicine Cleveland, Ohio With more than 1000 illustrations
Table of Contents
Cover image
Title page
Health Alerts
Copyright
Contributors
Reviewers
Preface
Organization and Content: What's New in the Sixth Edition
Features to Promote Learning
Art Program
Teaching/Learning Package
Acknowledgments
Introduction to Pathophysiology Part One Basic Concepts of Pathophysiology
Unit 1 The Cell
1 Cellular Biology
Prokaryotes and Eukaryotes
Cellular Functions
Structure and Function of Cellular Components
Cell-to-Cell Adhesions
Cellular Communication and Signal Transduction
Cellular Metabolism
Membrane Transport: Cellular Intake and Output
Cellular Reproduction: the Cell Cycle
Tissues
Did You Understand?
Key Terms
References
2 Genes and Genetic Diseases
DNA, RNA, and Proteins: Heredity at the Molecular Level
Chromosomes
Elements of Formal Genetics
Transmission of Genetic Diseases
Linkage Analysis and Gene Mapping
Multifactorial Inheritance
Did You Understand?
Key Terms
References
3 Epigenetics and Disease
Epigenetic Mechanisms
Epigenetics and Human Development
Genomic Imprinting
Long-Term and Multigenerational Persistence of Epigenetic States Induced by Stochastic and
Environmental Factors
Epigenetics and Cancer
Future Directions
Did You Understand?
Key Terms
References
4 Altered Cellular and Tissue Biology
Cellular Adaptation
Cellular Injury
Manifestations of Cellular Injury: Accumulations
Cellular Death
Aging and Altered Cellular and Tissue Biology
Somatic Death
Did You Understand?
Key Terms
References
5 Fluids and Electrolytes, Acids and Bases
Distribution of Body Fluids and Electrolytes
Alterations in Water Movement
Sodium, Chloride, and Water Balance
Alterations in Sodium, Chloride, and Water Balance
Alterations in Potassium and Other Electrolytes
Acid-Base Balance
Did You Understand?
Key Terms
References
Unit 2 Mechanisms of Self-Defense
6 Innate Immunity: Inflammation and Wound Healing
Human Defense Mechanisms
Innate Immunity
Acute and Chronic Inflammation
Wound Healing
Did You Understand?
Key Terms
References
7 Adaptive Immunity
Third Line of Defense: Adaptive Immunity
Antigens and Immunogens
Antibodies
Immune Response: Collaboration of B Cells and T Cells
Cell-Mediated Immunity
Did You Understand?
Key Terms
References
8 Infection and Defects in Mechanisms of Defense
Infection
Deficiencies in Immunity
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity
Did You Understand?
Key Terms
References
9 Stress and Disease
Historical Background and General Concepts
The Stress Response
Stress, Personality, Coping, and Illness
Did You Understand?
Key Terms
References
Unit 3 Cellular Proliferation: Cancer
10 Biology of Cancer
Cancer Terminology and Characteristics
The Biology of Cancer Cells
Clinical Manifestations of Cancer
Diagnosis, Characterization, and Treatment of Cancer
Did You Understand?
Key Terms
References
11 Cancer Epidemiology
Genetics, Epigenetics, and Tissue
In Utero and Early Life Conditions
Environmental-Lifestyle Factors
Did You Understand?
In Utero and Early Life Conditions
Key Terms
References
12 Cancer in Children and Adolescents
Incidence, Etiology, and Types of Childhood Cancer
Prognosis
Did You Understand?
Key Terms
References
Part Two Body Systems and Diseases
Unit 4 The Neurologic System
13 Structure and Function of the Neurologic System
Overview and Organization of the Nervous System
Cells of the Nervous System
The Nerve Impulse
The Central Nervous System
The Peripheral Nervous System
The Autonomic Nervous System
Did You Understand?
Key Terms
References
14 Pain, Temperature, Sleep, and Sensory Function
Pain
Temperature Regulation
Sleep
The Special Senses
Somatosensory Function
Geriatric Considerations
Geriatric Considerations
Did You Understand?
Key Terms
References
15 Alterations in Cognitive Systems, Cerebral Hemodynamics, and Motor Function
Alterations in Cognitive Systems
Alterations in Cerebral Hemodynamics
Alterations in Neuromotor Function
Alterations in Complex Motor Performance
Extrapyramidal Motor Syndromes
Did You Understand?
Key Terms
References
16 Disorders of the Central and Peripheral Nervous Systems and Neuromuscular Junction
Central Nervous System Disorders
Peripheral Nervous System and Neuromuscular Junction Disorders
Tumors of the Central Nervous System
Did You Understand?
Key Terms
References
17 Alterations of Neurologic Function in Children
Development of the Nervous System in Children
References
Structural Malformations
Alterations in Function: Encephalopathies
Cerebrovascular Disease in Children
Childhood Tumors
Did You Understand?
Key Terms
References
Unit 5 The Endocrine System
18 Mechanisms of Hormonal Regulation
Mechanisms of Hormonal Regulation
Structure and Function of the Endocrine Glands
Geriatric Considerations
Did You Understand?
Key Terms
References
19 Alterations of Hormonal Regulation
Mechanisms of Hormonal Alterations
Alterations of the Hypothalamic-Pituitary System
Alterations of Thyroid Function
Alterations of Parathyroid Function
Dysfunction of the Endocrine Pancreas: Diabetes Mellitus
Alterations of Adrenal Function
Did You Understand?
Key Terms
References
Unit 6 The Hematologic System
20 Structure and Function of the Hematologic System
Components of the Hematologic System
Development of Blood Cells
Mechanisms of Hemostasis
Pediatrics & Hematologic Value Changes
Aging & Hematologic Value Changes
Did You Understand?
Key Terms
References
21 Alterations of Hematologic Function
Alterations of Erythrocyte Function
Myeloproliferative Red Cell Disorders
Alterations of Leukocyte Function
Alterations of Lymphoid Function
Alterations of Splenic Function
Hemorrhagic Disorders and Alterations of Platelets and Coagulation
Did You Understand?
Key Terms
References
22 Alterations of Hematologic Function in Children
Disorders of Erythrocytes
Disorders of Coagulation and Platelets
Neoplastic Disorders
Did You Understand?
Key Terms
References
Unit 7 The Cardiovascular and Lymphatic Systems
23 Structure and Function of the Cardiovascular and Lymphatic Systems
The Circulatory System
The Heart
The Systemic Circulation
The Lymphatic System
Did You Understand?
Key Terms
References
24 Alterations of Cardiovascular Function
Diseases of the Veins
Diseases of the Arteries
Disorders of the Heart Wall
Manifestations of Heart Disease
Shock
Did You Understand?
Key Terms
References
25 Alterations of Cardiovascular Function in Children
Congenital Heart Disease
Acquired Cardiovascular Disorders
Did You Understand?
Key Terms
References
Unit 8 The Pulmonary System
26 Structure and Function of the Pulmonary System
Structures of the Pulmonary System
Function of the Pulmonary System
Geriatric Considerations
Did you Understand?
Key Terms
References
27 Alterations of Pulmonary Function
Clinical Manifestations of Pulmonary Alterations
Pulmonary Disorders
Did You Understand?
Key Terms
References
28 Alterations of Pulmonary Function in Children
Disorders of the Upper Airways
Disorders of the Lower Airways
Sudden Infant Death Syndrome (SIDS)
Did You Understand?
Key Terms
References
Unit 9 The Renal and Urologic Systems
29 Structure and Function of the Renal and Urologic Systems
Structures of the Renal System
Renal Blood Flow
Kidney Function
Tests of Renal Function
Pediatric Considerations
Geriatric Considerations
Did You Understand?
Key Terms
References
30 Alterations of Renal and Urinary Tract Function
Urinary Tract Obstruction
Urinary Tract Infection
Glomerular Disorders
Acute Kidney Injury
Chronic Kidney Disease
Did You Understand?
Key Terms
References
31 Alterations of Renal and Urinary Tract Function in Children
Structural Abnormalities
Glomerular Disorders
Nephroblastoma
Bladder Disorders
Urinary Incontinence
Did You Understand?
Key Terms
References
Unit 10 The Reproductive Systems
32 Structure and Function of the Reproductive Systems
Development of the Reproductive Systems
The Female Reproductive System
Structure and Function of the Breast
The Male Reproductive System
Aging & Reproductive Function
Did You Understand?
Key Terms
References
33 Alterations of the Female Reproductive System
Abnormalities of the Female Reproductive Tract
Alterations of Sexual Maturation
Disorders of the Female Reproductive System
References
Disorders of the Female Breast
Did You Understand?
Key Terms
References
34 Alterations of the Male Reproductive System
Alterations of Sexual Maturation
Disorders of the Male Reproductive System
References
Disorders of the Male Breast
Sexually Transmitted Diseases
Did You Understand?
Key Terms
References
Unit 11 The Digestive System
35 Structure and Function of the Digestive System
The Gastrointestinal Tract
Accessory Organs of Digestion
Geriatric Considerations
Did You Understand?
Key Terms
References
36 Alterations of Digestive Function
Disorders of the Gastrointestinal Tract
Disorders of the Accessory Organs of Digestion
Cancer of the Digestive System
Did You Understand?
Key Terms
References
37 Alterations of Digestive Function in Children
Disorders of the Gastrointestinal Tract
Disorders of the Liver
Did You Understand?
Key Terms
References
Unit 12 The Musculoskeletal and Integumentary Systems
38 Structure and Function of the Musculoskeletal System
Structure and Function of Bones
Structure and Function of Joints
Structure and Function of Skeletal Muscles
Aging & the Musculoskeletal System
Did You Understand?
Key Terms
References
39 Alterations of Musculoskeletal Function
Musculoskeletal Injuries
Disorders of Bones
Disorders of Joints
Disorders of Skeletal Muscle
Musculoskeletal Tumors
Did You Understand?
Key Terms
References
40 Alterations of Musculoskeletal Function in Children
Congenital Defects
Bone Infection
Juvenile Idiopathic Arthritis
Osteochondroses
Scoliosis
Muscular Dystrophy
Musculoskeletal Tumors
Nonaccidental Trauma
Did You Understand?
Key Terms
References
41 Structure, Function, and Disorders of the Integument
Structure and Function of the Skin
Disorders of the Skin
Disorders of the Hair
Disorders of the Nail
Geriatric Considerations
Did You Understand?
Key Terms
References
42 Alterations of the Integument in Children
Acne Vulgaris
Dermatitis
Infections of the Skin
Insect Bites and Parasites
Cutaneous Hemangiomas and Vascular Malformations
Other Skin Disorders
Did You Understand?
Key Terms
References
Glossary
Index
Prefixes and Suffixes Used in Medical Terminology
Word Roots Commonly Used in Medical Terminology
Health Alerts
Gene Therapy, 57
The Percentage of Child Medication–Related Poisoning Deaths Is Increasing, 85
Air Pollution Reported as Largest Single Environmental Health Risk, 87
Low-Level Lead Exposure Harms Children: A Renewed Call for Primary Prevention, 89
Alcohol: Global Burden, Adolescent Onset, Chronic or Binge Drinking, 92
Unintentional Injury Errors in Health Care and Patient Safety, 93
Hyponatremia and the Elderly, 121
Potassium Intake: Hypertension and Stroke, 122
Risk of HIV Transmission Associated with Sexual Practices, 194
Glucocorticoids, Insulin, Inflammation, and Obesity, 220
Psychosocial Stress and Progression to Coronary Heart Disease, 221
Acute Emotional Stress and Adverse Heart Effects, 226
Partner's Survival and Spouse's Hospitalizations and/or Death, 226
Global Cancer Statistics and Risk Factors Associated with Causes of Cancer Death, 273
Increasing Use of Computed Tomography Scans and Risks, 285
Rising Incidence of HPV-Associated Oropharyngeal Cancers, 291
Radiation Risks and Pediatric Computed Tomography (CT): Data from the National Cancer Institute, 305
Magnetic Fields and Development of Pediatric Cancer, 305
Neuroplasticity, 311
Biomarkers and Neurodegenerative Dementia, 372
Tourette Syndrome, 378
Prevention of Stroke in Women, 403
West Nile Virus, 410
Alcohol-Related Neurodevelopmental Disorder (ARND), 423
Growth Hormone (GH) and Insulin-like Growth Factor (IGF) in Aging, 447
Vitamin D, 450
Immunotherapy for the Prevention and Treatment of Type 1 Diabetes, 474
Incretin Hormones for Type 2 Diabetes Mellitus Therapy, 476
Sticky Platelets, Genetic Variations, and Cardiovascular Complications, 505
A Significant Number of Children Develop and Suffer from Severe Iron Deficiency Anemia, 555
Myocardial Regeneration, 571
Regression of Myocardial Hypertrophy, 579
The Renin-Angiotensin-Aldosterone System (RAAS) and Cardiovascular Disease, 601
Obesity and Hypertension, 602
New Insights and Guidelines into the Management of Dyslipidemia for the Prevention of Coronary Artery Disease, 612
Mediterranean Diet, 612
Women and Microvascular Angina, 614
Metabolic Changes in Heart Failure, 634
Gene Therapy for Heart Failure, 635
Central Line–Associated Bloodstream Infection, 645
The Surviving Sepsis Guidelines, 646
Endocarditis Risk, 658
U.S. Childhood Obesity and Its Association with Cardiovascular Disease, 668
Changes in the Chemical Control of Breathing During Sleep, 678
The Microbiome and Asthma, 698
Ventilator-Associated Pneumonia (VAP), 704
Molecular Targets in Lung Cancer Treatment, 711
Exercise-Induced Bronchoconstriction, 724
Newborn Screening for Cystic Fibrosis, 726
The Many Effects of Erythropoietin (Epo), 742
Urinary Tract Infection and Antibiotic Resistance, 754
Childhood Urinary Tract Infections, 775
Nutrition and Premenstrual Syndrome, 810
Vaginal Mesh, 814
Screening with the Papanicolaou (Pap) Test and with the Human Papillomavirus (HPV) DNA Test: Benefits and Harms from Cervical Cancer Screening (PDQ®), 820
Cervical Cancer Primary Prevention, 823
Breast Cancer Screening Mammography, 834
Paracetamol (Acetaminophen) and Acute Liver Failure, 900
Clostridium difficile and Fecal Microbiome Transplant, 908
Types of Adipose Tissue and Obesity, 925
Childhood Obesity and Nonalcoholic Fatty Liver Disease, 963
Tendon and Ligament Repair, 987
Managing Tendinopathy, 997
Osteoporosis Facts and Figures at a Glance, 1002
Calcium, Vitamin D, and Bone Health, 1005
New Treatments for Osteoporosis, 1006
Musculoskeletal Molecular Imaging, 1015
Psoriasis and Comorbidities, 1063
Melanoma in Non-White People, 1073
Copyright
3251 Riverport Lane St. Louis, Missouri 63043
UNDERSTANDING PATHOPHYSIOLOGY, SIXTH EDITION ISBN: 978-0-323- 35409-7 Copyright © 2017, Elsevier Inc. All rights reserved. Previous editions copyrighted 2012, 2008, 2004, 2000, 1996.
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Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and
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Library of Congress Cataloging-in-Publication Data Names: Huether, Sue E., editor. | McCance, Kathryn L., editor. Title: Understanding pathophysiology / [edited by] Sue E. Huether, Kathryn L. McCance ; section editors, Valentina L. Brashers, Neal S. Rote. Description: Sixth edition. | St. Louis, Missouri : Elsevier, [2017] | Includes bibliographical references and index. Identifiers: LCCN 2015037586 | ISBN 9780323354097 (pbk. : alk. paper) Subjects: | MESH: Pathology—Nurses' Instruction. | Disease—Nurses' Instruction. | Physiology—Nurses' Instruction. Classification: LCC RB113 | NLM QZ 4 | DDC 616.07—dc23 LC record available at http://lccn.loc.gov/2015037586
ABOUT THE COVER
Microbiome. This colored scanning electron micrograph of Escherichia coli bacteria (red rods) was taken from the small intestine of a child. E. coli are part of the normal flora or microbiota of
the human gut and many normal flora are essential for health. The terms microbiota or microbiome refer to the community of microbes that normally reside on and within the human
body. The microbiome also means the full collection of genes of all the microbes in the community. DNA-sequencing tools have helped define the microbiome and they outnumber our
own cells by about 10 to 1. These resident microbes are highly skilled and provide crucial functions—they sense what food is present, if pathogens are lurking, and the inflammatory
state of the gut. Shifts in the bacterial composition of the gut microbiota have been correlated with intestinal dysfunctions such as inflammatory bowel disease, antibiotic-associated diarrhea and metabolic dysfunction including obesity. Gut microflora have protective, metabolic, growth,
and immunologic functions because the microbiota interact with both innate and adaptive immune systems. If the overall interaction is flawed autoimmune or inflammatory diseases may
occur. We acquire our microbiomes from the environment at birth. Our microbial profiles change with aging because microbial populations shift with changes in the environment. Credit:
STEPHANIE SCHULLER/SCIENCE PHOTO LIBRARY
Executive Content Strategist: Kellie White Content Development Manager: Laurie Gower Senior Content Development Specialist: Karen C. Turner Publishing Services Manager: Jeffrey Patterson Senior Project Managers: Jeanne Genz and Tracey Schriefer Designer: Margaret Reid
Printed in the United States of America
Last digit is the print number: 9 8 7 6 5 4 3 2 1
Contributors
Barbara J. Boss RN, PHD, CFNP, CANP Retired Professor of Nursing University of Mississippi Medical Center Jackson, Mississippi
Kristen Lee Carroll MD Chief of Staff Medical Staff/Orthopedics Shriners Hospital for Children Professor of Orthopedics University of Utah Salt Lake City, Utah
Margaret F. Clayton PhD, APRN Associate Professor and Assistant Dean for the PhD Program College of Nursing University of Utah Salt Lake City, Utah
Christy L. Crowther-Radulewicz RN, MS, CRNP Nurse Practitioner Orthopedic Surgery Anne Arundel Orthopedic Surgeons Annapolis, Maryland
Susanna G. Cunningham BSN, MA, PhD, RN, FAHA, FAAN Professor Emeritus Department of Biobehavioral Nursing School of Nursing University of Washington Seattle, Washington
Sara J. Fidanza MS, RN, CNS-BC, CPNP-BC Digestive Health Institute Children's Hospital Colorado Clinical Faculty
University of Colorado College of Nursing Aurora, Colorado
Diane P. Genereux PhD Assistant Professor Department of Biology Westfield State Westfield, Massachusetts
Todd Cameron Grey MD Chief Medical Examiner Office of the Medical Examiner State of Utah Salt Lake City, Utah
Robert E. Jones MD, FACP, FACE Professor of Medicine Endocrinology Division University of Utah School of Medicine Salt Lake City, Utah
Lynn B. Jorde PhD H.A. and Edna Benning Presidential Professor and Chair Department of Human Genetics University of Utah School of Medicine Salt Lake City, Utah
Lynne M. Kerr MD, PhD Associate Professor Department of Pediatrics, Division of Pediatric Neurology University of Utah Medical Center Salt Lake City, Utah
Nancy E. Kline PhD, RN, CPNP, FAAN † Director, Nursing Research, Medicine Patient Services/Emergency Department Boston Children's Hospital Boston, Massachusetts
Lauri A. Linder PhD, APRN, CPON
Assistant Professor College of Nursing University of Utah Clinical Nurse Specialist Cancer Transplant Center Primary Children's Hospital Salt Lake City, Utah
Sue Ann McCann MSN, RN, DNC Programmatic Nurse Specialist Nursing Clinical Research Coordinator Dermatology University of Pittsburgh Medical Center Pittsburgh, Pennsylvania
Nancy L. McDaniel MD Associate Professor of Pediatrics University of Virginia Charlottesville, Virginia
Afsoon Moktar PhD, EMBA, CT (ASCP) Associate Professor School of Physician Assistant Studies Massachusetts College of Pharmacy and Health Sciences University Boston, Massachusetts
Noreen Heer Nicol PhD, RN, FNP, NEA-BC Associate Professor College of Nursing University of Colorado Denver, Colorado
Nancy Pike PhD, RN, CPNP-AC, FAAN Assistant Professor UCLA School of Nursing Pediatric Nurse Practitioner Cardiothoracic Surgery Children's Hospital Los Angeles Los Angeles, California
Patricia Ring RN, MSN, PNP, BC Pediatric Nephrology Children's Hospital of Wisconsin Wauwatosa, Wisconsin
Anna E. Roche MSN, RN, CPNP, CPON Pediatric Nurse Practitioner Dana Farber/Boston Children’s Cancer and Blood Disorders Center Boston, Massachusetts
George W. Rodway PhD, APRN Associate Clinical Professor Betty Irene Moore School of Nursing at UC Davis Sacramento, California
Sharon Sables-Baus PhD, MPA, RN, PCNS-BC Associate Professor University of Colorado College of Nursing and School of Medicine Department of Pediatrics Pediatric Nurse Scientist Children's Hospital Colorado Aurora, Colorado
Anna Schwartz PhD, FNP-C, FAAN Associate Professor School of Nursing Northern Arizona University Flagstaff, Arizona; Affiliate Associate Professor Biobehavioral Nursing and Health Systems University of Washington Seattle, Washington
Joan Shea MSN, RN, CPON Staff Nurse III Hematology/Oncology/Clinical Research Boston Children's Hospital Boston, Massachusetts
Lorey K. Takahashi PhD Professor of Psychology Department of Psychology University of Hawaii at Manoa Honolulu, Hawaii
David M. Virshup MD Professor and Director Program in Cancer and Stem Cell Biology Duke-NUS Graduate Medical School Singapore; Professor of Pediatrics Duke University School of Medicine Durham, North Carolina †Deceased.
Reviewers
Deborah Cipale RN, MSN Nursing Resource Lab Coordinator Des Moines Area Community College Ankeny, Iowa
David J. Derrico RN, MSN Clinical Assistant Professor Department of Adult and Elderly Nursing University of Florida College of Nursing Gainesville, Florida
Sandra L. Kaminski MS, PA-C Adjunct Professor Physician Assistant Program Pace University New York, New York
Stephen D. Krau PhD, RN, CNE Associate Professor Vanderbilt University School of Nursing Nashville, Tennessee
Lindsay McCrea PhD, RN, FNP-BC, CWOCN Professor Nursing Program Assistant Director California State University, East Bay Hayward, California
Afsoon Moktar PhD, EMBA, CT (ASCP) Associate Professor School of Physician Assistant Studies Massachusetts College of Pharmacy and Health Sciences University Boston, Massachusetts
Kathleen S. Murtaugh RN, MSN, CAN Assistant Professor of Nursing
Saint Joseph College—St. Elizabeth School of Nursing Cooperative Program Rensselaer, Indiana
Judith L. Myers MSN, RN Assistant Professor of Nursing Grand View University Nursing Department Des Moines, Iowa
Holldrid Odreman MScN-Ed, BScN, RN Professor Program Coordinator of Nursing Niagara College Canada Certified Simulationist SIMone Ontario Simulation Network Welland, Ontario, Canada
Jay Schulkin PhD Director of Research The American Congress of Obstetricians and Gynecologists Washington, DC; Acting Professor Obstetrics & Gynecology University of Washington School of Medicine Seattle, Washington
Crystal R. Sherman DNP, RN, APHN-BC Associate Professor of Nursing Shawnee State University Portsmouth, Ohio
Lorey K. Takahashi PhD Professor of Psychology Department of Psychology University of Hawaii at Manoa Honolulu, Hawaii
Cheryl A. Tucker MSN, RN, CNE Senior Lecturer and Undergraduate Theory Coordinator Baylor University
Louise Herrington School of Nursing Dallas, Texas
Linda Turchin MSN, CNE Associate Professor of Nursing Fairmont State University Fairmont, West Virginia
Jo A. Voss PhD, RN, CNS Associate Professor South Dakota State University West River Department of Nursing Rapid City, South Dakota
Kim Webb MN, RN Part-time Nursing Instructor Pioneer Technology Center Ponca City, Oklahoma
Preface
The sixth edition of Understanding Pathophysiology, like other editions, has been rigorously updated and revised with consideration of the rapid advances in molecular and cell biology. Many sections have been rewritten or reorganized to provide a foundation for better understanding of the mechanisms of disease. Integrated throughout the text are concepts from the basic sciences, including genetics, epigenetics, gene–environment interaction, immunity, and inflammation. The text has been written to assist students with the translation of the concepts and processes of pathophysiology into clinical practice and to promote lifelong learning.
Although the primary focus of the text is pathophysiology, we continue to include discussions of the following interconnected topics to highlight their importance for clinical practice:
• A life-span approach that includes special sections on aging and separate chapters on children
• Epidemiology and incidence rates showing regional and worldwide differences that reflect the importance of environmental and lifestyle factors on disease initiation and progression
• Sex differences that affect epidemiology and pathophysiology • Molecular biology—mechanisms of normal cell function and how their alteration leads to disease
• Clinical manifestations, summaries of treatment, and health promotion/risk reduction
Organization and Content: What's New in the Sixth Edition The book is organized into two parts: Part One, Basic Concepts of Pathophysiology, and Part Two, Body Systems and Diseases. Two new chapters have been added.
Part One: Basic Concepts of Pathophysiology Part One introduces basic principles and processes that are important for a contemporary understanding of the pathophysiology of common diseases. The concepts include descriptions of cellular communication; forms of cell injury; genes and genetic disease; epigenetics; fluid and electrolytes and acid and base balance; immunity and inflammation; mechanisms of infection; stress, coping, and illness; and tumor biology. A new chapter, Epigenetics and Disease (Chapter 3), has been added since significant progress is emerging that explains the way heritable changes in gene expression—phenotype without a change in genotype—are influenced by several factors, including age, environment/lifestyle, and disease state.
Significant revisions to Part One also include new or updated information on the following topics:
• Updated content on cell membranes, cell junctions, intercellular communication, transport by vesicles, and stem cells (Chapter 1)
• New chapter on epigenetics and disease (Chapter 3) • Updated content on cellular adaptations, oxidative stress, chemical injury, types of cell death, and aging (Chapter 4)
• Updates regarding mechanisms of human defense—characteristics of innate and adaptive immunity (Chapters 6 and 7)
• Updated content on mechanisms of infection, antibiotic-resistant disease, and alterations in immune defense (Chapter 8)
• Updated content on stress, inflammation, hormones, and disease (Chapter 9) • Extensive entire chapter revisions and reorganization of tumor biology (Chapter 10)
• Extensive entire chapter revisions and updated epidemiology of cancer (Chapter 11)
Part Two: Body Systems and Diseases Part Two presents the pathophysiology of the most common alterations according to body system. To promote readability and comprehension, we have used a logical sequence and uniform approach in presenting the content of the units and chapters. Each unit focuses on a specific organ system and contains chapters related to anatomy and physiology, the pathophysiology of the most common diseases, and common alterations in children. The anatomy and physiology content is presented as a review to enhance the learner's understanding of the structural and functional changes inherent in pathophysiology. A brief summary of normal aging effects is included at the end of these review chapters. The general organization of each disease/disorder discussion includes an introductory paragraph on relevant risk factors and epidemiology, a significant focus on pathophysiology and clinical manifestations, and then a brief review of evaluation and treatment.
The information on reproductive pathophysiology is now presented in two chapters, with a new chapter, Alterations of the Male Reproductive System. Other significant revisions to Part Two include new and/or updated information on the following topics:
• Mechanisms of pain transmission, pain syndromes, and categories of sleep disorders (Chapter 14)
• Alterations in levels of consciousness, seizure disorders, and delirium. Pathogenesis of degenerative brain diseases, the dementias, movement disorders, traumatic brain and spinal cord injury, stroke syndromes, headache, and infections and structural malformations of the CNS (Chapters 15, 16, 17)
• The pathogenesis of type 2 diabetes mellitus (Chapter 19) • Platelet function and coagulation; anemias, alterations of leukocyte function and myeloid and lymphoid tumors (Chapters 20 and 21)
• Extensive chapter revisions of alterations of hematologic function in children (Chapter 22)
• Extensive chapter revisions on structure and function of the cardiovascular and lymphatic systems (Chapter 23)
• Mechanisms of atherosclerosis, hypertension, coronary artery disease, heart failure, and shock (Chapter 24)
• Pediatric valvular disorders, heart failure, hypertension, obesity, and heart disease (Chapter 25)
• Pathophysiology of acute lung injury, asthma, pneumonia, lung cancer, respiratory distress in the newborn, and cystic fibrosis (Chapters 27 and 28)
• Mechanisms of kidney stone formation, immune processes of glomerulonephritis, and acute and chronic kidney injury (Chapters 30 and 31)
• Female and male reproductive disorders, female and male reproductive cancers, breast diseases and mechanisms of breast cancer, prostate cancer, male breast cancer, and sexually transmitted infections (Chapters 33 and 34)
• Gastroesophageal reflux, nonalcoholic liver disease, inflammatory bowel disease, viral hepatitis, obesity, gluten-sensitive enteropathy, and necrotizing enterocolitis (Chapters 36 and 37)
• Bone cells, bone remodeling, joint and tendon diseases, osteoporosis, rheumatoid arthritis, and osteoarthritis (Chapters 38 and 39)
• Congenital and acquired musculoskeletal disorders, and muscular dystrophies in children (Chapter 40)
• Psoriasis, discoid lupus erythematosus, and atopic dermatitis (Chapters 41 and 42) Cancer of the various organ systems was updated for all of the chapters.
Features to Promote Learning A number of features are incorporated into this text that guide and support learning and understanding, including:
• Chapter Outlines including page numbers for easy reference • Quick Check questions strategically placed throughout each chapter to help readers confirm their understanding of the material; answers are included on the textbook's Evolve website
• Health Alerts with concise discussions of the latest research • Risk Factors boxes for selected diseases • End-of-chapter Did You Understand? summaries that condense the major concepts of each chapter into an easy-to-review list format; printable versions of these are available on the textbook's Evolve website
• Key Terms set in blue boldface in text and listed, with page numbers, at the end of each chapter
• Special boxes for Aging and Pediatrics content that highlight discussions of life- span alterations
Art Program All of the figures and photographs have been carefully reviewed, revised, or updated. This edition features approximately 100 new or heavily revised illustrations and photographs with a total of approximately 1000 images. The figures are designed to help students visually understand sometimes difficult and complex material. Hundreds of high-quality photographs show clinical manifestations, pathologic specimens, and clinical imaging techniques. Micrographs show normal and abnormal cellular structure. The combination of illustrations, algorithms, photographs, and use of color for tables and boxes allows a more precise understanding of essential information.
Teaching/Learning Package For Students The free electronic Student Resources on Evolve include review questions and answers, numerous animations, answers to the Quick Check questions in the book, printable key points, and bonus case studies with questions and answers. A comprehensive Glossary for the textbook of more than 600 terms helps students with the often difficult terminology related to pathophysiology; this is available both on Evolve and in the electronic version of the textbook. These electronic resources enhance learning options for students. Go to http://evolve.elsevier.com/Huether.
The newly rewritten Study Guide includes many different question types, aiming to help the broad spectrum of student learners. Question types include the following:
• Choose the Correct Words • Complete These Sentences • Categorize These Clinical Examples • Explain the Pictures • Teach These People about Pathophysiology • Plus many more… Answers are found in the back of the Study Guide for easy reference for
students.
For Instructors The electronic Instructor Resources on Evolve are available free to instructors with qualified adoptions of the textbook and include the following: TEACH Lesson Plans with case studies to assist with clinical application; a Test Bank of more than 1200 items; PowerPoint Presentations for each chapter, with integrated images, audience response questions, and case studies; and an Image Collection of approximately 1000 key figures from the text. All of these teaching resources are also available to instructors on the book's Evolve site. Plus the Evolve Learning System provides a comprehensive suite of course communication and organization tools that allow you to upload your class calendar and syllabus, post scores and announcements, and more. Go to http://evolve.elsevier.com/Huether.
The most exciting part of the learning support package is Pathophysiology Online, a complete set of online modules that provide thoroughly developed lessons
on the most important and difficult topics in pathophysiology supplemented with illustrations, animations, interactive activities, interactive algorithms, self- assessment reviews, and exams. Instructors can use it to enhance traditional classroom lecture courses or for distance and online-only courses. Students can use it as a self-guided study tool.
Acknowledgments This book would not be possible without the knowledge and expertise of our contributors, both those who have worked with us through previous editions and the new members of our team. Their reviews and synthesis of the evidence and clear concise presentation of information is a strength of the text. We thank them.
Nancy Kline, PhD, RN was a highly respected colleague, researcher, nurse, and contributor to our textbooks. We dedicate this edition to her memory and the many contributions she made to nursing research, medicine, patient services, and children’s health. We will miss her.
The reviewers for this edition provided excellent recommendations for focus of content and revisions. We appreciate their insightful work.
For more than 30 years Sue Meeks has been the rock of our manuscript preparation. She is masterful at managing details of the numerous revisions, maintains the correct formatting, provides helpful recommendations, and manages the complexity and chaos—all with a wonderful sense of humor. We cannot thank her enough.
Tina Brashers, MD, and Neal Rote, PhD, continued to serve as section editors and contributing authors. Tina is a distinguished teacher and has received numerous awards for her teaching and work with nursing and medical students and faculty. She is nationally known for her leadership and development in promoting and teaching interprofessional collaboration. Tina brings innovation and clarity to the subject of pathophysiology. Her contributions to the online course continue to be intensive and creative, and a significant learning enhancement for students. Thank you, Tina, for the outstanding quality of your work. Neal has major expertise, passion, and hard- to-find precision in the topics of immunity, reproductive biology, and human defenses. His expertise was well placed to rewrite and update the challenging tumor biology chapter. Neal has held many appointments, including department chair, associate dean, and professor in both reproductive biology and pathology. He is a top-notch researcher and reviewer of grants and has received numerous awards and recognition for his teaching. Neal has a gift for creating images that bring clarity to the complex content of immunology. He also completely updated the glossary. Thank you, Neal, for your persistence in promoting understanding and for your continuing devotion to students.
Karen Turner was our excellent Senior Content Development Specialist. Always gracious and efficient, Karen guided us through the hardest times and even the redo times. Thank you, Karen, especially for another set of “eagle eyes.” Jeanne Genz
retired as the Project Manager during the preparation of this edition and we will miss her expertise. Always dedicated and an amazing “can do” attitude, we thank you, Jeanne. Tracey Schriefer picked up the reins without missing a step. Thank you, Tracey, for such diligence—finding and correcting obscure errors. We also thank Beth Welch, who has copyedited our last four editions. Kellie White was our Executive Content Strategist and was responsible for overseeing the entire project. Very organized and a delightful sense of humor, we thank you Kellie. The internal layout, selection of colors, and design of the cover highlight the pedagogy and were done by our Designer, Margaret Reid. Thanks to the team from Graphic World, who created many new images and managed the cleanup and scanning of artwork obtained from many resources.
We thank the Department of Dermatology at the University of Utah School of Medicine, which provided numerous photos of skin lesions. Thank you to our many colleagues and friends at the University of Utah College of Nursing, School of Medicine, Eccles Medical Library, and College of Pharmacy for their helpfulness, suggestions, and critiques.
We extend gratitude to those who contributed to the book supplements. Linda Felver has created an all new inventive and resourceful Study Guide. Thank you, Linda, for your very astute edits. Additional thanks to the reviewers of the Study Guide, Janie Corbitt, Kathleen Murtaugh, and Linda Turchin. A special thanks to Linda Turchin, Joanna Cain, Stephen Krau, and Melanie Cole for their thorough approach in preparing the materials for the Evolve website, and to Linda Turchin, Kim Webb, and Lauren Mussig for the valuable reviews of these resources. Tina Brashers, Nancy Burruss, Mandi Counters, Joe Gordon, Melissa Geist, Kay Gaehle, Stephen Krau, Jason Mott, and Kim Webb also updated the interactive online lessons and activities for Pathophysiology Online.
Special thanks to faculty and nursing students and other health science students for your questions and suggestions. It is because of you, the future clinicians, that we are so motivated to put our best efforts into this work.
Sincerely and with great affection we thank our families, especially Mae and John. Always supportive, you make the work possible!
Sue E. Huether
Kathryn L. McCance
Introduction to Pathophysiology
The word root “patho” is derived from the Greek word pathos, which means suffering. The Greek word root “logos” means discourse or, more simply, system of formal study, and “physio” refers to functions of an organism. Altogether, pathophysiology is the study of the underlying changes in body physiology (molecular, cellular, and organ systems) that result from disease or injury. Important, however, is the inextricable component of suffering and the psychological, spiritual, social, cultural, and economic implications of disease. The science of pathophysiology seeks to provide an understanding of the
mechanisms of disease and to explain how and why alterations in body structure and function lead to the signs and symptoms of disease. Understanding pathophysiology guides healthcare professionals in the planning, selection, and evaluation of therapies and treatments. Knowledge of human anatomy and physiology and the interrelationship among
the various cells and organ systems of the body is an essential foundation for the study of pathophysiology. Review of this subject matter enhances comprehension of pathophysiologic events and processes. Understanding pathophysiology also entails the utilization of principles, concepts, and basic knowledge from other fields of study including pathology, genetics, epigenetics, immunology, and epidemiology. A number of terms are used to focus the discussion of pathophysiology; they may be used interchangeably at times, but that does not necessarily indicate that they have the same meaning. Those terms are reviewed here for the purpose of clarification. Pathology is the investigation of structural alterations in cells, tissues, and
organs, which can help identify the cause of a particular disease. Pathology differs from pathogenesis, which is the pattern of tissue changes associated with the development of disease. Etiology refers to the study of the cause of disease. Diseases may be caused by infection, heredity, gene–environment interactions, alterations in immunity, malignancy, malnutrition, degeneration, or trauma. Diseases that have no identifiable cause are termed idiopathic. Diseases that occur as a result of medical treatment are termed iatrogenic (for example, some antibiotics can injure the kidney and cause renal failure). Diseases that are acquired as a consequence of being in a hospital environment are called nosocomial. An infection that develops as a result of a person's immune system being depressed after receiving cancer treatment during a hospital stay would be defined as a nosocomial infection. Diagnosis is the naming or identification of a disease. A diagnosis is made from
an evaluation of the evidence accumulated from the presenting signs and symptoms, health and medical history, physical examination, laboratory tests, and imaging. A prognosis is the expected outcome of a disease. Acute disease is the sudden appearance of signs and symptoms that last only a short time. Chronic disease develops more slowly and the signs and symptoms last for a long time, perhaps for a lifetime. Chronic diseases may have a pattern of remission and exacerbation. Remissions are periods when symptoms disappear or diminish significantly. Exacerbations are periods when the symptoms become worse or more severe. A complication is the onset of a disease in a person who is already coping with another existing disease (for example, a person who has undergone surgery to remove a diseased appendix may develop the complication of a wound infection or pneumonia). Sequelae are unwanted outcomes of having a disease or are the result of trauma, such as paralysis resulting from a stroke or severe scarring resulting from a burn. Clinical manifestations are the signs and symptoms or evidence of disease. Signs
are objective alterations that can be observed or measured by another person, measures of bodily functions such as pulse rate, blood pressure, body temperature, or white blood cell count. Some signs are local, such as redness or swelling, and other signs are systemic, such as fever. Symptoms are subjective experiences reported by the person with disease, such as pain, nausea, or shortness of breath; and they vary from person to person. The prodromal period of a disease is the time during which a person experiences vague symptoms such as fatigue or loss of appetite before the onset of specific signs and symptoms. The term insidious symptoms describes vague or nonspecific feelings and an awareness that there is a change within the body. Some diseases have a latent period, a time during which no symptoms are readily apparent in the affected person, but the disease is nevertheless present in the body; an example is the incubation phase of an infection or the early growth phase of a tumor. A syndrome is a group of symptoms that occur together and may be caused by several interrelated problems or a specific disease; severe acute respiratory syndrome (SARS), for example, presents with a set of symptoms that include headache, fever, body aches, an overall feeling of discomfort, and sometimes dry cough and difficulty breathing. A disorder is an abnormality of function; this term also can refer to an illness or a particular problem such as a bleeding disorder. Epidemiology is the study of tracking patterns or disease occurrence and
transmission among populations and by geographic areas. Incidence of a disease is the number of new cases occurring in a specific time period. Prevalence of a disease is the number of existing cases within a population during a specific time period.
Risk factors, also known as predisposing factors, increase the probability that disease will occur, but these factors are not the cause of disease. Risk factors include heredity, age, gender, race, environment, and lifestyle. A precipitating factor is a condition or event that does cause a pathologic event or disorder. For example, asthma is precipitated by exposure to an allergen, or angina (pain) is precipitated by exertion. Pathophysiology is an exciting field of study that is ever-changing as new
discoveries are made. Understanding pathophysiology empowers healthcare professionals with the knowledge of how and why disease develops and informs their decision making to ensure optimal healthcare outcomes. Embedded in the study of pathophysiology is understanding that suffering is a personal, individual experience and a major component of disease.
PART ONE Basic Concepts of Pathophysiology
OUTLINE Unit 1 The Cell Unit 2 Mechanisms of Self-Defense Unit 3 Cellular Proliferation: Cancer
UNIT 1 The Cell
OUTLINE 1 Cellular Biology 2 Genes and Genetic Diseases 3 Epigenetics and Disease 4 Altered Cellular and Tissue Biology 5 Fluids and Electrolytes, Acids and Bases
1
Cellular Biology Kathryn L. McCance
CHAPTER OUTLINE
Prokaryotes and Eukaryotes, 1 Cellular Functions, 1 Structure and Function of Cellular Components, 2
Nucleus, 2 Cytoplasmic Organelles, 2 Plasma Membranes, 2 Cellular Receptors, 9
Cell-to-Cell Adhesions, 10
Extracellular Matrix, 10 Specialized Cell Junctions, 11
Cellular Communication and Signal Transduction, 12 Cellular Metabolism, 14
Role of Adenosine Triphosphate, 16 Food and Production of Cellular Energy, 16 Oxidative Phosphorylation, 16
Membrane Transport: Cellular Intake and Output, 17
Electrolytes as Solutes, 18 Transport by Vesicle Formation, 21 Movement of Electrical Impulses: Membrane Potentials, 24
Cellular Reproduction: The Cell Cycle, 25
Phases of Mitosis and Cytokinesis, 26 Rates of Cellular Division, 26 Growth Factors, 26
Tissues, 27
Tissue Formation, 27 Types of Tissues, 27
All body functions depend on the integrity of cells. Therefore an understanding of cellular biology is increasingly necessary to comprehend disease processes. An overwhelming amount of information reveals how cells behave as a multicellular “social” organism. At the heart of it all is cellular communication (cellular “crosstalk”)—how messages originate and are transmitted, received, interpreted, and used by the cell. Streamlined conversation between, among, and within cells maintains cellular function and specialization. Cells must demonstrate a “chemical fondness” for other cells to maintain the integrity of the entire organism. When they no longer tolerate this fondness, the conversation breaks down, and cells either adapt (sometimes altering function) or become vulnerable to isolation, injury, or disease.
Prokaryotes and Eukaryotes Living cells generally are divided into eukaryotes and prokaryotes. The cells of higher animals and plants are eukaryotes, as are the single-celled organisms, fungi, protozoa, and most algae. Prokaryotes include cyanobacteria (blue-green algae), bacteria, and rickettsiae. Prokaryotes traditionally were studied as core subjects of molecular biology. Today, emphasis is on the eukaryotic cell; much of its structure and function have no counterpart in bacterial cells. Eukaryotes (eu = good; karyon = nucleus; also spelled eucaryotes) are larger
and have more extensive intracellular anatomy and organization than prokaryotes. Eukaryotic cells have a characteristic set of membrane-bound intracellular compartments, called organelles, that includes a well-defined nucleus. The prokaryotes contain no organelles, and their nuclear material is not encased by a nuclear membrane. Prokaryotic cells are characterized by lack of a distinct nucleus. Besides having structural differences, prokaryotic and eukaryotic cells differ in
chemical composition and biochemical activity. The nuclei of prokaryotic cells carry genetic information in a single circular chromosome, and they lack a class of proteins called histones, which in eukaryotic cells bind with deoxyribonucleic acid (DNA) and are involved in the supercoiling of DNA. Eukaryotic cells have several or many chromosomes. Protein production, or synthesis, in the two classes of cells also differs because of major structural differences in ribonucleic acid (RNA)– protein complexes. Other distinctions include differences in mechanisms of transport across the outer cellular membrane and in enzyme content.
Cellular Functions Cells become specialized through the process of differentiation, or maturation, so that some cells eventually perform one kind of function and other cells perform other functions. Cells with a highly developed function, such as movement, often lack some other property, such as hormone production, which is more highly developed in other cells. The eight chief cellular functions are as follows:
1. Movement. Muscle cells can generate forces that produce motion. Muscles that are attached to bones produce limb movements, whereas those muscles that enclose hollow tubes or cavities move or empty contents when they contract (e.g., the colon).
2. Conductivity. Conduction as a response to a stimulus is manifested by a wave of excitation, an electrical potential that passes along the surface of the cell to reach its other parts. Conductivity is the chief function of nerve cells.
3. Metabolic absorption. All cells can take in and use nutrients and other substances from their surroundings.
4. Secretion. Certain cells, such as mucous gland cells, can synthesize new substances from substances they absorb and then secrete the new substances to serve as needed elsewhere.
5. Excretion. All cells can rid themselves of waste products resulting from the metabolic breakdown of nutrients. Membrane-bound sacs (lysosomes) within cells contain enzymes that break down, or digest, large molecules, turning them into waste products that are released from the cell.
6. Respiration. Cells absorb oxygen, which is used to transform nutrients into energy in the form of adenosine triphosphate (ATP). Cellular respiration, or oxidation, occurs in organelles called mitochondria.
7. Reproduction. Tissue growth occurs as cells enlarge and reproduce themselves. Even without growth, tissue maintenance requires that new cells be produced to replace cells that are lost normally through cellular death. Not all cells are capable of continuous division (see Chapter 4).
8. Communication. Communication is vital for cells to survive as a society of cells.
Appropriate communication allows the maintenance of a dynamic steady state.
Structure and Function of Cellular Components Figure 1-1, A, shows a “typical” eukaryotic cell, which consists of three components: an outer membrane called the plasma membrane, or plasmalemma; a fluid “filling” called cytoplasm (Figure 1-1, B); and the “organs” of the cell—the membrane-bound intracellular organelles, among them the nucleus.
FIGURE 1-1 Typical Components of a Eukaryotic Cell and Structure of the Cytoplasm. A, Artist's interpretation of cell structure. Note the many mitochondria known as the “power plants of the
cell.” B, Color-enhanced electron micrograph of a cell. The cell is crowded. Note, too, the innumerable dots bordering the endoplasmic reticulum. These are ribosomes, the cell's
“protein factories.” (B, from Patton KT, Thibodeau GA: Anatomy & physiology, ed 8, St Louis, 2013, Mosby.)
Nucleus The nucleus, which is surrounded by the cytoplasm and generally is located in the
center of the cell, is the largest membrane-bound organelle. Two pliable membranes compose the nuclear envelope (Figure 1-2, A). The nuclear envelope is pockmarked with pits, called nuclear pores, which allow chemical messages to exit and enter the nucleus (see Figure 1-2). The outer membrane is continuous with membranes of the endoplasmic reticulum (see Figure 1-1). The nucleus contains the nucleolus (a small dense structure composed largely of ribonucleic acid), most of the cellular DNA, and the DNA-binding proteins (i.e., the histones) that regulate its activity. The DNA “chain” in eukaryotic cells is so long that it is easily broken. Therefore the histones that bind to DNA cause DNA to fold into chromosomes (Figure 1-2, C), which decreases the risk of breakage and is essential for cell division in eukaryotes.
FIGURE 1-2 The Nucleus. The nucleus is composed of a double membrane, called a nuclear envelope, that encloses the fluid-filled interior, called nucleoplasm. The chromosomes are
suspended in the nucleoplasm (illustrated here much larger than actual size to show the tightly packed DNA strands). Swelling at one or more points of the chromosome, shown in A, occurs at
a nucleolus where genes are being copied into RNA. The nuclear envelope is studded with pores. B, The pores are visible as dimples in this freeze-etch of a nuclear envelope. C, Histone-
folding DNA in chromosomes. (B, from Raven PH, Johnson GB: Biology, St Louis, 1992, Mosby.)
The primary functions of the nucleus are cell division and control of genetic information. Other functions include the replication and repair of DNA and the transcription of the information stored in DNA. Genetic information is transcribed into ribonucleic acid (RNA), which can be processed into messenger, transport, and ribosomal RNAs and introduced into the cytoplasm, where it directs cellular activities. Most of the processing of RNA occurs in the nucleolus. (The roles of DNA and RNA in protein synthesis are discussed in Chapter 2.)
Cytoplasmic Organelles Cytoplasm is an aqueous solution (cytosol) that fills the cytoplasmic matrix—the
space between the nuclear envelope and the plasma membrane. The cytosol represents about half the volume of a eukaryotic cell. It contains thousands of enzymes involved in intermediate metabolism and is crowded with ribosomes making proteins (see Figure 1-1, B). Newly synthesized proteins remain in the cytosol if they lack a signal for transport to a cell organelle.1 The organelles suspended in the cytoplasm are enclosed in biologic membranes, so they can simultaneously carry out functions requiring different biochemical environments. Many of these functions are directed by coded messages carried from the nucleus by RNA. The functions include synthesis of proteins and hormones and their transport out of the cell, isolation and elimination of waste products from the cell, performance of metabolic processes, breakdown and disposal of cellular debris and foreign proteins (antigens), and maintenance of cellular structure and motility. The cytosol is a storage unit for fat, carbohydrates, and secretory vesicles. Table 1-1 lists the principal cytoplasmic organelles.
Quick Check 1-1
1. Why is the process of differentiation essential to specialization? Give an example.
2. Describe at least two cellular functions.
TABLE 1-1 Principal Cytoplasmic Organelles
Organelle Characteristics and Description Ribosomes RNA-protein complexes (nucleoproteins) synthesized in nucleolus and secreted into cytoplasm. Provide sites for cellular protein synthesis. Endoplasmic reticulum
Network of tubular channels (cisternae) that extend throughout outer nuclear membrane. Specializes in synthesis and transport of protein and lipid components of most organelles.
Golgi complex
Network of smooth membranes and vesicles located near nucleus. Responsible for processing and packaging proteins onto secretory vesicles that break away from the complex and migrate to various intracellular and extracellular destinations, including plasma membrane. Best- known vesicles are those that have coats largely made of the protein clathrin. Proteins in the complex bind to the cytoskeleton, generating tension that helps organelle function and keep its stretched shape intact.
Lysosomes Saclike structures that originate from Golgi complex and contain enzymes for digesting most cellular substances to their basic form, such as amino acids, fatty acids, and carbohydrates (sugars). Cellular injury leads to release of lysosomal enzymes that cause cellular self-destruction.
Peroxisomes Similar to lysosomes but contain several oxidative enzymes (e.g., catalase, urate oxidase) that produce hydrogen peroxide; reactions detoxify various wastes.
Mitochondria Contain metabolic machinery needed for cellular energy metabolism. Enzymes of respiratory chain (electron-transport chain), found in inner membrane of mitochondria, generate most of cell's ATP (oxidative phosphorylation). Have a role in osmotic regulation, pH control, calcium homeostasis, and cell signaling.
Cytoskeleton “Bone and muscle” of cell. Composed of a network of protein filaments, including microtubules and actin filaments (microfilaments); forms cell extensions (microvilli, cilia, flagella).
Caveolae Tiny indentations (caves) that can capture extracellular material and shuttle it inside the cell or across the cell. Vaults Cytoplasmic ribonucleoproteins shaped like octagonal barrels. Thought to act as “trucks,” shuttling molecules from nucleus to elsewhere in
cell.
Plasma Membranes Every cell is contained within a membrane with gates, channels, and pumps. Membranes surround the cell or enclose an intracellular organelle and are exceedingly important to normal physiologic function because they control the composition of the space, or compartment, they enclose. Membranes can allow or exclude various molecules and, because of selective transport systems, they can move molecules in or out of the space (Figure 1-3). By controlling the movement of substances from one compartment to another, membranes exert a powerful influence on metabolic pathways. Directional transport is facilitated by polarized domains, distinct apical and basolateral domains. Cell polarity, the direction of cellular transport, maintains normal cell and tissue structure for numerous functions (for example, movement of nutrients in and out of the cell) and becomes altered with diseases (Figure 1-4). The plasma membrane also has an important role in cell- to-cell recognition. Other functions of the plasma membrane include cellular mobility and the maintenance of cellular shape (Table 1-2).
FIGURE 1-3 Functions of Plasma Membrane Proteins. The plasma membrane proteins illustrated here show a variety of functions performed by the different types of plasma
membranes. (From Raven PH, Johnson GB: Understanding biology, ed 3, Dubuque, Iowa, 1995, Brown.)
FIGURE 1-4 Cell Polarity of Epithelial Cells. Schematic of cell polarity (cell direction) of epithelial cells. Shown are the directions of the basal side and the apical side. Organelles and
cytoskeleton are also arranged directionally to enable, for example, intestinal cell secretion and absorption. (Adapted from Life science web textbook, The University of Tokyo.)
TABLE 1-2 Plasma Membrane Functions
Cellular Mechanism
Membrane Functions
Structure Usually thicker than membranes of intracellular organelles Containment of cellular organelles Maintenance of relationship with cytoskeleton, endoplasmic reticulum, and other organelles Maintenance of fluid and electrolyte balance Outer surfaces of plasma membranes in many cells are not smooth but are dimpled with cavelike indentations called caveolae; they are also studded with cilia or even smaller cylindrical projections called microvilli; both are capable of movement
Protection Barrier to toxic molecules and macromolecules (proteins, nucleic acids, polysaccharides) Barrier to foreign organisms and cells
Activation of cell
Hormones (regulation of cellular activity) Mitogens (cellular division; see Chapter 2) Antigens (antibody synthesis; see Chapter 6) Growth factors (proliferation and differentiation; see Chapter 10)
Storage Storage site for many receptors Transport Diffusion and exchange diffusion Endocytosis (pinocytosis, phagocytosis) Exocytosis (secretion) Active transport
Cell-to-cell interaction
Communication and attachment at junctional complexes Symbiotic nutritive relationships Release of enzymes and antibodies to extracellular environment Relationships with extracellular matrix
Modified from King DW, Fenoglio CM, Lefkowitch JH: General pathology: principles and dynamics, Philadelphia, 1983, Lea & Febiger.
Membrane Composition The basic structure of cell membranes is the lipid bilayer, composed of two apposing leaflets and proteins that span the bilayer or interact with the lipids on either side of the two leaflets (Figure 1-5). Lipid research is growing and principles of membrane organization are being overhauled.2 In short, the main constituents of cell membranes are lipids and proteins. Historically, the plasma membrane was described as a fluid lipid bilayer (fluid mosaic model) composed of a uniform lipid distribution with inserted moving proteins. It now appears that the lipid bilayer is a much more complex structure where lipids and proteins are not uniformly distributed but can separate into discrete units called microdomains, differing in their protein and lipid compositions.3 Different membranes have varying percentages of lipids and proteins. Intracellular membranes may have a higher percentage of proteins than do plasma membranes, presumably because most enzymatic activity occurs within organelles. The membrane organization is achieved through noncovalent bonds that allow different physical states called phases. The lipid bilayer can be structured in three main phases: solid gel phase, fluid liquid-crystalline phase, and liquid-ordered phase (Figure 1-5, B). These phases can change under physiologic factors such as temperature and pressure
fluctuations. Carbohydrates are mainly associated with plasma membranes, in which they are chemically combined with lipids, forming glycolipids, and with proteins, forming glycoproteins (see Figure 1-5).
FIGURE 1-5 Lipid Bilayer Membranes. A, Concepts of biologic membranes have markedly changed in the last two decades, from the classic fluid mosaic model to the current model that lipids and proteins are not evenly distributed but can isolate into microdomains, differing in their protein and lipid composition. B, An example of a microdomain is lipid rafts (yellow). Rafts are dynamic domain structures composed of cholesterol, sphingolipids, and membrane proteins important in different cellular processes. Various models exist to clarify the functions of
domains. The three major phases of lipid bilayer organization include a solid gel phase (e.g., with low temperatures), a liquid-ordered phase (high temperatures), and a fluid liquid-crystalline (or liquid-disordered) phase. Some membrane-associated proteins are integrated into the lipid bilayer; other proteins are loosely attached to the outer and inner surfaces of the membrane.
Transmembrane proteins protrude through the entire outer and inner surfaces of the membrane, and they can be attracted to microdomains through specific interactions with lipids. Interaction of the membrane proteins with distinct lipids depends on the hydrophobic thickness of the membrane, the lateral pressures of the membrane (mechanical force may shift protein channels from an open to closed state), the polarity or electrical charges at the lipid-protein interface, and the presence on the protein side of amino acid side chains. Important for pathophysiology is the proposal that protein-lipid interactions can be critical for correct
insertion, folding, and orientation of membrane proteins. For example, diseases related to lipids that interfere with protein folding are becoming more prevalent. C, The cell membrane is not static but is always moving. Observed for the first time from measurements taken at the National Institute of Standards and Technology (NIST) and France's Institut Laue-Langevin
(ILL). (Adapted from Bagatolli LA et al: Prog Lipid Res 49[4]:378-389, 2010; Contreras FX et al: Cold Spring Harb Perspect Biol 3[6]:pii a004705,
2011; Cooper GM: The cell—a molecular approach, ed 2, Sunderland (MA): Sinauer Associates, 2000; Defamie N, Mesnil M: Biochim Biophys Acta 1818(8):1866-1869, 2012; W oodka AC et al: Phys Rev Lett 9(5):058102, 2012.)
The outer surface of the plasma membrane in many types of cells, especially endothelial cells and adipocytes, is not smooth but dimpled with flask-shaped invaginations known as caveolae (“tiny caves”). Caveolae serve as a storage site for many receptors, provide a route for transport into the cell, and act as the initiator for relaying signals from several extracellular chemical messengers into the cell's interior (see p. 24).
Lipids. Each lipid molecule is said to be polar, or amphipathic, which means that one part is hydrophobic (uncharged, or “water hating”) and another part is hydrophilic (charged, or “water loving”) (Figure 1-6). The membrane spontaneously organizes itself into two layers because of these two incompatible solubilities. The hydrophobic region (hydrophobic tail) of each lipid molecule is protected from water, whereas the hydrophilic region (hydrophilic head) is immersed in it. The bilayer serves as a barrier to the diffusion of water and hydrophilic substances, while allowing lipid-soluble molecules, such as oxygen (O2) and carbon dioxide (CO2), to diffuse through the membrane readily.
FIGURE 1-6 Structure of a Phospholipid Molecule. A, Each phospholipid molecule consists of a phosphate functional group and two fatty acid chains attached to a glycerol molecule. B, The fatty acid chains and glycerol form nonpolar, hydrophobic “tails,” and the phosphate functional group forms the polar, hydrophilic “head” of the phospholipid molecule. When placed in water, the hydrophobic tails of the molecule face inward, away from the water, and the hydrophilic head faces outward, toward the water. (From Raven PH, Johnson GB: Understanding biology, ed 3, Dubuque, Iowa, 1995,
Brown.)
A major component of the plasma membrane is a bilayer of lipid molecules— glycerophospholipids, sphingolipids, and sterols (for example, cholesterol). The
most abundant lipids are phospholipids. Phospholipids have a phosphate-containing hydrophilic head connected to a hydrophobic tail. Phospholipids and glycolipids form self-sealing lipid bilayers. Lipids along with protein assemblies act as “molecular glue” for the structural integrity of the membrane. Investigators are studying the concept of lipid rafts. Membrane lipid rafts (MLRs) appear to be structurally and functionally distinct regions of the plasma membrane4,5 and consist of cholesterol and sphingolipid-dependent microdomains that form a network of lipid-lipid, protein-protein, and protein-lipid interactions (Figures 1-5, B, and 1-7) Although discrepancies between experimental results exist, two main types of MLRs are hypothesized: those that contain the cholesterol-binding protein caveolin (see p. 24) and those that do not.4 Researchers hypothesized there are lipid rafts that have several functions, including (1) providing cellular polarity and organization of signaling trafficking; (2) acting as platforms for extracellular matrix (ECM) adhesion and intracellular cytoskeletal tethering to the plasma membrane through cellular adhesion molecules (CAMs, see p. 8); (3) enabling signaling across the membrane, which can rearrange cytoskeletal architecture and regulate cell growth, migration, and other functions; and (4) allowing entry of viruses, bacteria, toxins, and nanoparticles.4
FIGURE 1-7 Lipid Rafts. The plasma membrane is composed of many lipids, including sphingomyelin (SM) and cholesterol, shown here as a small raft in the external leaflet. GS,
Glycosphingolipid; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine. (From Pollard TD, Ernshaw W C: Cell biology, St Louis, 2004, Saunders Elsevier.)
Proteins. A protein is made from a chain of amino acids known as polypeptides. There are 20 types of amino acids in proteins and each type of protein has a unique sequence
of amino acids. Proteins are the major workhorses of the cell. After translation (the synthesis of protein from RNA, see Chapter 2) of a protein, posttranslational modifications (PTMs) are the methods used to diversify the limited numbers of proteins generated. These modifications alter the activity and functions of proteins and have become very important in understanding diseases. Researchers have known for decades that pathogens interfere with the host's PTMs.6 New approaches are being used to understand changes in proteins—a field called proteomics is the study of the proteome, or entire set of proteins expressed by a genome from synthesis, translocation, and modification (e.g., folding), and the analysis of the roles of proteomes in a staggering number of diseases. Membrane proteins associate with the lipid bilayer in different ways (Figure 1-8),
including (1) transmembrane proteins that extend across the bilayer and exposed to an aqueous environment on both sides of the membrane (see Figure 1-8, A); (2) proteins located almost entirely in the cytosol and associated with the cytosolic half of the lipid bilayer by an α helix exposed on the surface of the protein (see Figure 1- 8, B); (3) proteins that exist outside the bilayer, on one side or the other, and attached to the membrane by one or more covalently attached lipid groups (see Figure 1-8, C); and (4) proteins bound indirectly to one or the other bilayer membrane face and held in place by their interactions with other proteins (see Figure 1-8, D).1
FIGURE 1-8 Proteins Attach to the Plasma Membrane in Different Ways. A, Transmembrane proteins extend through the membrane as a single α helix, as multiple α helices, or as a rolled up barrel-like sheet called a β barrel. B, Some membrane proteins are anchored to the cytosolic side of the lipid bilayer by an amphipathic α helix. C, Some proteins are linked on either side of
the membrane by a covalently attached lipid molecule. D, Proteins are attached by weak noncovalent interactions with other membrane proteins. All are integral membrane proteins
except. (D, adapted from Alberts B: Essential cell biology, ed 4, New York, 2014, Garland.)
Proteins directly attached to the membrane bilayer can be removed by dissolving the bilayer with detergents called integral membrane proteins. The remaining
proteins that can be removed by gentler procedures that interfere with protein- protein interactions but do not dissolve the bilayer are known as peripheral membrane proteins. Proteins exist in densely folded molecular configurations rather than straight
chains; so most hydrophilic units are at the surface of the molecule and most hydrophobic units are inside. Membrane proteins, like other proteins, are synthesized by the ribosome and then make their way, called trafficking, to different membrane locations of a cell.7 Trafficking places unique demands on membrane proteins for folding, translocation, and stability.7 Thus, much research is now being done to understand misfolded proteins (for example, as a cause of disease; Box 1- 1).
Box 1-1 Endoplasmic Reticulum, Protein Folding, and ER Stress Protein folding in the endoplasmic reticulum (ER) is critical for us. As the biologic workhorses, proteins perform vital functions in every cell. To do these tasks proteins must fold into complex three-dimensional structures (see figure). Most secreted proteins fold and are modified in an error-free manner, but ER or cell stress, mutations, or random (stochastic) errors during protein synthesis can decrease the folding amount or the rate of folding. Pathophysiologic processes, such as viral infections, environmental toxins, and mutant protein expression, can perturb the sensitive ER environment. Natural processes also can perturb the environment, such as the large protein-synthesizing load placed on the ER. These perturbations cause the accumulation of immature and abnormal proteins in cells, leading to ER stress. Fortunately, the ER is loaded with protective ways to help folding; for example, protein chaperones facilitate folding and prevent the formation of off-pathway types. Because specialized cells produce large amounts of secreted proteins, the movement or flux through the ER is tremendous. Therefore misfolded proteins not repaired in the ER are observed in some diseases and can initiate apoptosis or cell death. It has recently been shown that the endoplasmic reticulum mediates intracellular signaling pathways in response to the accumulation of unfolded or misfolded proteins; collectively, the pathways are known as the unfolded-protein response (UPR). Investigators are studying UPR- associated inflammation and how the UPR is coupled to inflammation in health and disease. Specific diseases include Alzheimer disease, Parkinson disease, prion disease, amyotrophic lateral sclerosis, and diabetes mellitus. Additionally being
studied is ER stress and how it may accelerate age-related dysfunction.
Protein Folding. Each protein exists as an unfolded polypeptide (left) or a random coil after the process of translation from a sequence of mRNA to a linear string of amino acids. From amino acids interacting with each other they produce a three-dimensional structure called the folded
protein (right) that is its native state.
Data from Brodsky J, Skach WR: Curr Opin Cell Biol 23:464-475, 2011; Jäger R et al: Biol Cell 104(5):259- 270,2012; Ron D, Walter P: Nat Rev Mol Cell Biol 8:519-529, 2007.
Although membrane structure is determined by the lipid bilayer, membrane functions are determined largely by proteins. Proteins act as (1) recognition and binding units (receptors) for substances moving into and out of the cell; (2) pores or transport channels for various electrically charged particles, called ions or electrolytes, and specific carriers for amino acids and monosaccharides; (3) specific enzymes that drive active pumps to promote concentration of certain ions, particularly potassium (K+), within the cell while keeping concentrations of other ions (for example, sodium, Na+), less than concentrations found in the extracellular environment; (4) cell surface markers, such as glycoproteins (proteins attached to carbohydrates), that identify a cell to its neighbor; (5) cell adhesion molecules (CAMs), or proteins that allow cells to hook together and form attachments of the cytoskeleton for maintaining cellular shape; and (6) catalysts of chemical reactions (for example, conversion of lactose to glucose; see Figure 1-3). Membrane proteins are key components of energy transduction, converting chemical energy into electrical energy, or electrical energy into either mechanical energy or synthesis of ATP.7 Investigators are studying ATP enzymes and the changes in shape of biologic membranes, particularly mitochondrial membranes, and their relationship to aging and disease.8-10
In animal cells, the plasma membrane is stabilized by a meshwork of proteins attached to the underside of the membrane called the cell cortex. Human red blood cells have a cell cortex that maintains their flattened biconcave shape.1
Protein regulation in a cell: protein homeostasis. The cellular protein pool is in constant change or flux. The number of copies of a protein in a cell depends on how quickly it is made and how long it survives or is broken down. This adaptable system of protein homeostasis is defined by the “proteostasis” network comprised of ribosomes (makers); chaperones (helpers); and two protein breakdown systems or proteolytic systems—lysosomes and the ubiquitin-proteasome system (UPS). These systems regulate protein homeostasis under a large variety of conditions, including variations in nutrient supply, the existence of oxidative stress or cellular differentiation, changes in temperature, and the presence of heavy metal ions and other sources of stress.11 Malfunction or failure of the proteostasis network is associated with human disease12 (Figure 1-9).
FIGURE 1-9 Protein Homeostasis System and Outcomes. A main role of the protein homeostasis network (proteostasis) is to minimize protein misfolding and protein aggregation. The network includes ribosome-mediated protein synthesis, chaperone (folding helpers in the ER) and enzyme mediated folding, breakdown systems of lysosome and proteasome-mediated protein degradation, and vesicular trafficking. The network integrates biologic pathways that balance folding, trafficking, and protein degradation depicted by arrows b, d, e, f, g, h, and i. ER,
Endoplasmic reticulum. (Adapted from Lindquist SL, Kelly JW : Cold Spring Harb Perspect Biol 3[12]:pii: a004507, 2011.)
Carbohydrates. The short chains of sugars or carbohydrates (oligosaccharides) contained within the plasma membrane are generally bound to membrane proteins (glycoproteins) and lipids (glycolipids). Long polysaccharide chains attached to membrane proteins are called proteoglycans. All of the carbohydrate on the glycoproteins, proteoglycans, and glycolipids is located on the outside of the plasma membrane and the carbohydrate coating is called the glycocalyx. The glycocalyx helps protect the cell from mechanical damage.1 Additionally, the layer of carbohydrate gives the cell a slimy surface that assists the mobility of other cells, like leukocytes, to squeeze through the narrow spaces.1 The functions of carbohydrates are more than protection and lubrication and include specific cell-cell recognition and adhesion. Intercellular recognition is an important function of membrane oligosaccharides; for example, the transmembrane proteins called lectins, which bind to a particular oligosaccharide, recognize neutrophils at the site of bacterial infection. This recognition allows the neutrophil to adhere to the blood vessel wall and migrate from the blood into the infected tissue to help eliminate the invading bacteria.1
Cellular Receptors Cellular receptors are protein molecules on the plasma membrane, in the cytoplasm, or in the nucleus that can recognize and bind with specific smaller molecules called ligands (from the Latin ligare, “to bind”) (Figure 1-10). The region of a protein that associates with a ligand is called its binding site. Hormones, for example, are ligands. Recognition and binding depend on the chemical configuration of the receptor and its smaller ligand, which must fit together somewhat like pieces of a jigsaw puzzle (see Chapter 18). Binding selectively to a protein receptor with high affinity to a ligand depends on formation of weak, noncovalent interactions—hydrogen bonds, electrostatic attractions, and van der Waals attractions—and favorable hydrophobic forces.1 Numerous receptors are found in most cells, and ligand binding to receptors activates or inhibits the receptor's associated signaling or biochemical pathway (see p. 12).
FIGURE 1-10 Cellular Receptors. (A) 1, Plasma membrane receptor for a ligand (here, a hormone molecule) on the surface of an integral protein. A neurotransmitter can exert its effect on a postsynaptic cell by means of two fundamentally different types of receptor proteins: 2, channel-linked receptors, and 3, non–channel-linked receptors. Channel-linked receptors are also known as ligand-gated channels. (B) Example of ligand-receptor interaction. Insulin-like
growth factor 1 (IGF-1) is a ligand and binds to the insulin-like growth factor 1 receptor (IGF-1R). With binding at the cell membrane the intracellular signaling pathway is activated, causing
translation of new proteins to act as intracellular communicators. This pathway is important for cancer growth. Researchers are developing pharmacologic strategies to reduce signaling at and downstream of the insulin-like growth factor 1 receptor (IGF-1R), hoping this will lead to
compounds useful in cancer treatment.
Plasma membrane receptors protrude from or are exposed at the external surface of the membrane and are important for cellular uptake of ligands (see Figure 1-10). The ligands that bind with membrane receptors include hormones, neurotransmitters, antigens, complement components, lipoproteins, infectious agents, drugs, and metabolites. Many new discoveries concerning the specific interactions of cellular receptors with their respective ligands have provided a basis for understanding disease. Although the chemical nature of ligands and their receptors differs, receptors are
classified based on their location and function. Cellular type determines overall cellular function, but plasma membrane receptors determine which ligands a cell will bind with and how the cell will respond to the binding. Specific processes also control intracellular mechanisms. Receptors for different drugs are found on the plasma membrane, in the
cytoplasm, and in the nucleus. Membrane receptors have been found for certain anesthetics, opiates, endorphins, enkephalins, antibiotics, cancer chemotherapeutic agents, digitalis, and other drugs. Membrane receptors for endorphins, which are opiate-like peptides isolated from the pituitary gland, are found in large quantities in pain pathways of the nervous system (see Chapters 13 and 14). With binding to the receptor, the endorphins (or drugs such as morphine) change the cell's permeability to ions, increase the concentration of molecules that regulate intracellular protein synthesis, and initiate molecular events that modulate pain perception. Receptors for infectious microorganisms, or antigen receptors, bind bacteria,
viruses, and parasites to the cell membrane. Antigen receptors on white blood cells (lymphocytes, monocytes, macrophages, granulocytes) recognize and bind with antigenic microorganisms and activate the immune and inflammatory responses (see Chapter 6).
Cell-to-Cell Adhesions Cells are small and squishy, not like bricks. They are enclosed only by a flimsy membrane, yet the cell depends on the integrity of this membrane for its survival. How can cells be connected strongly, with their membranes intact, to form a muscle that can lift this textbook? Plasma membranes not only serve as the outer boundaries of all cells but also allow groups of cells to be held together robustly, in cell-to-cell adhesions, to form tissues and organs. Once arranged, cells are linked by three different means: (1) cell adhesion molecules in the cell's plasma membrane (see p. 8), (2) the extracellular matrix, and (3) specialized cell junctions.
Extracellular Matrix Cells can be united by attachment to one another or through the extracellular matrix (including the basement membrane), which the cells secrete around themselves. The extracellular matrix is an intricate meshwork of fibrous proteins embedded in a watery, gel-like substance composed of complex carbohydrates (Figure 1-11). The matrix is similar to glue; however, it provides a pathway for diffusion of nutrients, wastes, and other water-soluble substances between the blood and tissue cells. Interwoven within the matrix are three groups of macromolecules: (1) fibrous structural proteins, including collagen and elastin; (2) adhesive glycoproteins, such as fibronectin; and (3) proteoglycans and hyaluronic acid.
1. Collagen forms cablelike fibers or sheets that provide tensile strength or resistance to longitudinal stress. Collagen breakdown, such as occurs in osteoarthritis, destroys the fibrils that give cartilage its tensile strength.
2. Elastin is a rubber-like protein fiber most abundant in tissues that must be capable of stretching and recoiling, such as found in the lungs.
3. Fibronectin, a large glycoprotein, promotes cell adhesion and cell anchorage. Reduced amounts have been found in certain types of cancerous cells; this allows cancer cells to travel, or metastasize, to other parts of the body. All of these macromolecules occur in intercellular junctions and cell surfaces and may assemble into two different components: interstitial matrix and basement membrane (BM) (see Figure 1-11).
FIGURE 1-11 Extracellular Matrix. A, Tissues are not just cells but also extracellular space. The extracellular space is an intricate network of macromolecules called the extracellular matrix
(ECM). The macromolecules that constitute the ECM are secreted locally (by mostly fibroblasts) and assembled into a meshwork in close association with the surface of the cell that produced
them. Two main classes of macromolecules include proteoglycans, which are bound to polysaccharide chains called glycosaminoglycans, and fibrous proteins (e.g., collagen, elastin,
fibronectin, and laminin), which have structural and adhesive properties. Together the proteoglycan molecules form a gel-like ground substance in which the fibrous proteins are
embedded. The gel permits rapid diffusion of nutrients, metabolites, and hormones between the blood and the tissue cells. Matrix proteins modulate cell-matrix interactions, including normal tissue remodeling (which can become abnormal, for example, with chronic inflammation).
Disruptions of this balance result in serious diseases such as arthritis, tumor growth, and other pathologic conditions. B, Scanning electron micrograph of a chick embryo where a portion of the epithelium has been removed, exposing the curtain-like extracellular matrix. (A, adapted from
Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders; B, © Robert L Trelstad; from Gartner LP, Hiatt JL: Color textbook of histology, ed 3, St Louis, 2006, Saunders/Elsevier.)
The basement membrane is a thin, tough layer of extracellular matrix (connective tissue) underlying the epithelium of many organs and is also called the basal lamina (see Figure 1-11, B). The extracellular matrix is secreted by fibroblasts (“fiber formers”) (Figure 1-
12), local cells that are present in the matrix. The matrix and the cells within it are known collectively as connective tissue because they interconnect cells to form
tissues and organs. Human connective tissues are enormously varied. They can be hard and dense, like bone; flexible, like tendons or the dermis of the skin; resilient and shock absorbing, like cartilage; or soft and transparent, similar to the jelly-like substance that fills the eye. In all these examples, the majority of the tissue is composed of extracellular matrix, and the cells that produce the matrix are scattered within it like raisins in a pudding (see Figure 1-12).
FIGURE 1-12 Fibroblasts in Connective Tissue. This micrograph shows tissue from the cornea of a rat. The extracellular matrix surrounds the fibroblasts (F). (From Nishida T et al: The extracellular matrix of
animal connective tissues, Invest Ophthalmol Vis Sci 29:1887-1880, 1998.)
The matrix is not just passive scaffolding for cellular attachment but also helps regulate the function of the cells with which it interacts. The matrix helps regulate such important functions as cell growth and differentiation.
Specialized Cell Junctions
Cells in direct physical contact with neighboring cells are often interconnected at specialized plasma membrane regions called cell junctions. Cell junctions are classified by their function: (1) some hold cells together and form a tight seal (tight junctions); (2) some provide strong mechanical attachments (adherens junctions, desmosomes, hemidesmosomes); (3) some provide a special type of chemical communication (for example, inorganic ions and small water-soluble molecules to move from the cytosol of one cell to the cytosol of another cell), such as those causing an electrical wave (gap junctions); and (4) some maintain apico-basal polarity of individual epithelial cells (tight junctions) (Figure 1-13). Overall, cell junctions make the epithelium leak-proof and mediate mechanical attachment of one cell to another, allow communicating tunnels and maintaining cell polarity.
FIGURE 1-13 Junctional Complex. A, Schematic drawing of a belt desmosome between epithelial cells. This junction, also called the zonula adherens, encircles each of the interacting cells. The spot desmosomes and hemidesmosomes, like the belt desmosomes, are adhering
junctions. This tight junction is an impermeable junction that holds cells together but seals them in such a way that molecules cannot leak between them. The gap junction, as a communicating junction, mediates the passage of small molecules from one interacting cell to the other. B, Connexons. The connexin gap junction proteins have four transmembrane domains and they
play a vital role in maintaining cell and tissue function and homeostasis. Cells connected by gap junctions are considered ionically (electrically) and metabolically coupled. Gap junctions
coordinate the activities of adjacent cells; for example, they are important for synchronizing contractions of heart muscle cells through ionic coupling and for permitting action potentials to spread rapidly from cell to cell in neural tissues. The reason gap junctions occur in tissues that
are not electrically active is unknown. Although most gap junctions are associated with junctional complexes, they sometimes exist as independent structures. C, Electron micrograph of desmosomes. (A and C from Raven PH, Johnson GB: Biology, St Louis, 1992, Mosby; B, adapted from Gartner LP, Hiatt JL: Color textbook of histology, ed 3, St Louis, 2006, Saunders Elsevier; Sherwood L: Learning, ed 8, Belmont, Calif, 2013, Brooks/Cole CENGAGE.)
Cell junctions can be classified as symmetric and asymmetric. Symmetric junctions include tight junctions, the belt desmosome (zonula adherens), desmosomes (macula adherens), and gap junctions (also called intercellular channel
or communicating junctions).13 An asymmetric junction is the hemidesmosome (see Figure 1-13). Together they form the junctional complex. Desmosomes unite cells either by forming continuous bands or belts of epithelial sheets or by developing button-like points of contact. Desmosomes also act as a system of braces to maintain structural stability. Tight junctions are barriers to diffusion, prevent the movement of substances through transport proteins in the plasma membrane, and prevent the leakage of small molecules between the plasma membranes of adjacent cells. Gap junctions are clusters of communicating tunnels or connexons that allow small ions and molecules to pass directly from the inside of one cell to the inside of another. Connexons are hemichannels that extend outward from each of the adjacent plasma membranes (Figure 1-13, C). Multiple factors regulate gap junction intercellular communication, including
voltage across the junction, intracellular pH, intracellular Ca++ concentration, and protein phosphorylation. The most abundant human connexin is connexin 43 (Cx43).14 Investigators recently showed that loss of Cx43 expression in colorectal tumors is correlated with a shorter cancer-free survival rate.15 This study is the first evidence that Cx43 acts as a tumor suppressor for colorectal cancer (enhances apoptosis) and therefore may be an important prognostic marker and target for therapy.15 Investigators also recently reported that glycyrrhizic acid (GA), a glycoside of licorice root extracts, may be a strong chemopreventive agent against carcinogens; induced colon cancer in rats and Cx43 is one target.16 Too much GA often in humans may lead to hypokalemia and hypertension.17 The junctional complex is a highly permeable part of the plasma membrane. Its
permeability is controlled by a process called gating. Increased levels of cytoplasmic calcium cause decreased permeability at the junctional complex. Gating enables uninjured cells to protect themselves from injured neighbors. Calcium is released from injured cells.
Cellular Communication and Signal Transduction Cells need to communicate with each other to maintain a stable internal environment, or homeostasis; to regulate their growth and division; to oversee their development and organization into tissues; and to coordinate their functions. Cells communicate by using hundreds of kinds of signal molecules, for example, insulin (see Figure 1-10, B). Cells communicate in three main ways: (1) they display plasma membrane–bound signaling molecules (receptors) that affect the cell itself and other cells in direct physical contact (Figure 1-14, A); (2) they affect receptor proteins inside the target cell and the signal molecule has to enter the cell to bind to them (Figure 1-14, B); and (3) they form protein channels (gap junctions) that directly coordinate the activities of adjacent cells (Figure 1-14, C). Alterations in cellular communication affect disease onset and progression. In fact, if a cell cannot perform gap junctional intercellular communication, normal growth control and cell differentiation is compromised, thereby favoring cancerous tumor development (see Chapter 10). (Communication through gap junctions was discussed earlier, and contact signaling by plasma membrane–bound molecules is discussed on this page and on p. 15.) Secreted chemical signals involve communication locally and at a distance. Primary modes of intercellular signaling are contact-dependent, paracrine, hormonal, neurohormonal, and neurotransmitter. Autocrine stimulation occurs when the secreting cell targets itself (Figure 1-15).
FIGURE 1-14 Cellular Communication. Three primary ways cells communicate with one another. (B adapted from Alberts B et al: Molecular biology of the cell, ed 5, New York, 2008, Garland.)
FIGURE 1-15 Primary Modes of Chemical Signaling. Five forms of signaling mediated by secreted molecules. Hormones, paracrines, neurotransmitters, and neurohormones are all intercellular messengers that accomplish communication between cells. Autocrines bind to receptors on the same cell. Not all neurotransmitters act in the strictly synaptic mode shown; some act in a contact-dependent mode as local chemical mediators that influence multiple
target cells in the area.
Contact-dependent signaling requires cells to be in close membrane-membrane contact. In paracrine signaling, cells secrete local chemical mediators that are quickly taken up, destroyed, or immobilized. Paracrine signaling usually involves different cell types; however, cells also can produce signals to which they alone respond, called autocrine signaling (see Figure 1-15). For example, cancer cells use this form of signaling to stimulate their survival and proliferation. The mediators act only on nearby cells. Hormonal signaling involves specialized endocrine cells that secrete chemicals called hormones; hormones are released by one set of cells and travel through the bloodstream to produce a response in other sets of cells (see Chapter 18). In neurohormonal signaling hormones are released into the blood by neurosecretory neurons. Like endocrine cells, neurosecretory neurons release blood-borne chemical messengers, whereas ordinary neurons secrete short-range neurotransmitters into a small discrete space (i.e., synapse). Neurons communicate directly with the cells they innervate by releasing chemicals or neurotransmitters at specialized junctions called chemical synapses; the neurotransmitter diffuses across the synaptic cleft and acts on the postsynaptic target cell (see Figure 1-15). Many of these same signaling molecules are receptors used in hormonal, neurohormonal, and paracrine signaling. Important differences lie in
the speed and selectivity with which the signals are delivered to their targets.1 Plasma membrane receptors belong to one of three classes that are defined by the
signaling (transduction) mechanism used. Table 1-3 summarizes these classes of receptors. Cells respond to external stimuli by activating a variety of signal transduction pathways, which are communication pathways, or signaling cascades (Figure 1-16, C). Signals are passed between cells when a particular type of molecule is produced by one cell—the signaling cell—and received by another—the target cell—by means of a receptor protein that recognizes and responds specifically to the signal molecule (Figure 1-16, A and B). In turn, the signaling molecules activate a pathway of intracellular protein kinases that results in various responses, such as grow and reproduce, die, survive, or differentiate (Figure 1-16, D). If deprived of appropriate signals, most cells undergo a form of cell suicide known as programmed cell death, or apoptosis (see p. 104).
TABLE 1-3 Classes of Plasma Membrane Receptors
Type of Receptor
Description
Ion channel coupled
Also called transmitter-gated ion channels; involve rapid synaptic signaling between electrically excitable cells. Channels open and close briefly in response to neurotransmitters, changing ion permeability of plasma membrane of postsynaptic cell.
Enzyme coupled
Once activated by ligands, function directly as enzymes or associate with enzymes.
G-protein coupled
Indirectly activate or inactivate plasma membrane enzyme or ion channel; interaction mediated by GTP-binding regulatory protein (G- protein). May also interact with inositol phospholipids, which are significant in cell signaling, and with molecules involved in inositol- phospholipid transduction pathway.
FIGURE 1-16 Schematic of a Signal Transduction Pathway. Like a telephone receiver that converts an electrical signal into a sound signal, a cell converts an extracellular signal, A, into an intracellular signal, B. C, An extracellular signal molecule (ligand) bonds to a receptor protein
located on the plasma membrane, where it is transduced into an intracellular signal. This process initiates a signaling cascade that relays the signal into the cell interior, amplifying and distributing it during transit. Amplification is often achieved by stimulating enzymes. Steps in the cascade can be modulated by other events in the cell. D, Different cell behaviors rely on multiple
extracellular signals.
Cellular Metabolism All of the chemical tasks of maintaining essential cellular functions are referred to as cellular metabolism. The energy-using process of metabolism is called anabolism (ana = upward), and the energy-releasing process is known as catabolism (kata = downward). Metabolism provides the cell with the energy it needs to produce cellular structures. Dietary proteins, fats, and starches (i.e., carbohydrates) are hydrolyzed in the
intestinal tract into amino acids, fatty acids, and glucose, respectively. These constituents are then absorbed, circulated, and incorporated into the cell, where they may be used for various vital cellular processes, including the production of ATP. The process by which ATP is produced is one example of a series of reactions called a metabolic pathway. A metabolic pathway involves several steps whose end products are not always detectable. A key feature of cellular metabolism is the directing of biochemical reactions by protein catalysts or enzymes. Each enzyme has a high affinity for a substrate, a specific substance converted to a product of the reaction.
Role of Adenosine Triphosphate Best known about ATP is its role as a universal “fuel” inside living cells. This fuel or energy drives biologic reactions necessary for cells to function. For a cell to function, it must be able to extract and use the chemical energy in organic molecules. When 1 mole (mol) of glucose metabolically breaks down in the presence of oxygen into carbon dioxide and water, 686 kilocalories (kcal) of chemical energy are released. The chemical energy lost by one molecule is transferred to the chemical structure of another molecule by an energy-carrying or energy-transferring molecule, such as ATP. The energy stored in ATP can be used in various energy-requiring reactions and in the process is generally converted to adenosine diphosphate (ADP) and inorganic phosphate (Pi). The energy available as a result of this reaction is about 7 kcal/mol of ATP. The cell uses ATP for muscle contraction and active transport of molecules across cellular membranes. ATP not only stores energy but also transfers it from one molecule to another. Energy stored by carbohydrate, lipid, and protein is catabolized and transferred to ATP. Emerging understandings are the role of ATP outside cells—as a messenger. In
animal studies, using the newly developed ATP probe, ATP has been measured in pericellular spaces. New research is clarifying the role of ATP as an extracellular messenger and its role in many physiologic processes, including inflammation.18-20
Food and Production of Cellular Energy Catabolism of the proteins, lipids, and polysaccharides found in food can be divided into the following three phases (Figure 1-17):
Phase 1: Digestion. Large molecules are broken down into smaller subunits: proteins into amino acids, polysaccharides into simple sugars (i.e., monosaccharides), and fats into fatty acids and glycerol. These processes occur outside the cell and are activated by secreted enzymes.
Phase 2: Glycolysis and oxidation. The most important part of phase 2 is glycolysis, the splitting of glucose. Glycolysis produces two molecules of ATP per glucose molecule through oxidation, or the removal and transfer of a pair of electrons. The total process is called oxidative cellular metabolism and involves ten biochemical reactions (Figure 1-18).
Phase 3: Citric acid cycle (Krebs cycle, tricarboxylic acid cycle). Most of the ATP is generated during this final phase, which begins with the citric acid cycle and ends with oxidative phosphorylation. About two thirds of the total oxidation of carbon compounds in most cells is accomplished during this phase. The major end products are carbon dioxide (CO2) and two dinucleotides—reduced nicotinamide adenine dinucleotide (NADH) and the reduced form of flavin adenine dinucleotide (FADH2)—both of which transfer their electrons into the electron-transport chain.
FIGURE 1-17 Three Phases of Catabolism, Which Lead from Food to Waste Products. These reactions produce adenosine triphosphate (ATP), which is used to power other processes in the
cell.
FIGURE 1-18 Glycolysis. Sugars are important for fuel or energy and they are oxidized in small steps to carbon dioxide (CO2) and water. Glycolysis is the process for oxidizing sugars or
glucose. Breakdown of glucose. A, Anaerobic catabolism, to lactic acid and little ATP. B, Aerobic catabolism, to carbon dioxide, water, and lots of ATP. (From Herlihy B: The human body in health and illness, ed 5, St
Louis, 2015, Saunders.)
Oxidative Phosphorylation Oxidative phosphorylation occurs in the mitochondria and is the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP. During the breakdown (catabolism) of foods, many reactions involve the removal of electrons from various intermediates. These reactions generally require a coenzyme (a nonprotein carrier molecule), such as nicotinamide adenine dinucleotide (NAD), to transfer the electrons and thus are called transfer reactions. Molecules of NAD and flavin adenine dinucleotide (FAD) transfer electrons they
have gained from the oxidation of substrates to molecular oxygen, O2. The
electrons from reduced NAD and FAD, NADH and FADH2, respectively, are transferred to the electron-transport chain on the inner surfaces of the mitochondria with the release of hydrogen ions. Some carrier molecules are brightly colored, iron-containing proteins known as cytochromes that accept a pair of electrons. These electrons eventually combine with molecular oxygen. If oxygen is not available to the electron-transport chain, ATP will not be formed
by the mitochondria. Instead, an anaerobic (without oxygen) metabolic pathway synthesizes ATP. This process, called substrate phosphorylation or anaerobic glycolysis, is linked to the breakdown (glycolysis) of carbohydrate (see Figure 1- 18). Because glycolysis occurs in the cytoplasm of the cell, it provides energy for cells that lack mitochondria. The reactions in anaerobic glycolysis involve the conversion of glucose to pyruvic acid (pyruvate) with the simultaneous production of ATP. With the glycolysis of one molecule of glucose, two ATP molecules and two molecules of pyruvate are liberated. If oxygen is present, the two molecules of pyruvate move into the mitochondria, where they enter the citric acid cycle (Figure 1-19).
FIGURE 1-19 What Happens to Pyruvate, the Product of Glycolysis? In the presence of oxygen, pyruvate is oxidized to acetyl coenzyme A (Acetyl CoA) and enters the citric acid cycle. In the absence of oxygen, pyruvate instead is reduced, accepting the electrons extracted during
glycolysis and carried by reduced nicotinamide adenine dinucleotide (NADH). When pyruvate is reduced directly, as it is in muscles, the product is lactic acid. When CO2 is first removed from
pyruvate and the remainder is reduced, as it is in yeasts, the resulting product is ethanol.
If oxygen is absent, pyruvate is converted to lactic acid, which is released into the extracellular fluid. The conversion of pyruvic acid to lactic acid is reversible; therefore once oxygen is restored, lactic acid is quickly converted back to either pyruvic acid or glucose. The anaerobic generation of ATP from glucose through glycolysis is not as efficient as the aerobic generation process. Adding an oxygen- requiring stage to the catabolic process (phase 3; see Figure 1-17) provides cells with a much more powerful method for extracting energy from food molecules.
Membrane Transport: Cellular Intake and Output Cell survival and growth depend on the constant exchange of molecules with their environment. Cells continually import nutrients, fluids, and chemical messengers from the extracellular environment and expel metabolites, or the products of metabolism, and end products of lysosomal digestion. Cells also must regulate ions in their cytosol and organelles. Simple diffusion across the lipid bilayer of the plasma membrane occurs for such important molecules as O2 and CO2. However, the majority of molecular transfer depends on specialized membrane transport proteins that span the lipid bilayer and provide private conduits for select molecules.1 Membrane transport proteins occur in many forms and are present in all cell membranes.1 Transport by membrane transport proteins is sometimes called mediated transport. Most of these transport proteins allow selective passage (for example, Na+ but not K+ or K+ but not Na+). Each type of cell membrane has its own transport proteins that determine which solute can pass into and out of the cell or organelle.1 The two main classes of membrane transport proteins are transporters and channels. These transport proteins differ in the type of solute—small particles of dissolved substances—they transport. A transporter is specific, allowing only those ions that fit the unique binding sites on the protein (Figure 1-20, A). A transporter undergoes conformational changes to enable membrane transport. A channel, when open, forms a pore across the lipid bilayer that allows ions and selective polar organic molecules to diffuse across the membrane (see Figure 1-20, B). Transport by a channel depends on the size and electrical charge of the molecule. Some channels are controlled by a gate mechanism that determines which solute can move into it. Ion channels are responsible for the electrical excitability of nerve and muscle cells and play a critical role in the membrane potential.
FIGURE 1-20 Inorganic Ions and Small, Polar Organic Molecules Can Cross a Cell Membrane Through Either a Transporter or a Channel. (Adapted from Alberts B: Essential cell biology, ed 4, New York, 2014, Garland.)
The mechanisms of membrane transport depend on the characteristics of the substance to be transported. In passive transport, water and small, electrically uncharged molecules move easily through pores in the plasma membrane's lipid bilayer (see Figure 1-20). This process occurs naturally through any semipermeable barrier. Molecules will easily flow “downhill” from a region of high concentration to a region of low concentration; this movement is called passive because it does not require expenditure of energy or a driving force. It is driven by osmosis, hydrostatic pressure, and diffusion, all of which depend on the laws of physics and do not require life. Other molecules are too large to pass through pores or are ligands bound to
receptors on the cell's plasma membrane. Some of these molecules are moved into and out of the cell by active transport, which requires life, biologic activity, and the cell's expenditure of metabolic energy (see Figure 1-20). Unlike passive transport, active transport occurs across only living membranes that have to drive the flow “uphill” by coupling it to an energy source (see p. 21). Movement of a solute against its concentration gradient occurs by special types of transporters called pumps (see Figure 1-20). These transporter pumps must harness an energy source to power the transport process. Energy can come from ATP hydrolysis, a transmembrane ion gradient, or sunlight (Figure 1-21). The best-known energy source is the Na+-K+–dependent adenosine triphosphatase (ATPase) pump (see Figure 1-26). It continuously regulates the cell's volume by controlling leaks through pores or protein channels and maintaining the ionic concentration gradients needed for cellular excitation and membrane conductivity (see p. 24). The maintenance of intracellular K+ concentrations is required also for enzyme activity, including enzymes involved in protein synthesis (see Figure 1-21). Large molecules (macromolecules), along with fluids, are transported by endocytosis (taking in) and
exocytosis (expelling) (see p. 21). Receptor-macromolecule complexes enter the cell by means of receptor-mediated endocytosis (see p. 24).
FIGURE 1-21 Pumps Carry Out Active Transport in Three Ways. 1, Coupled pumps link the uphill transport of one solute to the downhill transport of another solute. 2, ATP-driven pumps drive uphill transport from hydrolysis of ATP. 3, Light-driven pumps are mostly found in bacteria and use energy from sunlight to drive uphill transport. (Adapted from Alberts B: Essential cell biology, ed 4, New York,
2014, Garland.)
Mediated transport systems can move solute molecules singly or two at a time. Two molecules can be moved simultaneously in one direction (a process called symport; for example, sodium-glucose in the digestive tract) or in opposite directions (called antiport; for example, the sodium-potassium pump in all cells), or a single molecule can be moved in one direction (called uniport; for example, glucose) (Figure 1-22).
FIGURE 1-22 Mediated Transport. Illustration shows simultaneous movement of a single solute molecule in one direction (Uniport), of two different solute molecules in one direction (Symport),
and of two different solute molecules in opposite directions (Antiport).
Electrolytes as Solutes Body fluids are composed of electrolytes, which are electrically charged and dissociate into constituent ions when placed in solution, and nonelectrolytes, such as glucose, urea, and creatinine, which do not dissociate. Electrolytes account for approximately 95% of the solute molecules in body water. Electrolytes exhibit polarity by orienting themselves toward the positive or negative pole. Ions with a positive charge are known as cations and migrate toward the negative pole, or cathode, if an electrical current is passed through the electrolyte solution. Anions carry a negative charge and migrate toward the positive pole, or anode, in the presence of electrical current. Anions and cations are located in both the intracellular fluid (ICF) and the extracellular fluid (ECF) compartments, although their concentration depends on their location. (Fluid and electrolyte balance between body compartments is discussed in Chapter 5.) For example, sodium (Na+) is the predominant extracellular cation, and potassium (K+) is the principal intracellular cation. The difference in ICF and ECF concentrations of these ions is important to the transmission of electrical impulses across the plasma membranes of nerve and muscle cells. Electrolytes are measured in milliequivalents per liter (mEq/L) or milligrams per
deciliter (mg/dl). The term milliequivalent indicates the chemical-combining
activity of an ion, which depends on the electrical charge, or valence, of its ions. In abbreviations, valence is indicated by the number of plus or minus signs. One milliequivalent of any cation can combine chemically with 1 mEq of any anion: one monovalent anion will combine with one monovalent cation. Divalent ions combine more strongly than monovalent ions. To maintain electrochemical balance, one divalent ion will combine with two monovalent ions (e.g., Ca++ + 2Cl− ⇌ CaCl2).
Passive Transport: Diffusion, Filtration, and Osmosis
Diffusion. Diffusion is the movement of a solute molecule from an area of greater solute concentration to an area of lesser solute concentration. This difference in concentration is known as a concentration gradient. Although particles in a solution move randomly in any direction, if the concentration of particles in one part of the solution is greater than that in another part, the particles distribute themselves evenly throughout the solution. According to the same principle, if the concentration of particles is greater on one side of a permeable membrane than on the other side, the particles diffuse spontaneously from the area of greater concentration to the area of lesser concentration until equilibrium is reached. The higher the concentration on one side, the greater the diffusion rate. The diffusion rate is influenced by differences of electrical potential across the
membrane (see p. 24). Because the pores in the lipid bilayer are often lined with Ca++, other cations (e.g., Na+ and K+) diffuse slowly because they are repelled by positive charges in the pores. The rate of diffusion of a substance depends also on its size (diffusion
coefficient) and its lipid solubility (Figure 1-23). Usually, the smaller the molecule and the more soluble it is in oil, the more hydrophobic or nonpolar it is and the more rapidly it will diffuse across the bilayer. Oxygen, carbon dioxide, and steroid hormones (for example, androgens and estrogens) are all nonpolar molecules. Water-soluble substances, such as glucose and inorganic ions, diffuse very slowly, whereas uncharged lipophilic (“lipid-loving”) molecules, such as fatty acids and steroids, diffuse rapidly. Ions and other polar molecules generally diffuse across cellular membranes more slowly than lipid-soluble substances.
FIGURE 1-23 Passive Diffusion of Solute Molecules Across the Plasma Membrane. Oxygen, nitrogen, water, urea, glycerol, and carbon dioxide can diffuse readily down the concentration gradient. Macromolecules are too large to diffuse through pores in the plasma membrane. Ions may be repelled if the pores contain substances with identical charges. If the pores are lined with cations, for example, other cations will have difficulty diffusing because the positive
charges will repel one another. Diffusion can still occur, but it occurs more slowly.
Water readily diffuses through biologic membranes because water molecules are small and uncharged. The dipolar structure of water allows it to rapidly cross the regions of the bilayer containing the lipid head groups. The lipid head groups constitute the two outer regions of the lipid bilayer.
Filtration: hydrostatic pressure. Filtration is the movement of water and solutes through a membrane because of a greater pushing pressure (force) on one side of the membrane than on the other side. Hydrostatic pressure is the mechanical force of water pushing against cellular membranes (Figure 1-24, A). In the vascular system, hydrostatic pressure is the blood pressure generated in vessels when the heart contracts. Blood reaching the capillary bed has a hydrostatic pressure of 25 to 30 mm Hg, which is sufficient force to push water across the thin capillary membranes into the interstitial space. Hydrostatic pressure is partially balanced by osmotic pressure, whereby water moving out of the capillaries is partially balanced by osmotic forces that tend to pull water into the capillaries (Figure 1-24, B). Water that is not osmotically attracted back into the capillaries moves into the lymph system (see the discussion of Starling forces in Chapter 5).
FIGURE 1-24 Hydrostatic Pressure and Oncotic Pressure in Plasma. 1, Hydrostatic pressure in plasma. 2, Oncotic pressure exerted by proteins in the plasma usually tends to pull water into the circulatory system. 3, Individuals with low protein levels (e.g., starvation) are unable to
maintain a normal oncotic pressure; therefore water is not reabsorbed into the circulation and, instead, causes body edema.
Osmosis. Osmosis is the movement of water “down” a concentration gradient—that is, across a semipermeable membrane from a region of higher water concentration to one of lower concentration. For osmosis to occur, (1) the membrane must be more permeable to water than to solutes and (2) the concentration of solutes on one side of the membrane must be greater than that on the other side so that water moves more easily. Osmosis is directly related to both hydrostatic pressure and solute concentration but not to particle size or weight. For example, particles of the plasma protein albumin are small but are more concentrated in body fluids than the larger and heavier particles of globulin. Therefore albumin exerts a greater osmotic force than does globulin. Osmolality controls the distribution and movement of water between body
compartments. The terms osmolality and osmolarity often are used interchangeably in reference to osmotic activity, but they define different measurements. Osmolality measures the number of milliosmoles per kilogram (mOsm/kg) of water, or the concentration of molecules per weight of water. Osmolarity measures the number of milliosmoles per liter of solution, or the concentration of molecules per volume of solution. In solutions that contain only dissociable substances, such as sodium and
chloride, the difference between the two measurements is negligible. When considering all the different solutes in plasma (e.g., proteins, glucose, lipids), however, the difference between osmolality and osmolarity becomes more significant. Less of plasma's weight is water, and the overall concentration of particles is therefore greater. The osmolality will be greater than the osmolarity because of the smaller proportion of water. Osmolality is thus preferred in human clinical assessment. The normal osmolality of body fluids is 280 to 294 mOsm/kg. The osmolalities
of intracellular and extracellular fluids tend to equalize, providing a measure of body fluid concentration and thus the body's hydration status. Hydration is affected also by hydrostatic pressure because the movement of water by osmosis can be opposed by an equal amount of hydrostatic pressure. The amount of hydrostatic pressure required to oppose the osmotic movement of water is called the osmotic pressure of the solution. Factors that determine osmotic pressure are the type and thickness of the plasma membrane, the size of the molecules, the concentration of molecules or the concentration gradient, and the solubility of molecules within the membrane. Effective osmolality is sustained osmotic activity and depends on the
concentration of solutes remaining on one side of a permeable membrane. If the solutes penetrate the membrane and equilibrate with the solution on the other side of the membrane, the osmotic effect will be diminished or lost. Plasma proteins influence osmolality because they have a negative charge (see
Figure 1-24, B). The principle involved is known as Gibbs-Donnan equilibrium; it occurs when the fluid in one compartment contains small, diffusible ions, such as Na+ and chloride (Cl−), together with large, nondiffusible, charged particles, such as plasma proteins. Because the body tends to maintain an electrical equilibrium, the nondiffusible protein molecules cause asymmetry in the distribution of small ions. Anions such as Cl− are thus driven out of the cell or plasma, and cations such as Na+ are attracted to the cell. The protein-containing compartment maintains a state of electroneutrality, but the osmolality is higher. The overall osmotic effect of colloids, such as plasma proteins, is called the oncotic pressure, or colloid osmotic pressure.
Tonicity describes the effective osmolality of a solution. (The terms osmolality and tonicity may be used interchangeably.) Solutions have relative degrees of tonicity. An isotonic solution (or isosmotic solution) has the same osmolality or concentration of particles (285 mOsm) as the ICF or ECF. A hypotonic solution has a lower concentration and is thus more dilute than body fluids (Figure 1-25). A hypertonic solution has a concentration of more than 285 to 294 mOsm/kg. The concept of tonicity is important when correcting water and solute imbalances by administering different types of replacement solutions (see Figure 1-25) (see Chapter 5).
Quick Check 1-2
1. What does glycolysis produce?
2. Define membrane transport proteins.
3. What are the differences between passive and active transport?
4. Why do water and small, electrically charged molecules move easily through pores in the plasma membrane?
FIGURE 1-25 Tonicity. Tonicity is important, especially for red blood cell function. A, Isotonic solution. B, Hypotonic solution. C, Hypertonic solution. (From W augh A, Grant A: Ross and Wilson anatomy and
physiology in health and illness, ed 12, London, 2012, Churchill Livingstone.)
Active Transport of Na+ and K+
The active transport system for Na+ and K+ is found in virtually all mammalian cells. The Na+-K+–antiport system (i.e., Na+ moving out of the cell and K+ moving into the cell) uses the direct energy of ATP to transport these cations. The transporter protein is ATPase, which requires Na+, K+, and magnesium (Mg++) ions. The concentration of ATPase in plasma membranes is directly related to Na+-K+– transport activity. Approximately 60% to 70% of the ATP synthesized by cells, especially muscle and nerve cells, is used to maintain the Na+-K+–transport system. Excitable tissues have a high concentration of Na+-K+ ATPase, as do other tissues that transport significant amounts of Na+. For every ATP molecule hydrolyzed, three molecules of Na+ are transported out of the cell, whereas only two molecules of K+ move into the cell. The process leads to an electrical potential and is called electrogenic, with the inside of the cell more negative than the outside. Although the exact mechanism for this transport is uncertain, it is possible that ATPase induces the transporter protein to undergo several conformational changes, causing Na+ and K+ to move short distances (Figure 1-26). The conformational change lowers the affinity for Na+ and K+ to the ATPase transporter, resulting in the release of the cations after transport.
FIGURE 1-26 Active Transport and the Sodium-Potassium Pump. 1, Three Na+ ions bind to sodium-binding sites on the carrier's inner face. 2, At the same time, an energy-containing
adenosine triphosphate (ATP) molecule produced by the cell's mitochondria binds to the carrier. The ATP dissociates, transferring its stored energy to the carrier. 3 and 4, The carrier then changes shape, releases the three Na+ ions to the outside of the cell, and attracts two
potassium (K+) ions to its potassium-binding sites. 5, The carrier then returns to its original shape, releasing the two K+ ions and the remnant of the ATP molecule to the inside of the cell.
The carrier is now ready for another pumping cycle.
Table 1-4 summarizes the major mechanisms of transport through pores and protein transporters in the plasma membranes. Many disease states are caused or manifested by loss of these membrane transport systems.
TABLE 1-4 Major Transport Systems in Mammalian Cells
Substance Transported Mechanism of Transport* Tissues Carbohydrates Glucose Passive: protein channel
Active: symport with Na+ Most tissues
Fructose Active: symport with Na+ Small intestines and renal tubular cells Passive Intestines and liver
Amino Acids Amino acid specific transporters Coupled channels Intestines, kidney, and liver All amino acids except proline Active: symport with Na+ Liver Specific amino acids Active: group translocation Small intestine
Passive Other Organic Molecules Cholic acid, deoxycholic acid, and taurocholic acid Active: symport with Na+ Intestines Organic anions (e.g., malate, α-ketoglutarate, glutamate)
Antiport with counter–organic anion Mitochondria of liver cells
ATP-ADP Antiport transport of nucleotides; can be active Mitochondria of liver cells Inorganic Ions Na+ Passive Distal renal tubular cells Na+/H+ Active antiport, proton pump Proximal renal tubular cells and small
intestines Na+/K+ Active: ATP driven, protein channel Plasma membrane of most cells Ca++ Active: ATP driven, antiport with Na+ All cells, antiporter in red cells H+/K+ Active Parietal cells of gastric cells secreting H+
(perhaps other anions) Mediated: antiport (anion transporter–band 3 protein)
Erythrocytes and many other cells
Water Osmosis passive All tissues
*NOTE: The known transport systems are listed here; others have been proposed. Most transport systems have been studied in only a few tissues and their sites of activity may be more limited than indicated. ADP, Adenosine diphosphate; ATP, adenosine triphosphate.
Data from Alberts B et al: Molecular biology of the cell, ed 4, New York, 2001, Wiley; Alberts B et al: Essential cell biology, ed 4, New York, 2014, Garland, Devlin TM, editor: Textbook of biochemistry: with clinical correlations, ed 3, New York, 1992, Wiley; Raven PH, Johnson GB: Understanding biology, ed 3, Dubuque, IA, 1995, Brown.
Transport by Vesicle Formation Endocytosis and Exocytosis The active transport mechanisms by which the cells move large proteins,
polynucleotides, or polysaccharides (macromolecules) across the plasma membrane are very different from those that mediate small solute and ion transport. Transport of macromolecules involves the sequential formation and fusion of membrane-bound vesicles. In endocytosis, a section of the plasma membrane enfolds substances from
outside the cell, invaginates (folds inward), and separates from the plasma membrane, forming a vesicle that moves into the cell (Figure 1-27, A). Two types of endocytosis are designated based on the size of the vesicle formed. Pinocytosis (cell drinking) involves the ingestion of fluids, bits of the plasma membrane, and solute molecules through formation of small vesicles; and phagocytosis (cell eating) involves the ingestion of large particles, such as bacteria, through formation of large vesicles (vacuoles).
FIGURE 1-27 Endocytosis and Exocytosis. A, Endocytosis and fusion with lysosome and exocytosis. B, Electron micrograph of exocytosis. (B from Raven PH, Johnson GB: Biology, ed 5, New York, 1999,
McGraw-Hill.)
Because most cells continually ingest fluid and solutes by pinocytosis, the terms pinocytosis and endocytosis often are used interchangeably. In pinocytosis, the vesicle containing fluids, solutes, or both fuses with a lysosome, and lysosomal enzymes digest the vesicle's contents for use by the cell. Vesicles that bud from
membranes have a particular protein coat on their cytosolic surface and are called coated vesicles. The best studied are those that have an outer coat of bristlelike structures—the protein clathrin. Pinocytosis occurs mainly by the clathrin-coated pits and vesicles (Figure 1-28). After the coated pits pinch off from the plasma membrane, they quickly shed their coats and fuse with an endosome. An endosome is a vesicle pinched off from the plasma membrane from which its contents can be recycled to the plasma membrane or sent to lysosomes for digestion. In phagocytosis, the large molecular substances are engulfed by the plasma membrane and enter the cell so that they can be isolated and destroyed by lysosomal enzymes (see Chapter 6). Substances that are not degraded by lysosomes are isolated in residual bodies and released by exocytosis. Both pinocytosis and phagocytosis require metabolic energy and often involve binding of the substance with plasma membrane receptors before membrane invagination and fusion with lysosomes in the cell. New data are revealing that endocytosis has an even larger and more important role than previously known (Box 1-2).
FIGURE 1-28 Ligand Internalization by Means of Receptor-Mediated Endocytosis. A, The ligand attaches to its surface receptor (through the bristle coat or clathrin coat) and, through receptor-
mediated endocytosis, enters the cell. The ingested material fuses with a lysosome and is processed by hydrolytic lysosomal enzymes. Processed molecules can then be transferred to other cellular components. B, Electron micrograph of a coated pit showing different sizes of
filaments of the cytoskeleton (×82,000). (B from Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Box 1-2 The New Endocytic Matrix
An explosion of new data is disclosing a much more involved role for endocytosis than just a simple way to internalize nutrients and membrane-associated molecules. These new data show that endocytosis not only is a master organizer of signaling pathways but also has a major role in managing signals in time and space. Endocytosis appears to control signaling; therefore it determines the net output of biochemical pathways. This occurs because endocytosis modulates the presence of receptors and their ligands as well as effectors at the plasma membrane or at intermediate stations of the endocytic route. The overall processes and anatomy of these new functions are sometimes called the “endocytic matrix.” All of these functions ultimately have a large impact on almost every cellular process, including the nucleus.
In eukaryotic cells, secretion of macromolecules almost always occurs by exocytosis (see Figure 1-27). Exocytosis has two main functions: (1) replacement of portions of the plasma membrane that have been removed by endocytosis and (2) release of molecules synthesized by the cells into the extracellular matrix.
Receptor-Mediated Endocytosis The internalization process, called receptor-mediated endocytosis (ligand internalization), is rapid and enables the cell to ingest large amounts of receptor- macromolecule complexes in clathrin-coated vesicles without ingesting large volumes of extracellular fluid (see Figure 1-28). The cellular uptake of cholesterol, for example, depends on receptor-mediated endocytosis. Additionally, many essential metabolites (for example, vitamin B12 and iron) depend on receptor- mediated endocytosis and, unfortunately, the influenza flu virus.
Caveolae The outer surface of the plasma membrane is dimpled with tiny flask-shaped pits (cavelike) called caveolae. Caveolae are thought to form from membrane microdomains or lipid rafts. Caveolae are cholesterol- and glycosphingolipid-rich microdomains where the protein caveolin is thought to be involved in several processes, including clathrin-independent endocytosis, cellular cholesterol regulation and transport, and cellular communication. Many proteins, including a variety of receptors, cluster in these tiny chambers. Caveolae are not only uptake vehicles but also important sites for signal
transduction, a tedious process in which extracellular chemical messages or signals are communicated to the cell's interior for execution. For example, in vitro evidence now exists that plasma membrane estrogen receptors can localize in caveolae, and
crosstalk with estradiol facilitates several intracellular biologic actions.21
Movement of Electrical Impulses: Membrane Potentials All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in electrical charge, or voltage, is known as the resting membrane potential and is about −70 to −85 millivolts (mV). The difference in voltage across the plasma membrane results from the differences in ionic composition of ICF and ECF. Sodium ions are more concentrated in the ECF, and potassium ions are in greater concentration in the ICF. The concentration difference is maintained by the active transport of Na+ and K+ (the sodium- potassium pump), which transports sodium outward and potassium inward (Figure 1-29). Because the resting plasma membrane is more permeable to K+ than to Na+, K+ diffuses easily from the ICF to the ECF. Because both sodium and potassium are cations, the net result is an excess of anions inside the cell, resulting in the resting membrane potential.
FIGURE 1-29 Sodium-Potassium Pump and Propagation of an Action Potential. A, Concentration difference of sodium (Na+) and potassium (K+) intracellularly and extracellularly. The direction of active transport by the sodium-potassium pump is also shown. B, The left
diagram represents the polarized state of a neuronal membrane when at rest. The middle and right diagrams represent changes in sodium and potassium membrane permeabilities with
depolarization and repolarization.
Nerve and muscle cells are excitable and can change their resting membrane potential in response to electrochemical stimuli. Changes in resting membrane potential convey messages from cell to cell. When a nerve or muscle cell receives a stimulus that exceeds the membrane threshold value, a rapid change occurs in the resting membrane potential, known as the action potential. The action potential carries signals along the nerve or muscle cell and conveys information from one cell to another in a domino-like fashion. Nerve impulses are described in Chapter 13. When a resting cell is stimulated through voltage-regulated channels, the cell membranes become more permeable to sodium, so a net movement of sodium into the cell occurs and the membrane potential decreases, or moves forward, from a negative value (in millivolts) to zero. This decrease is known as depolarization. The depolarized cell is more positively charged, and its polarity is neutralized. To generate an action potential and the resulting depolarization, the threshold
potential must be reached. Generally this occurs when the cell has depolarized by 15 to 20 millivolts. When the threshold is reached, the cell will continue to depolarize with no further stimulation. The sodium gates open, and sodium rushes into the cell, causing the membrane potential to drop to zero and then become positive (depolarization). The rapid reversal in polarity results in the action potential.
During repolarization, the negative polarity of the resting membrane potential is reestablished. As the voltage-gated sodium channels begin to close, voltage-gated potassium channels open. Membrane permeability to sodium decreases and potassium permeability increases, so potassium ions leave the cell. The sodium gates close, and with the loss of potassium the membrane potential becomes more negative. The Na+, K+ pump then returns the membrane to the resting potential by pumping potassium back into the cell and sodium out of the cell. During most of the action potential, the plasma membrane cannot respond to an
additional stimulus. This time is known as the absolute refractory period and is related to changes in permeability to sodium. During the latter phase of the action potential, when permeability to potassium increases, a stronger-than-normal stimulus can evoke an action potential; this time is known as the relative refractory period. When the membrane potential is more negative than normal, the cell is in a
hyperpolarized state (less excitable: decreased K+ levels within the cell). A stronger-than-normal stimulus is then required to reach the threshold potential and generate an action potential. When the membrane potential is more positive than normal, the cell is in a hypopolarized state (more excitable than normal: increased K+ levels within the cell) and a weaker-than-normal stimulus is required to reach the threshold potential. Changes in the intracellular and extracellular concentrations of ions or a change in membrane permeability can cause these alterations in membrane excitability.
Quick Check 1-3
1. Identify examples of molecules transported in one direction (symport) and opposite directions (antiport).
2. If oxygen is no longer available to make ATP, what happens to the transport of Na+?
3. Define the differences between pinocytosis, phagocytosis, and receptor-mediated endocytosis.
Cellular Reproduction: the Cell Cycle Human cells are subject to wear and tear, and most do not last for the lifetime of the individual. In most tissues, new cells are created as fast as old cells die. Cellular reproduction is therefore necessary for the maintenance of life. Reproduction of gametes (sperm and egg cells) occurs through a process called meiosis, described in Chapter 2. The reproduction, or division, of other body cells (somatic cells) involves two sequential phases—mitosis, or nuclear division, and cytokinesis, or cytoplasmic division. Before a cell can divide, however, it must double its mass and duplicate all its contents. Separation for division occurs during the growth phase, called interphase. The alternation between mitosis and interphase in all tissues with cellular turnover is known as the cell cycle. The four designated phases of the cell cycle (Figure 1-30) are (1) the S phase (S
= synthesis), in which DNA is synthesized in the cell nucleus; (2) the G2 phase (G = gap), in which RNA and protein synthesis occurs, namely, the period between the completion of DNA synthesis and the next phase (M); (3) the M phase (M = mitosis), which includes both nuclear and cytoplasmic division; and (4) the G1 phase, which is the period between the M phase and the start of DNA synthesis.
FIGURE 1-30 Interphase and the Phases of Mitosis. A, The G1/S checkpoint is to “check” for cell size, nutrients, growth factors, and DNA damage. See text for resting phases. The G2/M checkpoint checks for cell size and DNA replication. B, The orderly progression through the phases of the cell cycle is regulated by cyclins (so called because levels rise and fall) and
cyclin-dependent protein kinases (CDKs) and their inhibitors. When cyclins are complexed with CDKs, cell cycle events are triggered.
Phases of Mitosis and Cytokinesis Interphase (the G1, S, and G2 phases) is the longest phase of the cell cycle. During interphase, the chromatin consists of very long, slender rods jumbled together in the nucleus. Late in interphase, strands of chromatin (the substance that gives the nucleus its granular appearance) begin to coil, causing shortening and thickening. The M phase of the cell cycle, mitosis and cytokinesis, begins with prophase, the
first appearance of chromosomes. As the phase proceeds, each chromosome is seen as two identical halves called chromatids, which lie together and are attached by a spindle site called a centromere. (The two chromatids of each chromosome, which are genetically identical, are sometimes called sister chromatids.) The nuclear membrane, which surrounds the nucleus, disappears. Spindle fibers are microtubules formed in the cytoplasm. They radiate from two centrioles located at opposite poles of the cell and pull the chromosomes to opposite sides of the cell, beginning metaphase. Next, the centromeres become aligned in the middle of the spindle, which is called the equatorial plate (or metaphase plate) of the cell. In this
stage, chromosomes are easiest to observe microscopically because they are highly condensed and arranged in a relatively organized fashion. Anaphase begins when the centromeres split and the sister chromatids are pulled
apart. The spindle fibers shorten, causing the sister chromatids to be pulled, centromere first, toward opposite sides of the cell. When the sister chromatids are separated, each is considered to be a chromosome. Thus the cell has 92 chromosomes during this stage. By the end of anaphase, there are 46 chromosomes lying at each side of the cell. Barring mitotic errors, each of the 2 groups of 46 chromosomes is identical to the original 46 chromosomes present at the start of the cell cycle. During telophase, the final stage, a new nuclear membrane is formed around
each group of 46 chromosomes, the spindle fibers disappear, and the chromosomes begin to uncoil. Cytokinesis causes the cytoplasm to divide into almost equal parts during this phase. At the end of telophase, two identical diploid cells, called daughter cells, have been formed from the original cell.
Rates of Cellular Division Although the complete cell cycle lasts 12 to 24 hours, about 1 hour is required for the four stages of mitosis and cytokinesis. All types of cells undergo mitosis during formation of the embryo, but many adult cells—such as nerve cells, lens cells of the eye, and muscle cells—lose their ability to replicate and divide. The cells of other tissues, particularly epithelial cells (e.g., cells of the intestine, lung, or skin), divide continuously and rapidly, completing the entire cell cycle in less than 10 hours. The difference between cells that divide slowly and cells that divide rapidly is the
length of time spent in the G1 phase of the cell cycle. Once the S phase begins, however, progression through mitosis takes a relatively constant amount of time. The mechanisms that control cell division depend on the integrity of genetic,
epigenetic (heritable changes in genome function that occur without alterations in the DNA sequence; see Chapter 3), and protein growth factors. Protein growth factors govern the proliferation of different cell types. Individual cells are members of a complex cellular society in which survival of the entire organism is key—not survival or proliferation of just the individual cells. When a need arises for new cells, as in repair of injured cells, previously nondividing cells must be triggered rapidly to reenter the cell cycle. With continual wear and tear, the cell birth rate and the cell death rate must be kept in balance.
Growth Factors
Growth factors, also called cytokines, are peptides (protein fractions) that transmit signals within and between cells. They have a major role in the regulation of tissue growth and development (Table 1-5). Having nutrients is not enough for a cell to proliferate; it must also receive stimulatory chemical signals (growth factors) from other cells, usually its neighbors or the surrounding supporting tissue called stroma. These signals act to overcome intracellular braking mechanisms that tend to restrain cell growth and block progress through the cell cycle (Figure 1-31).
TABLE 1-5 Examples of Growth Factors and Their Actions
Growth Factor Physiologic Actions Platelet-derived growth factor (PDGF) Stimulates proliferation of connective tissue cells and neuroglial cells Epidermal growth factor (EGF) Stimulates proliferation of epidermal cells and other cell types Insulin-like growth factor 1 (IGF-1) Collaborates with PDGF and EGF; stimulates proliferation of fat cells and connective tissue cells Vascular endothelial growth factor (VEGF) Mediates functions of endothelial cells; proliferation, migration, invasion, survival, and permeability Insulin-like growth factor 2 (IGF-2) Collaborates with PDGF and EGF; stimulates or inhibits response of most cells to other growth factors;
regulates differentiation of some cell types (e.g., cartilage) Transforming growth factor-beta (TGF-β; multiple subtypes)
Stimulates or inhibits response of most cells to other growth factors; regulates differentiation of some cell types (e.g., cartilage)
Fibroblast growth factor (FGF; multiple subtypes)
Stimulates proliferation of fibroblasts, endothelial cells, myoblasts, and other multiple subtypes
Interleukin-2 (IL-2) Stimulates proliferation of T lymphocytes Nerve growth factor (NGF) Promotes axon growth and survival of sympathetic and some sensory and central nervous system (CNS)
neurons Hematopoietic cell growth factors (IL-3, GM- CSF, G-CSF, erythropoietin)
Promote proliferation of blood cells
G-CSF, Granulocyte colony-stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor.
FIGURE 1-31 How Growth Factors Stimulate Cell Proliferation. A, Resting cell. With the absence of growth factors, the retinoblastoma (Rb) protein is not phosphorylated; thus it holds the gene regulatory proteins in an inactive state. The gene regulatory proteins are required to stimulate the transcription of genes needed for cell proliferation. B, Proliferating cell. Growth factors bind to the cell surface receptors and activate intracellular signaling pathways, leading to activation of intracellular proteins. These intracellular proteins phosphorylate and thereby inactivate the Rb protein. The gene regulatory proteins are now free to activate the transcription of genes,
leading to cell proliferation.
An example of a brake that regulates cell proliferation is the retinoblastoma (Rb) protein, first identified through studies of a rare childhood eye tumor called retinoblastoma, in which the Rb protein is missing or defective. The Rb protein is abundant in the nucleus of all vertebrate cells. It binds to gene regulatory proteins, preventing them from stimulating the transcription of genes required for cell proliferation (see Figure 1-31). Extracellular signals, such as growth factors, activate intracellular signaling pathways that inactivate the Rb protein, leading to cell proliferation. Different types of cells require different growth factors; for example, platelet-
derived growth factor (PDGF) stimulates the production of connective tissue cells. Table 1-5 summarizes the most significant growth factors. Evidence shows that some growth factors also regulate other cellular processes, such as cellular differentiation. In addition to growth factors that stimulate cellular processes, there are factors that inhibit these processes; these factors are not well understood. Cells that are starved of growth factors come to a halt after mitosis and enter the arrested (resting) (G0) state of the cell cycle (see p. 25 for cell cycle).
1
Tissues Cells of one or more types are organized into tissues, and different types of tissues compose organs. Finally, organs are integrated to perform complex functions as tracts or systems. All cells are in contact with a network of extracellular macromolecules known as
the extracellular matrix (see p. 10). This matrix not only holds cells and tissues together but also provides an organized latticework within which cells can migrate and interact with one another.
Tissue Formation To form tissues, cells must exhibit intercellular recognition and communication, adhesion, and memory. Specialized cells sense their environment through signals, such as growth factors, from other cells. This type of communication ensures that new cells are produced only when and where they are required. Different cell types have different adhesion molecules in their plasma membranes, sticking selectively to other cells of the same type. They can also adhere to extracellular matrix components. Because cells are tiny and squishy and enclosed by a flimsy membrane, it is remarkable that they form a strong human being. Strength can occur because of the extracellular matrix and the strength of the cytoskeleton with cell-cell adhesions to neighboring cells. Cells have memory because of specialized patterns of gene expression evoked by signals that acted during embryonic development. Memory allows cells to autonomously preserve their distinctive character and pass it on to their progeny.1 Fully specialized or terminally differentiated cells that are lost are regenerated
from proliferating precursor cells. These precursor cells have been derived from a smaller number of stem cells.1 Stem cells are cells with the potential to develop into many different cell types during early development and growth. In many tissues, stem cells serve as an internal repair and maintenance system, dividing indefinitely. These cells can maintain themselves over very long periods of time, called self- renewal, and can generate all the differentiated cell types of the tissue or multipotency. This stem cell–driven tissue renewal is very evident in the epithelial lining of the intestine, stomach, blood cells, and skin, which is continuously exposed to environmental factors. A class of extracellular signaling proteins, known as Wnt signals, sustain tissue renewal and enable tissue to be continuously replenished and maintained over a lifetime.22 When a stem cell divides, each daughter cell has a choice: it can remain as a stem cell or it can follow a pathway that results in terminal differentiation (Figure 1-32).
FIGURE 1-32 Properties of Stem Cell Systems. A, Stem cells have three characteristics: self- renewal, proliferation, and differentiation into mature cells. Stem cells are housed in niches
consisting of stromal cells that provide factors for their maintenance. Stem cells of the embryo can give rise to cell precursors that generate all the tissues of the body. This property defines stem cells as multipotent. Stem cells are difficult to identify anatomically. Their identification is
based on specific cell surface markers (cell surface antigens recognized by specific monoclonal antibodies) and on the lineage they generate following transplantation. B, Wnt
signaling fuels tissue renewal. (A, from Kierszenbaum A: Histology and cell biology: an introduction to pathology, ed 3, St Louis, 2012, Elsevier. B, from Clevers H, et al: Science 346(3), 2014.)
Types of Tissues The four basic types of tissues are nerve, epithelial, connective, and muscle. The structure and function of these four types underlie the structure and function of each organ system. Neural tissue is composed of highly specialized cells called neurons, which receive and transmit electrical impulses rapidly across junctions called synapses (see Figure 13-1). Different types of neurons have special characteristics that depend on their distribution and function within the nervous system. Epithelial, connective, and muscle tissues are summarized in Tables 1-6, 1-7, and 1-8, respectively.
Quick Check 1-4
1. What is the cell cycle?
2. Discuss the five types of intracellular communication.
3. Why is the extracellular matrix important for tissue cells?
TABLE 1-6 Characteristics of Epithelial Tissues
Simple Squamous Epithelium Structure Single layer of cells Location and Function Lining of blood vessels leads to diffusion and filtration Lining of pulmonary alveoli (air sacs) leads to separation of blood from fluids in tissues Bowman's capsule (kidney), where it filters substances from blood, forming urine
Simple Squamous Epithelial Cell. Photomicrograph of simple squamous epithelial cell in parietal wall of Bowman's capsule in kidney. (From Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Stratified Squamous Epithelium Structure Two or more layers, depending on location, with cells closest to basement membrane tending to be cuboidal Location and Function Epidermis of skin and linings of mouth, pharynx, esophagus, and anus provide protection and secretion
Cornified Stratified Squamous Epithelium. Diagram of stratified squamous epithelium of skin. (Copyright Ed Reschke. Used with permission.)
Transitional Epithelium Structure Vary in shape from cuboidal to squamous depending on whether basal cells of bladder are columnar or are composed of many layers; when bladder is full and stretched, the cells flatten and stretch like squamous cells Location and Function Linings of urinary bladder and other hollow structures stretch, allowing expansion of the hollow organs
Stratified Squamous Transitional Epithelium. Photomicrograph of stratified squamous transitional epithelium of urinary bladder. (Copyright Ed Reschke. Used with permission.)
Simple Cuboidal Epithelium Structure Simple cuboidal cells; rarely stratified (layered) Location and Function Glands (e.g., thyroid, sweat, salivary) and parts of the kidney tubules and outer covering of ovary secrete fluids
Simple Cuboidal Epithelium. Photomicrograph of simple cuboidal epithelium of pancreatic duct. (From Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Simple Columnar Epithelium Structure Large amounts of cytoplasm and cellular organelles Location and Function
Ducts of many glands and lining of digestive tract allow secretion and absorption from stomach to anus
Simple Columnar Epithelium. Photomicrograph of simple columnar epithelium. (Copyright Ed Reschke. Used with permission.)
Ciliated Simple Columnar Epithelium Structure Same as simple columnar epithelium but ciliated Location and Function Linings of bronchi of lungs, nasal cavity, and oviducts allow secretion, absorption, and propulsion of fluids and particles
Stratified Columnar Epithelium Structure Small and rounded basement membrane (columnar cells do not touch basement membrane) Location and Function Linings of epiglottis, part of pharynx, anus, and male urethra provide protection Pseudostratified Ciliated Columnar Epithelium Structure All cells in contact with basement membrane Nuclei found at different levels within cell, giving stratified appearance Free surface often ciliated Location and Function Linings of large ducts of some glands (parotid, salivary), male urethra, respiratory passages, and eustachian tubes of ears transport substances
Pseudostratified Ciliated Columnar Epithelium. Photomicrograph of pseudostratified ciliated columnar epithelium of trachea. (Copyright Robert L. Calentine. Used with permission.)
TABLE 1-7 Connective Tissues
Loose or Areolar Tissue Structure Unorganized; spaces between fibers Most fibers collagenous, some elastic and reticular Includes many types of cells (fibroblasts and macrophages most common) and large amount of intercellular fluid Location and Function Attaches skin to underlying tissue; holds organs in place by filling spaces between them; supports blood vessels Intercellular fluid transports nutrients and waste products Fluid accumulation causes swelling (edema)
Loose Areolar Connective Tissue. (Copyright Ed Reschke. Used with permission.)
Dense Irregular Tissue Structure Dense, compact, and areolar tissue, with fewer cells and greater number of closely woven collagenous fibers than in loose tissue Location and Function Dermis layer of skin; acts as protective barrier
Dense, Irregular Connective Tissue. (Copyright Ed Reschke. Used with permission.)
Dense, Regular (White Fibrous) Tissue Structure
Collagenous fibers and some elastic fibers, tightly packed into parallel bundles, with only fibroblast cells Location and Function Forms strong tendons of muscle, ligaments of joints, some fibrous membranes, and fascia that surrounds organs and muscles
Dense, Regular (W hite Fibrous) Connective Tissue. (Copyright Phototake. Used with permission.)
Elastic Tissue Structure Elastic fibers, some collagenous fibers, fibroblasts Location and Function Lends strength and elasticity to walls of arteries, trachea, vocal cords, and other structures
Elastic Connective Tissue. (From Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Adipose Tissue Structure Fat cells dispersed in loose tissues; each cell containing a large droplet of fat flattens nucleus and forces cytoplasm into a ring around cell's periphery Location and Function
Stores fat, which provides padding and protection
Adipose Tissue. A, Fat storage areas—distribution of fat in male and female bodies. B, Photomicrograph of adipose tissue. (A from Thibodeau GA, Patton KT: Anatomy & physiology, ed 6, St Louis, 2007, Mosby; B copyright Ed Reschke. Used with permission.)
Cartilage (Hyaline, Elastic, Fibrous) Structure Collagenous fibers embedded in a firm matrix (chondrin); no blood supply
Location and Function Gives form, support, and flexibility to joints, trachea, nose, ear, vertebral disks, embryonic skeleton, and many internal structures
Cartilage. A, Hyaline cartilage. B, Elastic cartilage. C, Fibrous cartilage. (A and C copyright Robert L. Calentine; B copyright Ed Reshke. Used with permission.)
Bone Structure Rigid connective tissue consisting of cells, fibers, ground substances, and minerals Location and Function Lends skeleton rigidity and strength
Bone. (Copyright Phototake. Used with permission.)
Special Connective Tissues Plasma Structure Fluid Location and Function Serves as matrix for blood cells Macrophages in Tissue, Reticuloendothelial, or Macrophage System Structure Scattered macrophages (phagocytes) called Kupffer cells (in liver), alveolar macrophages (in lungs), microglia (in central nervous system) Location and Function Facilitate inflammatory response and carry out phagocytosis in loose connective, lymphatic, digestive, medullary (bone marrow), splenic, adrenal, and pituitary tissues
TABLE 1-8 Muscle Tissues
Skeletal (Striated) Muscle Structure Characteristics of Cells Long, cylindrical cells that extend throughout length of muscles Striated myofibrils (proteins) Many nuclei on periphery Location and Function Attached to bones directly or by tendons and provide voluntary movement of skeleton and maintenance of posture
Skeletal (Striated) Muscle. (From Thibodeau GA, Patton KT: Anatomy & physiology, ed 6, St Louis, 2007, Mosby.)
Cardiac Muscle Structure Characteristics of Cells Branching networks throughout muscle tissue Striated myofibrils Location and Function Cells attached end-to-end at intercalated disks with tissue forming walls of heart (myocardium) to provide involuntary pumping action of heart
Cardiac Muscle. (Copyright Ed Reschke. Used with permission.)
Smooth (Visceral) Muscle Structure Characteristics of Cells Long spindles that taper to a point Absence of striated myofibrils Location and Function Walls of hollow internal structures, such as digestive tract and blood vessels (viscera), provide voluntary and involuntary contractions that move substances through hollow structures
Smooth (Visceral) Muscle. (Copyright Phototake. Used with permission.)
Did You Understand? Cellular Functions 1. Cells become specialized through the process of differentiation or maturation.
2. The eight specialized cellular functions are movement, conductivity, metabolic absorption, secretion, excretion, respiration, reproduction, and communication.
Structure and Function of Cellular Components 1. The eukaryotic cell consists of three general components: the plasma membrane, the cytoplasm, and the intracellular organelles.
2. The nucleus is the largest membrane-bound organelle and is found usually in the cell's center. The chief functions of the nucleus are cell division and control of genetic information.
3. Cytoplasm, or the cytoplasmic matrix, is an aqueous solution (cytosol) that fills the space between the nucleus and the plasma membrane.
4. The organelles are suspended in the cytoplasm and are enclosed in biologic membranes.
5. The endoplasmic reticulum is a network of tubular channels (cisternae) that extend throughout the outer nuclear membrane. It specializes in the synthesis and transport of protein and lipid components of most of the organelles.
6. The Golgi complex is a network of smooth membranes and vesicles located near the nucleus. The Golgi complex is responsible for processing and packaging proteins into secretory vesicles that break away from the Golgi complex and migrate to a variety of intracellular and extracellular destinations, including the plasma membrane.
7. Lysosomes are saclike structures that originate from the Golgi complex and contain digestive enzymes. These enzymes are responsible for digesting most cellular substances to their basic form, such as amino acids, fatty acids, and carbohydrates (sugars).
8. Cellular injury leads to a release of the lysosomal enzymes, causing cellular self-
digestion.
9. Peroxisomes are similar to lysosomes but contain several enzymes that either produce or use hydrogen peroxide.
10. Mitochondria contain the metabolic machinery necessary for cellular energy metabolism. The enzymes of the respiratory chain (electron-transport chain), found in the inner membrane of the mitochondria, generate most of the cell's ATP.
11. The cytoskeleton is the “bone and muscle” of the cell. The internal skeleton is composed of a network of protein filaments, including microtubules and actin filaments (microfilaments).
12. The plasma membrane encloses the cell and, by controlling the movement of substances across it, exerts a powerful influence on metabolic pathways. Principles of membrane structure are being overhauled.
13. Proteins are the major workhorses of the cell. Membrane proteins, like other proteins, are synthesized by the ribosome and then make their way, called trafficking, to different locations in the cell. Trafficking places unique demands on membrane proteins for folding, translocation, and stability. Misfolded proteins are emerging as an important cause of disease.
14. Protein regulation in a cell is called protein homeostasis and is defined by the proteostasis network. This network is composed of ribosomes (makers), chaperones (helpers), and protein breakdown or proteolytic systems. Malfunction of these systems is associated with disease.
15. Carbohydrates contained within the plasma membrane are generally bound to membrane proteins (glycoproteins) and lipids (glycolipids).
16. Protein receptors (recognition units) on the plasma membrane enable the cell to interact with other cells and with extracellular substances.
17. Membrane functions are determined largely by proteins. These functions include recognition by protein receptors and transport of substances into and out of the cell.
Cell-to-Cell Adhesions 1. Cell-to-cell adhesions are formed on plasma membranes, thereby allowing the
formation of tissues and organs. Cells are held together by three different means: (1) the extracellular membrane, (2) cell adhesion molecules in the cell's plasma membrane, and (3) specialized cell junctions.
2. The extracellular matrix includes three groups of macromolecules: (1) fibrous structural proteins (collagen and elastin), (2) adhesive glycoproteins, and (3) proteoglycans and hyaluronic acid. The matrix helps regulate cell growth, movement, and differentiation.
3. The basement membrane is a tough layer of extracellular matrix underlying the epithelium of many organs; it is also called the basal lamina.
4. Cell junctions can be classified as symmetric and asymmetric. Symmetric junctions include tight junctions, the belt desmosome, desmosomes, and gap junctions. An asymmetric junction is the hemidesmosome.
Cellular Communication and Signal Transduction 1. Cells communicate in three main ways: (1) they form protein channels (gap junctions); (2) they display receptors that affect intracellular processes or other cells in direct physical contact; and (3) they use receptor proteins inside the target cell.
2. Primary modes of intercellular signaling include contact-dependent, paracrine, hormonal, neurohormonal, and neurotransmitter.
3. Signal transduction involves signals or instructions from extracellular chemical messengers that are conveyed to the cell's interior for execution. If deprived of appropriate signals, cells undergo a form of cell suicide known as programmed cell death or apoptosis.
Cellular Metabolism 1. The chemical tasks of maintaining essential cellular functions are referred to as cellular metabolism. Anabolism is the energy-using process of metabolism, whereas catabolism is the energy-releasing process.
2. Adenosine triphosphate (ATP) functions as an energy-transferring molecule. It is fuel for cell survival. Energy is stored by molecules of carbohydrate, lipid, and
protein, which, when catabolized, transfers energy to ATP.
3. Oxidative phosphorylation occurs in the mitochondria and is the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP.
Membrane Transport: Cellular Intake and Output 1. Cell survival and growth depends on the constant exchange of molecules with their environment. The two main classes of membrane transport proteins are transporters and channels. The majority of molecular transfer depends on specialized membrane transport proteins.
2. Water and small, electrically uncharged molecules move through pores in the plasma membrane's lipid bilayer in the process called passive transport.
3. Passive transport does not require the expenditure of energy; rather, it is driven by the physical effect of osmosis, hydrostatic pressure, and diffusion.
4. Larger molecules and molecular complexes are moved into the cell by active transport, which requires the cell to expend energy (by means of ATP).
5. The largest molecules (macromolecules) and fluids are transported by the processes of endocytosis (ingestion) and exocytosis (expulsion). Endocytosis, or vesicle formation, is when the substance to be transported is engulfed by a segment of the plasma membrane, forming a vesicle that moves into the cell.
6. Pinocytosis is a type of endocytosis in which fluids and solute molecules are ingested through formation of small vesicles.
7. Phagocytosis is a type of endocytosis in which large particles, such as bacteria, are ingested through formation of large vesicles, called vacuoles.
8. In receptor-mediated endocytosis, the plasma membrane receptors are clustered, along with bristlelike structures, in specialized areas called coated pits.
9. Endocytosis occurs when coated pits invaginate, internalizing ligand-receptor complexes in coated vesicles.
10. Inside the cell, lysosomal enzymes process and digest material ingested by
endocytosis.
11. Two types of solutes exist in body fluids: electrolytes and nonelectrolytes. Electrolytes are electrically charged and dissociate into constituent ions when placed in solution. Nonelectrolytes do not dissociate when placed in solution.
12. Diffusion is the passive movement of a solute from an area of higher solute concentration to an area of lower solute concentration.
13. Filtration is the measurement of water and solutes through a membrane because of a greater pushing pressure.
14. Hydrostatic pressure is the mechanical force of water pushing against cellular membranes.
15. Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.
16. The amount of hydrostatic pressure required to oppose the osmotic movement of water is called the osmotic pressure of solution.
17. The overall osmotic effect of colloids, such as plasma proteins, is called the oncotic pressure or colloid osmotic pressure.
18. All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in voltage across the plasma membrane is the resting membrane potential.
19. When an excitable (nerve or muscle) cell receives an electrochemical stimulus, cations enter the cell and cause a rapid change in the resting membrane potential known as the action potential. The action potential “moves” along the cell's plasma membrane and is transmitted to an adjacent cell. This is how electrochemical signals convey information from cell to cell.
Cellular Reproduction: The Cell Cycle 1. Cellular reproduction in body tissues involves mitosis (nuclear division) and cytokinesis (cytoplasmic division).
2. Only mature cells are capable of division. Maturation occurs during a stage of
cellular life called interphase (growth phase).
3. The cell cycle is the reproductive process that begins after interphase in all tissues with cellular turnover. There are four phases of the cell cycle: (1) the S phase, during which DNA synthesis takes place in the cell nucleus; (2) the G2 phase, the period between the completion of DNA synthesis and the next phase (M); (3) the M phase, which involves both nuclear (mitotic) and cytoplasmic (cytokinetic) division; and (4) the G1 phase (growth phase), after which the cycle begins again.
4. The M phase (mitosis) involves four stages: prophase, metaphase, anaphase, and telophase.
5. The mechanisms that control cellular division depend on the integrity of genetic, epigenetic, and protein growth factors.
Tissues 1. Cells of one or more types are organized into tissues, and different types of tissues compose organs. Organs are organized to function as tracts or systems.
2. Three key factors that maintain the cellular organization of tissues are (1) recognition and cell communication, (2) selective cell-to-cell adhesion, and (3) memory.
3. Fully specialized or terminally differentiated cells that are lost are generated from proliferating precursor cells and they, in turn, have been derived from a smaller number of stem cells. Stem cells are cells with the potential to develop into many different cell types during early development and growth. In many tissues, stem cells serve as an internal repair and maintenance system dividing indefinitely. These cells can maintain themselves over very long periods of time, called self- renewal, and can generate all the differentiated cell types of the tissue or multipotency.
4. Tissue cells are linked at cell junctions, which are specialized regions on their plasma membranes. Cell junctions attach adjacent cells and allow small molecules to pass between them.
5. The four basic types of tissues are epithelial, muscle, nerve, and connective tissues.
6. Neural tissue is composed of highly specialized cells called neurons that receive and transmit electrical impulses rapidly across junctions called synapses.
7. Epithelial tissue covers most internal and external surfaces of the body. The functions of epithelial tissue include protection, absorption, secretion, and excretion.
8. Connective tissue binds various tissues and organs together, supporting them in their locations and serving as storage sites for excess nutrients.
9. Muscle tissue is composed of long, thin, highly contractile cells or fibers called myocytes. Muscle tissue that is attached to bones enables voluntary movement. Muscle tissue in internal organs enables involuntary movement, such as the heartbeat.
Key Terms Absolute refractory period, 25
Action potential, 24
Active transport, 17
Amphipathic, 3
Anabolism, 14
Anaphase, 26
Anion, 19
Antiport, 18
Arrested (resting) (G0) state, 27
Autocrine signaling, 12
Basal lamina, 10
Basement membrane, 10
Binding site, 9
Catabolism, 14
Cation, 19
Caveolae, 24
Cell adhesion molecule (CAM), 8
Cell cortex, 8
Cell cycle, 25
Cell junction, 11
Cell polarity, 2
Cell-to-cell adhesion, 10
Cellular metabolism, 14
Cellular receptor, 9
Centromere, 26
Channel, 17
Chemical synapse, 12
Chromatid, 26
Chromatin, 26
Citric acid cycle (Krebs cycle, tricarboxylic acid cycle), 16
Clathrin, 22
Coated vesicle, 22
Collagen, 10
Concentration gradient, 19
Connective tissue, 10
Connexon, 12
Contact-dependent signaling, 12
Cytokinesis, 25
Cytoplasm, 2
Cytoplasmic matrix, 2
Cytosol, 2
Daughter cell, 26
Depolarization, 24
Desmosome, 12
Differentiation, 1
Diffusion, 19
Digestion, 16
Effective osmolality, 20
Elastin, 10
Electrolyte, 18
Electron-transport chain, 16
Endocytosis, 22
Endosome, 22
Equatorial plate (metaphase plate), 26
ER stress, 8
Eukaryote, 1
Exocytosis, 22
Extracellular matrix, 10
Fibroblast, 10
Fibronectin, 10
Filtration, 19
G1 phase, 26
G2 phase, 25
Gap junction, 12
Gating, 12
Glycocalyx, 9
Glycolipid, 3
Glycolysis, 16
Glycoprotein, 3
Growth factor (cytokine), 26
Homeostasis, 12
Hormonal signaling, 12
Hydrostatic pressure, 19
Hyperpolarized state, 25
Hypopolarized state, 25
Integral membrane protein, 7
Interphase, 25
Ions, 7
Junctional complex, 12
Ligand, 9
Lipid bilayer, 2
M phase, 25
Macromolecule, 10
Mediated transport, 17
Membrane lipid raft (MLR), 5
Membrane transport protein, 17
Metabolic pathway, 16
Metaphase, 26
Mitosis, 25
Multipotency, 27
Neurohormonal signaling, 12
Neurotransmitter, 12
Nuclear envelope, 2
Nuclear pores, 2
Nucleolus, 2
Nucleus, 2
Oncotic pressure (colloid osmotic pressure), 20
Organelle, 2
Osmolality, 19
Osmolarity, 19
Osmosis, 19
Osmotic pressure, 20
Oxidation, 16
Oxidative phosphorylation, 16
Paracrine signaling, 12
Passive transport, 17
Peripheral membrane protein, 7
Phagocytosis, 22
Phospholipid, 5
Pinocytosis, 22
Plasma membrane (plasmalemma), 2
Plasma membrane receptor, 9
Platelet-derived growth factor (PDGF), 27
Polarity, 19
Polypeptide, 5
Posttranslational modification (PTM), 5
Prokaryote, 1
Prophase, 26
Protein, 5
Proteolytic, 9
Proteome, 7
Proteomic, 7
Receptor protein, 12
Receptor-mediated endocytosis (ligand internalization), 24
Relative refractory period, 25
Repolarization, 25
Resting membrane potential, 24
Retinoblastoma (Rb) protein, 26
Self-renewal, 27
S phase, 25
Signal transduction pathway, 12
Signaling cell, 12
Solute, 17
Spindle fiber, 26
Stem cell, 27
Stroma, 26
Substrate, 16
Substrate phosphorylation (anaerobic glycolysis), 16
Symport, 18
Target cell, 12
Telophase, 26
Terminally differentiated, 27
Threshold potential, 24
Tight junction, 12
Tonicity, 20
Transfer reaction, 16
Transmembrane protein, 7
Transporter, 17
Unfolded-protein response, 8
Uniport, 18
Valence, 19
Wnt signals, 27
References 1. Alberts B. Essential cell biology. ed 4. Garland: New York; 2014. 2. Simons K, Sampaio JL. Membrane organization and lipid rafts. Cold Spring Harb Perspect Biol. 2011;3(10):a004697.
3. Contreras FX, et al. Specificity of intramembrane protein-lipid interactions. Cold Spring Harb Perspec Biol. 2011;3(6) [pii a004705].
4. Head BP, et al. Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling. Biochim Biophys Acta. 2014;1838(2):532–545.
5. Karnovsky MJ, et al. The concept of lipid domains in membranes. J Cell Biol. 1982;94:1–6.
6. Ribert D, Cossart P. Pathogen-mediated postranslational modification: a re- emerging field. Cell. 2010;143:694–702.
7. Vinothkumar KR, Henderson R. Structure of membrane proteins. Q Rev Biophysics. 2010;43(1):65–158.
8. Cogliati S, et al. Mitochondrial cristae shape determines respiratory chain supercomplexes assembly and respiratory efficiency. Cell. 2013;155(1):160–171.
9. Daum B, et al. Age-dependent dissociation of ATP synthase dimers and loss of inner-membrane cristae in mitochondria. Proc Natl Acad Sci U S A. 2013;110(38):15301–15306.
10. Friedman JR, Nunnari J. Mitochondrial form and function. Nature. 2014;505:335–343.
11. Amm I, et al. Protein quality control and elimination of protein waste: the role of the ubiquitin-proteosome system. Biochim Biophys Acta. 2014;1843:182–196.
12. Lindquist SL, Kelly JW. Chemical and biological approaches for adapting proteostasis to ameliorate protein misfolding and aggregation diseases: progress and prognosis. Cold Spring Harb Perspect Biol. 2011;3(12).
13. Kierszenbaum AL, Tres LT. Histology and cell biology: an introduction to pathology. ed 3. Elsevier: St Louis; 2011.
14. Xu Q, et al. Gating of connexin 43 gap junctions by a cytoplasmic loop calmodulin binding domain. Am J Physiol Cell Physiol. 2012;302(10):C1548–C1556.
15. Sirnes S, et al. Connexin43 acts as a colorectal tumor suppressor and predicts disease outcome. Int J Cancer. 2012;131(3):570–581.
16. Khan R, et al. Glycyrrhizic acid suppresses the development of precancerous
lesions via regulating the hyperproliferation, inflammation, angiogenesis ad apoptosis in the colon of Wistar rats. PLoS One. 2013;8(2):e56020.
17. Zhang MZ, et al. Inhibition of 11β hydroxysteroid dehydrogenase type II selectively blocks the tumor COX-2 pathway and suppresses colon carcinogenesis in mice and humans. J Clin Invest. 2009;119:876–885.
18. Burnstock G. Physiology and pathophysiology in purinergic neurotransmission. Physiol Rev. 2007;87(2):659–797.
19. Falzoni S, et al. Detecting adenosine triphosphate in the pericellular space. Interface Focus. 2013;3(3):20120101.
20. Nurse CA, Piskuric NA. Signal processing at mammalian carotid body chemoreceptors. Semin Cell Dev Biol. 2012;24(1):22–30.
21. Chaudhri RA, et al. Role of ERα36 in membrane-associated signaling by estrogen. Steroids. 2014;81:74–80.
22. Clevers H, et al. Stem cell signaling. An integral program for tissue renewal and regeneration: Wnt signaling and stem cell control. Science. 2014;346(6205):1248012.
2
Genes and Genetic Diseases Lynn B. Jorde
CHAPTER OUTLINE
DNA, RNA, and Proteins: Heredity at the Molecular Level, 38
Definitions, 38 From Genes to Proteins, 39
Chromosomes, 42
Chromosome Aberrations and Associated Diseases, 42
Elements of Formal Genetics, 49
Phenotype and Genotype, 49 Dominance and Recessiveness, 49
Transmission of Genetic Diseases, 49
Autosomal Dominant Inheritance, 50 Autosomal Recessive Inheritance, 52 X-Linked Inheritance, 54
Linkage Analysis and Gene Mapping, 56
Classic Pedigree Analysis, 56 Complete Human Gene Map: Prospects and Benefits, 57
Multifactorial Inheritance, 57
Genetics occupies a central position in the entire study of biology. An understanding of genetics is essential to study human, animal, plant, or microbial life. Genetics is the study of biologic inheritance. In the nineteenth century, microscopic studies of cells led scientists to suspect the nucleus of the cell contained the important mechanisms of inheritance. Scientists found chromatin, the substance giving the nucleus a granular appearance, is observable in nondividing cells. Just before the cell divides, the chromatin condenses to form discrete, dark-staining organelles, which are called chromosomes. (Cell division is discussed in Chapter 1.) With the rediscovery of Mendel's important breeding experiments at the turn of the twentieth century, it soon became apparent the chromosomes contained genes, the basic units of inheritance (Figure 2-1).
FIGURE 2-1 Successive Enlargements from a Human to the Genetic Material.
The primary constituent of chromatin is deoxyribonucleic acid (DNA). Genes are composed of sequences of DNA. By serving as the blueprints of proteins in the body, genes ultimately influence all aspects of body structure and function. Humans have approximately 20,000 protein-coding genes and an additional 9000 to 10,000 genes that encode various types of RNA (see below) that are not translated into proteins. An error in one of these genes often leads to a recognizable genetic
disease. To date, more than 20,000 genetic traits and diseases have been identified and
cataloged. As infectious diseases continue to be more effectively controlled, the proportion of beds in pediatric hospitals occupied by children with genetic diseases has risen. In addition to children, many common diseases primarily affecting adults, such as hypertension, coronary heart disease, diabetes, and cancer, are now known to have important genetic components. Great progress is being made in the diagnosis of genetic diseases and in the
understanding of genetic mechanisms underlying them. With the huge strides being made in molecular genetics, “gene therapy”—the utilization of normal genes to correct genetic disease—has begun.
DNA, RNA, and Proteins: Heredity at the Molecular Level Definitions Composition and Structure of DNA Genes are composed of DNA, which has three basic components: the five-carbon monosaccharide deoxyribose; a phosphate molecule; and four types of nitrogenous bases. Two of the bases, cytosine and thymine, are single carbon-nitrogen rings called pyrimidines. The other two bases, adenine and guanine, are double carbon- nitrogen rings called purines. The four bases are commonly represented by their first letters: A (adenine), C (cytosine), T (thymine), and G (guanine). Watson and Crick demonstrated how these molecules are physically assembled as
DNA, proposing the double-helix model, in which DNA appears like a twisted ladder with chemical bonds as its rungs (Figure 2-2). The two sides of the ladder consist of deoxyribose and phosphate molecules, united by strong phosphodiester bonds. Projecting from each side of the ladder, at regular intervals, are the nitrogenous bases. The base projecting from one side is bound to the base projecting from the other by a weak hydrogen bond. Therefore the nitrogenous bases form the rungs of the ladder; adenine pairs with thymine, and guanine pairs with cytosine. Each DNA subunit—consisting of one deoxyribose molecule, one phosphate group, and one base—is called a nucleotide.
FIGURE 2-2 Watson-Crick Model of the DNA Molecule. The DNA structure illustrated here is based on that published by James Watson (photograph, left) and Francis Crick (photograph, right) in 1953. Note that each side of the DNA molecule consists of alternating sugar and phosphate groups. Each sugar group is bonded to the opposing sugar group by a pair of nitrogenous bases (adenine-thymine or cytosine-guanine). The sequence of these pairs
constitutes a genetic code that determines the structure and function of a cell. (Illustration from Herlihy B: The human body in health and illness, ed 5, St Louis, 2015, Saunders.)
DNA as the Genetic Code DNA directs the synthesis of all the body's proteins. Proteins are composed of one or more polypeptides (intermediate protein compounds), which in turn consist of sequences of amino acids. The body contains 20 different types of amino acids; they are specified by the 4 nitrogenous bases. To specify (code for) 20 different amino acids with only 4 bases, different combinations of bases, occurring in groups of 3 (triplets), are used. These triplets of bases are known as codons. Each codon specifies a single amino acid in a corresponding protein. Because there are 64 (4 × 4 × 4) possible codons but only 20 amino acids, there are many cases in which several codons correspond to the same amino acid. The genetic code is universal: all living organisms use precisely the same DNA
codes to specify proteins except for mitochondria, the cytoplasmic organelles in which cellular respiration takes place (see Chapter 1)—they have their own extranuclear DNA. Several codons of mitochondrial DNA encode different amino acids, as compared to the same nuclear DNA codons.
Replication of DNA DNA replication consists of breaking the weak hydrogen bonds between the bases, leaving a single strand with each base unpaired (Figure 2-3). The consistent pairing of adenine with thymine and of guanine with cytosine, known as complementary base pairing, is the key to accurate replication. The unpaired base attracts a free nucleotide only if the nucleotide has the proper complementary base. When replication is complete, a new double-stranded molecule identical to the original is formed. The single strand is said to be a template, or molecule on which a complementary molecule is built, and is the basis for synthesizing the new double strand.
FIGURE 2-3 Replication of DNA. The two chains of the double helix separate and each chain serves as the template for a new complementary chain. (From Herlihy B: The human body in health and illness, ed 5,
St Louis, 2015, Saunders.)
Several different proteins are involved in DNA replication. The most important of these proteins is an enzyme known as DNA polymerase. This enzyme travels along the single DNA strand, adding the correct nucleotides to the free end of the new strand and checking to ensure that its base is actually complementary to the template base. This mechanism of DNA proofreading substantially enhances the accuracy of DNA replication.
Mutation A mutation is any inherited alteration of genetic material. One type of mutation is the base pair substitution, in which one base pair replaces another. This replacement can result in a change in the amino acid sequence. However, because of the redundancy of the genetic code, many of these mutations do not change the amino acid sequence and thus have no consequence. Such mutations are called silent mutations. Base pair substitutions altering amino acids consist of two basic types: missense mutations, which produce a change (i.e., the “sense”) in a single amino acid; and nonsense mutations, which produce one of the three stop codons (UAA, UAG, or UGA) in the messenger RNA (mRNA) (Figure 2-4). Missense mutations (see Figure 2-4, A) produce a single amino acid change, whereas nonsense mutations (see Figure 2-4, B) produce a premature stop codon in the mRNA. Stop codons terminate translation of the polypeptide.
FIGURE 2-4 Base Pair Substitution. Missense mutations (A) produce a single amino acid change, whereas nonsense mutations (B) produce a stop codon in the mRNA. Stop codons
terminate translation of the polypeptide. (From Jorde L et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
The frameshift mutation involves the insertion or deletion of one or more base pairs of the DNA molecule. As Figure 2-5 shows, these mutations change the entire “reading frame” of the DNA sequence because the deletion or insertion is not a multiple of three base pairs (the number of base pairs in a codon). Frameshift mutations can thus greatly alter the amino acid sequence. (In-frame insertions or deletions, in which a multiple of three bases is inserted or lost, tend to have less severe disease consequences than do frameshift mutations.)
FIGURE 2-5 Frameshift Mutations. Frameshift mutations result from the addition or deletion of a number of bases that is not a multiple of 3. This mutation alters all of the codons downstream
from the site of insertion or deletion. (From Jorde L et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
Agents known as mutagens increase the frequency of mutations. Examples include radiation and chemicals such as nitrogen mustard, vinyl chloride, alkylating agents, formaldehyde, and sodium nitrite. Mutations are rare events. The rate of spontaneous mutations (those occurring
in the absence of exposure to known mutagens) in humans is about 10−4 to 10−7 per gene per generation. This rate varies from one gene to another. Some DNA sequences have particularly high mutation rates and are known as mutational hot spots.
From Genes to Proteins DNA is formed and replicated in the cell nucleus, but protein synthesis takes place in the cytoplasm. The DNA code is transported from nucleus to cytoplasm, and
subsequent protein is formed through two basic processes: transcription and translation. These processes are mediated by ribonucleic acid (RNA), which is chemically similar to DNA except the sugar molecule is ribose rather than deoxyribose, and uracil rather than thymine is one of the four bases. The other bases of RNA, as in DNA, are adenine, cytosine, and guanine. Uracil is structurally similar to thymine, so it also can pair with adenine. Whereas DNA usually occurs as a double strand, RNA usually occurs as a single strand.
Transcription In transcription, RNA is synthesized from a DNA template, forming messenger RNA (mRNA). RNA polymerase binds to a promoter site, a sequence of DNA that specifies the beginning of a gene. RNA polymerase then separates a portion of the DNA, exposing unattached DNA bases. One DNA strand then provides the template for the sequence of mRNA nucleotides. The sequence of bases in the mRNA is thus complementary to the template strand,
and except for the presence of uracil instead of thymine, the mRNA sequence is identical to that of the other DNA strand. Transcription continues until a termination sequence, codons that act as signals for the termination of protein synthesis, is reached. Then the RNA polymerase detaches from the DNA, and the transcribed mRNA is freed to move out of the nucleus and into the cytoplasm (Figures 2-6 and 2-7).
FIGURE 2-6 General Scheme of Ribonucleic Acid (RNA) Transcription. In transcription of messenger RNA (mRNA), a DNA molecule “unzips” in the region of the gene to be transcribed. RNA nucleotides already present in the nucleus temporarily attach themselves to exposed DNA
bases along one strand of the unzipped DNA molecule according to the principle of complementary pairing. As the RNA nucleotides attach to the exposed DNA, they bind to each other and form a chainlike RNA strand called a messenger RNA (mRNA) molecule. Notice that the new mRNA strand is an exact copy of the base sequence on the opposite side of the DNA molecule. As in all metabolic processes, the formation of mRNA is controlled by an enzyme—in this case, the enzyme is called RNA polymerase. (From Ignatavicius DD, W orkman LD: Medical-surgical nursing, ed 6, St
Louis, 2010, Saunders.)
FIGURE 2-7 Protein Synthesis. The site of transcription is the nucleus and the site of translation is the cytoplasm. See the text for details.
Gene Splicing When the mRNA is first transcribed from the DNA template, it reflects exactly the base sequence of the DNA. In eukaryotes, many RNA sequences are removed by nuclear enzymes, and the remaining sequences are spliced together to form the functional mRNA that migrates to the cytoplasm. The excised sequences are called introns (intervening sequences), and the sequences that are left to code for proteins
are called exons.
Translation In translation, RNA directs the synthesis of a polypeptide (see Figure 2-7), interacting with transfer RNA (tRNA), a cloverleaf-shaped strand of about 80 nucleotides. The tRNA molecule has a site where an amino acid attaches. The three- nucleotide sequence at the opposite side of the cloverleaf is called the anticodon. It undergoes complementary base pairing with an appropriate codon in the mRNA, which specifies the sequence of amino acids through tRNA. The site of actual protein synthesis is in the ribosome, which consists of
approximately equal parts of protein and ribosomal RNA (rRNA). During translation, the ribosome first binds to an initiation site on the mRNA sequence and then binds to its surface, so that base pairing can occur between tRNA and mRNA. The ribosome then moves along the mRNA sequence, processing each codon and translating an amino acid by way of the interaction of mRNA and tRNA. The ribosome provides an enzyme that catalyzes the formation of covalent
peptide bonds between the adjacent amino acids, resulting in a growing polypeptide. When the ribosome arrives at a termination signal on the mRNA sequence, translation and polypeptide formation cease; the mRNA, ribosome, and polypeptide separate from one another; and the polypeptide is released into the cytoplasm to perform its required function.
Chromosomes Human cells can be categorized into gametes (sperm and egg cells) and somatic cells, which include all cells other than gametes. Each somatic cell nucleus has 46 chromosomes in 23 pairs (Figure 2-8). These are diploid cells, and the individual's father and mother each donate one chromosome per pair. New somatic cells are formed through mitosis and cytokinesis. Gametes are haploid cells: they have only 1 member of each chromosome pair, for a total of 23 chromosomes. Haploid cells are formed from diploid cells by meiosis (Figure 2-9).
FIGURE 2-8 From Molecular Parts to the Whole Somatic Cell.
FIGURE 2-9 Phases of Meiosis and Comparison to Mitosis. (From Jorde LB et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
In 22 of the 23 chromosome pairs, the 2 members of each pair are virtually identical in microscopic appearance: thus they are homologous (Figure 2-10, B). These 22 chromosome pairs are homologous in both males and females and are termed autosomes. The remaining pair of chromosomes, the sex chromosomes, consists of two homologous X chromosomes in females and a nonhomologous pair, X and Y, in males.
FIGURE 2-10 Karyotype of Chromosomes. A, Human karyotype. B, Homologous chromosomes and sister chromatids. (From Raven PH et al: Biology, ed 8, New York, 2008, McGraw-Hill.)
Figure 2-10, A, illustrates a metaphase spread, which is a photograph of the chromosomes as they appear in the nucleus of a somatic cell during metaphase. (Chromosomes are easiest to visualize during this stage of mitosis.) In Figure 2-10, A, the chromosomes are arranged according to size, with the homologous chromosomes paired. The 22 autosomes are numbered according to length, with chromosome 1 being the longest and chromosome 22 the shortest. A karyotype, or karyogram, is an ordered display of chromosomes. Some natural variation in relative chromosome length can be expected from person to person, so it is not always possible to distinguish each chromosome by its length. Therefore the position of the centromere (region of DNA responsible for movement of the replicated chromosomes into the two daughter cells during mitosis and meiosis) also is used to classify chromosomes (Figures 2-10, B and 2-11).
FIGURE 2-11 Structure of Chromosomes. A, Human chromosomes 2, 5, and 13. Each is replicated and consists of two chromatids. Chromosome 2 is a metacentric chromosome
because the centromere is close to the middle; chromosome 5 is submetacentric because the centromere is set off from the middle; chromosome 13 is acrocentric because the centromere is at or very near the end. B, During mitosis, the centromere divides and the chromosomes move to opposite poles of the cell. At the time of centromere division, the chromatids are
designated as chromosomes.
The chromosomes in Figure 2-10 were stained with Giemsa stain, resulting in distinctive chromosome bands. These form various patterns in the different chromosomes so that each chromosome can be distinguished easily. Using banding techniques, researchers can number chromosomes and study individual variations. Missing or duplicated portions of chromosomes, which often result in serious diseases, also are readily identified. More recently, techniques have been devised permitting each chromosome to be visualized with a different color.
Chromosome Aberrations and Associated Diseases Chromosome abnormalities are the leading known cause of intellectual disability and miscarriage. Estimates indicate that a major chromosome aberration occurs in at least 1 in 12 conceptions. Most of these fetuses do not survive to term; about 50% of all recovered first-trimester spontaneous abortuses have major chromosome aberrations.1 The number of live births affected by these abnormalities is, however, significant; approximately 1 in 150 has a major diagnosable chromosome abnormality.1
Polyploidy
Cells with a multiple of the normal number of chromosomes are euploid cells (Greek eu = good or true). Because normal gametes are haploid and most normal somatic cells are diploid, they are both euploid forms. When a euploid cell has more than the diploid number of chromosomes, it is said to be a polyploid cell. Several types of body tissues, including some liver, bronchial, and epithelial tissues, are normally polyploid. A zygote that has three copies of each chromosome, rather than the usual two, has a form of polyploidy called triploidy. Nearly all triploid fetuses are spontaneously aborted or stillborn. The prevalence of triploidy among live births is approximately 1 in 10,000. Tetraploidy, a condition in which euploid cells have 92 chromosomes, has been found primarily in early abortuses, although occasionally affected infants have been born alive. Like triploid infants, however, they do not survive. Triploidy and tetraploidy are relatively common conditions, accounting for approximately 10% of all known miscarriages.2
Aneuploidy A cell that does not contain a multiple of 23 chromosomes is an aneuploid cell. A cell containing three copies of one chromosome is said to be trisomic (a condition termed trisomy) and is aneuploid. Monosomy, the presence of only one copy of a given chromosome in a diploid cell, is the other common form of aneuploidy. Among the autosomes, monosomy of any chromosome is lethal, but newborns with trisomy of chromosomes 13, 18, 21, or X can survive. This difference illustrates an important principle: in general, loss of chromosome material has more serious consequences than duplication of chromosome material. Aneuploidy of the sex chromosomes is less serious than that of the autosomes.
Very little genetic material—only about 40 genes—is located on the Y chromosome. For the X chromosome, inactivation of extra chromosomes (see p. 54) largely diminishes their effect. A zygote bearing no X chromosome, however, will not survive. Aneuploidy is usually the result of nondisjunction, an error in which
homologous chromosomes or sister chromatids fail to separate normally during meiosis or mitosis (Figure 2-12). Nondisjunction produces some gametes that have two copies of a given chromosome and others that have no copies of the chromosome. When such gametes unite with normal haploid gametes, the resulting zygote is monosomic or trisomic for that chromosome. Occasionally, a cell can be monosomic or trisomic for more than one chromosome.
FIGURE 2-12 Nondisjunction. Nondisjunction causes aneuploidy when chromosomes or sister chromatids fail to divide properly. (From Jorde LB et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
Autosomal aneuploidy. Trisomy can occur for any chromosome, but fetuses with other trisomies of chromosomes (other than 13, 18, 21, or X) do not survive to term. Trisomy 16, for example, is the most common trisomy among abortuses, but it is not seen in live births.3 Partial trisomy, in which only an extra portion of a chromosome is present in
each cell, can occur also. The consequences of partial trisomies are not as severe as those of complete trisomies. Trisomies may occur in only some cells of the body. Individuals thus affected are said to be chromosomal mosaics, meaning that the body has two or more different cell lines, each of which has a different karyotype. Mosaics are often formed by early mitotic nondisjunction occurring in one embryonic cell but not in others. The best-known example of aneuploidy in an autosome is trisomy of
chromosome 21, which causes Down syndrome (named after J. Langdon Down, who first described the syndrome in 1866). Down syndrome is seen in
approximately 1 in 800 to 1 in 1000 live births;4 its principal features are shown and outlined in Figure 2-13 and Table 2-1.
FIGURE 2-13 Child with Down Syndrome. (Courtesy Drs. A. Olney and M. MacDonald, University of Nebraska Medical Center, Omaha, Neb.)
TABLE 2-1 Characteristics of Various Chromosome Disorders
Disease/Disorder Features Down Syndrome Trisomy of Chromosome 21 IQ Usually ranges from 20 to 70 (intellectual disability) Male/female findings
Virtually all males are sterile; some females can reproduce
Face Distinctive: low nasal bridge, epicanthal folds, protruding tongue, low-set ears Musculoskeletal system
Poor muscle tone (hypotonia), short stature
Systemic disorders Congenital heart disease (one third to half of cases), reduced ability to fight respiratory tract infections, increased susceptibility to leukemia—overall reduced survival rate; by age 40 years usually develop symptoms similar to those of Alzheimer disease
Mortality About 75% of fetuses with Down syndrome abort spontaneously or are stillborn; 20% of infants die before age 10 years; those who live beyond 10 years have life expectancy of about 60 years
Causative factors 97% caused by nondisjunction during formation of one of parent's gametes or during early embryonic development; 3% result from translocations; in 95% of cases, nondisjunction occurs when mother's egg cell is formed; remainder involve paternal nondisjunction; 1% are mosaics—these have a large number of normal cells, and effects of trisomic cells are attenuated and symptoms are generally less severe
Turner Syndrome (45,X) Monosomy of X Chromosome IQ Not considered to be intellectually disabled, although some impairment of spatial and mathematical reasoning ability is found Male/female findings
Found only in females
Musculoskeletal system
Short stature common, characteristic webbing of neck, widely spaced nipples, reduced carrying angle at elbow
Systemic disorders Coarctation (narrowing) of aorta, edema of feet in newborns, usually sterile and have gonadal streaks rather than ovaries; streaks are sometimes susceptible to cancer
Mortality About 15-20% of spontaneous abortions with chromosome abnormalities have this karyotype, most common single-chromosome aberration; highly lethal during gestation, only about 0.5% of these conceptions survive to term
Causative factors 75% inherit X chromosome from mother, thus caused by meiotic error in father; frequency low compared with other sex chromosome aneuploidies (1 : 5000 newborn females); 50% have simple monosomy of X chromosome; remainder have more complex abnormalities; combinations of 45, X cells with XX or XY cells common
Klinefelter Syndrome (47,XXY) XXY Condition IQ Moderate degree of mental impairment may be present Male/female findings
Have a male appearance but usually sterile; 50% develop female-like breasts (gynecomastia); occurs in 1 : 1000 male births
Face Voice somewhat high pitched Systemic disorders Sparse body hair, sterile, small testicles Causative factors 50% of cases the result of nondisjunction of X chromosomes in mother, frequency rises with increasing maternal age; also involves
XXY and XXXY karyotypes with degree of physical and mental impairment increasing with each added X chromosome; mosaicism fairly common with most prevalent combination of XXY and XY cells
The risk of having a child with Down syndrome increases greatly with maternal age. As Figure 2-14 demonstrates, women younger than 30 years have a risk ranging from about 1 in 1000 births to 1 in 2000 births. The risk begins to rise substantially after 35 years of age, and reaches 3% to 5% for women older than 45 years. This dramatic increase in risk is caused by the age of maternal egg cells, which are held in an arrested state of prophase I from the time they are formed in the female embryo until they are shed in ovulation. Thus an egg cell formed by a 45-year-old woman is itself 45 years old. This long suspended state may allow defects to accumulate in the cellular proteins responsible for meiosis, leading to nondisjunction. The risk of Down syndrome, as well as other trisomies, does not increase with paternal age.4
FIGURE 2-14 Down Syndrome Increases with Maternal Age. Rate is per 1000 live births related to maternal age.
Sex chromosome aneuploidy. The incidence of sex chromosome aneuploidies is fairly high. Among live births, about 1 in 500 males and 1 in 900 females have a form of sex chromosome aneuploidy.5 Because these conditions are generally less severe than autosomal aneuploidies, all forms except complete absence of any X chromosome material allow at least some individuals to survive. One of the most common sex chromosome aneuploidies, affecting about 1 in
1000 newborn females, is trisomy X. Instead of two X chromosomes, these females have three X chromosomes in each cell. Most of these females have no overt physical abnormalities, although sterility, menstrual irregularity, or intellectual disability is sometimes seen. Some females have four X chromosomes, and they are more often intellectually disabled. Those with five or more X chromosomes generally have more severe intellectual disability and various physical defects. A condition that leads to somewhat more serious problems is the presence of a
single X chromosome and no homologous X or Y chromosome, so that the individual has a total of 45 chromosomes. The karyotype is usually designated 45,X, and it causes a set of symptoms known as Turner syndrome (Figure 2-15; see Table 2-1). Individuals with at least two X chromosomes and one Y chromosome in each cell (47,XXY karyotype) have a disorder known as Klinefelter syndrome (Figure 2- 16; see Table 2-1).
FIGURE 2-15 Turner Syndrome. A, A sex chromosome is missing, and the person's chromosomes are 45,X. Characteristic signs are short stature, female genitalia, webbed neck,
shieldlike chest with underdeveloped breasts and widely spaced nipples, and imperfectly developed ovaries. B, As this karyotype shows, Turner syndrome results from monosomy of sex chromosomes (genotype XO). (From Patton KT, Thibodeau GA: Anatomy & physiology, ed 8, St Louis, 2013, Mosby. Courtesy
Nancy S. W exler, PhD, Columbia University.)
FIGURE 2-16 Klinefelter Syndrome. This young man exhibits many characteristics of Klinefelter syndrome: small testes, some development of the breasts, sparse body hair, and long limbs.
This syndrome results from the presence of two or more X chromosomes with one Y chromosome (genotypes XXY or XXXY, for example). (From Patton KT, Thibodeau GA: Anatomy & physiology, ed 9, St
Louis, 2016, Mosby. Courtesy Nancy S. W exler, PhD, Columbia University.)
Abnormalities of Chromosome Structure In addition to the loss or gain of whole chromosomes, parts of chromosomes can be lost or duplicated as gametes are formed, and the arrangement of genes on chromosomes can be altered. Unlike aneuploidy and polyploidy, these changes sometimes have no serious consequences for an individual's health. Some of them can even remain entirely unnoticed, especially when very small pieces of chromosomes are involved. Nevertheless, abnormalities of chromosome structure can also produce serious disease in individuals or their offspring.
During meiosis and mitosis, chromosomes usually maintain their structural integrity, but chromosome breakage occasionally occurs. Mechanisms exist to “heal” these breaks and usually repair them perfectly with no damage to the daughter cell. However, some breaks remain or heal in a way that alters the chromosome's structure. The risk of chromosome breakage increases with exposure to harmful agents called clastogens (e.g., ionizing radiation, viral infections, or some types of chemicals).
Deletions. Broken chromosomes and lost DNA cause deletions (Figure 2-17). Usually, a gamete with a deletion unites with a normal gamete to form a zygote. The zygote thus has one chromosome with the normal complement of genes and one with some missing genes. Because many genes can be lost in a deletion, serious consequences result even though one normal chromosome is present. The most often cited example of a disease caused by a chromosomal deletion is the cri du chat syndrome. The term literally means “cry of the cat” and describes the characteristic cry of the affected child. Other symptoms include low birth weight, severe intellectual disability, microcephaly (smaller than normal head size), and heart defects. The disease is caused by a deletion of part of the short arm of chromosome 5.
FIGURE 2-17 Abnormalities of Chromosome Structure. A, Deletion occurs when a chromosome segment is lost. B, Normal crossing over. C, The generation of duplication and
deletion through unequal crossing over.
Duplications. A deficiency of genetic material is more harmful than an excess, so duplications usually have less serious consequences than deletions. For example, a deletion of a region of chromosome 5 causes cri du chat syndrome, but a duplication of the same region causes mental retardation but less serious physical defects.
Inversions. An inversion occurs when two breaks take place on a chromosome, followed by the reinsertion of the missing fragment at its original site but in inverted order. Therefore a chromosome symbolized as ABCDEFG might become ABEDCFG after an inversion. Unlike deletions and duplications, no loss or gain of genetic material occurs, so
inversions are “balanced” alterations of chromosome structure, and they often have no apparent physical effect. Some genes are influenced by neighboring genes, however, and this position effect, a change in a gene's expression caused by its position, sometimes results in physical defects in these persons. Inversions can cause serious problems in the offspring of individuals carrying the inversion because the inversion can lead to duplications and deletions in the chromosomes transmitted to the offspring.
Translocations. The interchange of genetic material between nonhomologous chromosomes is called translocation. A reciprocal translocation occurs when breaks take place in two different chromosomes and the material is exchanged (Figure 2-18, A). As with inversions, the carrier of a reciprocal translocation is usually normal, but his or her offspring can have duplications and deletions.
FIGURE 2-18 Normal and Abnormal Chromosome Translocation. A, Normal chromosomes and reciprocal translocation. B, Pairing at meiosis. C, Consequences of translocation in gametes; unbalanced gametes result in zygotes that are partially trisomic and partially monosomic and
consequently develop abnormally.
A second and clinically more important type of translocation is Robertsonian translocation. In this disorder, the long arms of two nonhomologous chromosomes fuse at the centromere, forming a single chromosome. Robertsonian translocations are confined to chromosomes 13, 14, 15, 21, and 22 because the short
arms of these chromosomes are very small and contain no essential genetic material. The short arms are usually lost during subsequent cell divisions. Because the carriers of Robertsonian translocations lose no important genetic material, they are unaffected although they have only 45 chromosomes in each cell. Their offspring, however, may have serious monosomies or trisomies. For example, a common Robertsonian translocation involves the fusion of the long arms of chromosomes 21 and 14. An offspring who inherits a gamete carrying the fused chromosome can receive an extra copy of the long arm of chromosome 21 and develop Down syndrome. Robertsonian translocations are responsible for approximately 3% to 5% of Down syndrome cases. Parents who carry a Robertsonian translocation involving chromosome 21 have an increased risk for producing multiple offspring with Down syndrome.
Fragile sites. A number of areas on chromosomes develop distinctive breaks and gaps (observable microscopically) when the cells are cultured. Most of these fragile sites do not appear to be related to disease. However, one fragile site, located on the long arm of the X chromosome, is associated with fragile X syndrome. The most important feature of this syndrome is intellectual disability. With a relatively high population prevalence (affecting approximately 1 in 4000 males and 1 in 8000 females), fragile X syndrome is the second most common genetic cause of intellectual disability (after Down syndrome). In fragile X syndrome, females who inherit the mutation do not necessarily
express the disease condition, but they can pass it on to descendants who do express it. Ordinarily, a male who inherits a disease gene on the X chromosome expresses the condition, because he has only one X chromosome. An uncommon feature of this disease is that about one third of carrier females are affected, although less severely than males. Unaffected transmitting males have been shown to have more than about 50 repeated DNA sequences near the beginning of the fragile X gene. These trinucleotide sequences, which consist of CGG sequences duplicated many times, cause fragile X syndrome when the number of copies exceeds 200.6 The number of these repeats can increase from generation to generation. More than 20 other genetic diseases, including Huntington disease and myotonic dystrophy, also are caused by this mechanism.7
Quick Check 2-1
1. What is the major composition of DNA?
2. Define the terms mutation, autosomes, and sex chromosomes.
3. What is the significance of mRNA?
4. What is the significance of chromosomal translocation?
Elements of Formal Genetics The mechanisms by which an individual's set of paired chromosomes produces traits are the principles of genetic inheritance. Mendel's work with garden peas first defined these principles. Later geneticists have refined Mendel's work to explain patterns of inheritance for traits and diseases that appear in families. Analysis of traits that occur with defined, predictable patterns has helped
geneticists to assemble the pieces of the human gene map. Current research focuses on determining the RNA or protein products of each gene and understanding the way they contribute to disease. Eventually, diseases and defects caused by single genes can be traced and therapies to prevent and treat such diseases can be developed. Traits caused by single genes are called mendelian traits (after Gregor Mendel).
Each gene occupies a position along a chromosome known as a locus. The genes at a particular locus can have different forms (i.e., they can be composed of different nucleotide sequences) called alleles. A locus that has two or more alleles that each occur with an appreciable frequency in a population is said to be polymorphic (or a polymorphism). Because humans are diploid organisms, each chromosome is represented twice,
with one member of the chromosome pair contributed by the father and one by the mother. At a given locus, an individual has one allele whose origin is paternal and one whose origin is maternal. When the two alleles are identical, the individual is homozygous at that locus. When the alleles are not identical, the individual is heterozygous at that locus.
Phenotype and Genotype The composition of genes at a given locus is known as the genotype. The outward appearance of an individual, which is the result of both genotype and environment, is the phenotype. For example, an infant who is born with an inability to metabolize the amino acid phenylalanine has the single-gene disorder known as phenylketonuria (PKU) and thus has the PKU genotype. If the condition is left untreated, abnormal metabolites of phenylalanine will begin to accumulate in the infant's brain and irreversible intellectual disability will occur. Intellectual disability is thus one aspect of the PKU phenotype. By imposing dietary restrictions to exclude food that contains phenylalanine, however, intellectual disability can be prevented. Foods high in phenylalanine include proteins found in milk, dairy products, meat, fish, chicken, eggs, beans, and nuts. Although the child still has the PKU genotype, a modification of the environment (in this case, the child's diet) produces an
outwardly normal phenotype.
Dominance and Recessiveness In many loci, the effects of one allele mask those of another when the two are found together in a heterozygote. The allele whose effects are observable is said to be dominant. The allele whose effects are hidden is said to be recessive (from the Latin root for “hiding”). Traditionally, for loci having two alleles, the dominant allele is denoted by an uppercase letter and the recessive allele is denoted by a lowercase letter. When one allele is dominant over another, the heterozygote genotype Aa has the same phenotype as the dominant homozygote AA. For the recessive allele to be expressed, it must exist in the homozygote form, aa. When the heterozygote is distinguishable from both homozygotes, the locus is said to exhibit codominance. A carrier is an individual who has a disease gene but is phenotypically normal.
Many genes for a recessive disease occur in heterozygotes who carry one copy of the gene but do not express the disease. When recessive genes are lethal in the homozygous state, they are eliminated from the population when they occur in homozygotes. By “hiding” in carriers, however, recessive genes for diseases are passed on to the next generation.
Transmission of Genetic Diseases The pattern in which a genetic disease is inherited through generations is termed the mode of inheritance. Knowing the mode of inheritance can reveal much about the disease-causing gene itself, and members of families with the disease can be given reliable genetic counseling. Gregor Mendel systematically studied modes of inheritance and formulated two
basic laws of inheritance. His principle of segregation states that homologous genes separate from one another during reproduction and that each reproductive cell carries only one copy of a homologous gene. Mendel's second law, the principle of independent assortment, states that the hereditary transmission of one gene does not affect the transmission of another. Mendel discovered these laws in the mid-nineteenth century by performing breeding experiments with garden peas, even though he had no knowledge of chromosomes. Early twentieth century geneticists found that chromosomal behavior essentially corresponds to Mendel's laws, which now form the basis for the chromosome theory of inheritance. The known single-gene diseases can be classified into four major modes of
inheritance: autosomal dominant, autosomal recessive, X-linked dominant, and X- linked recessive. The first two types involve genes known to occur on the 22 pairs of autosomes. The last two types occur on the X chromosome; very few disease- causing genes occur on the Y chromosome. The pedigree chart summarizes family relationships and shows which members
of a family are affected by a genetic disease (Figure 2-19). Generally, the pedigree begins with one individual in the family, the proband. This individual is usually the first person in the family diagnosed or seen in a clinic.
FIGURE 2-19 Symbols Commonly Used in Pedigrees. (From Jorde LB et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
Autosomal Dominant Inheritance Characteristics of Pedigrees Diseases caused by autosomal dominant genes are rare, with the most common occurring in fewer than 1 in 500 individuals. Therefore it is uncommon for two individuals who are both affected by the same autosomal dominant disease to produce offspring together. Figure 2-20, A, illustrates this unusual pattern. Affected offspring are usually produced by the union of a normal parent with an affected heterozygous parent. The Punnett square in Figure 2-20, B, illustrates this mating. The affected parent can pass either a disease-causing allele or a normal allele to the next generation. On average, half the children will be heterozygous and will express the disease, and half will be normal.
FIGURE 2-20 Punnett Square and Autosomal Dominant Traits. A, Punnett square for the mating of two individuals with an autosomal dominant gene. Here both parents are affected by the trait. B, Punnett square for the mating of a normal individual with a carrier for an autosomal dominant
gene.
The pedigree in Figure 2-21 shows the transmission of an autosomal dominant allele. Several important characteristics of this pedigree support the conclusion that the trait is caused by an autosomal dominant gene:
1. The two sexes exhibit the trait in approximately equal proportions; males and females are equally likely to transmit the trait to their offspring.
2. No generations are skipped. If an individual has the trait, one parent must also have it. If neither parent has the trait, none of the children have it (with the exception of new mutations, as discussed later).
3. Affected heterozygous individuals transmit the trait to approximately half their children, and because gamete transmission is subject to chance fluctuations, all or none of the children of an affected parent may have the trait. When large numbers of matings of this type are studied, however, the proportion of affected children
closely approaches one half.
FIGURE 2-21 Pedigree Illustrating the Inheritance Pattern of Postaxial Polydactyly, an Autosomal Dominant Disorder. Affected individuals are represented by shading. (From Jorde LB et al:
Medical genetics, ed 4, St Louis, 2010, Mosby.)
Recurrence Risks Parents at risk for producing children with a genetic disease nearly always ask the question, “What is the chance that our child will have this disease?” The probability that an individual will develop a genetic disease is termed the recurrence risk. When one parent is affected by an autosomal dominant disease (and is a heterozygote) and the other is unaffected, the recurrence risk for each child is one half. An important principle is that each birth is an independent event, much like a coin
toss. Thus, even though parents may have already had a child with the disease, their recurrence risk remains one half. Even if they have produced several children, all affected (or all unaffected) by the disease, the law of independence dictates the probability their next child will have the disease is still one half. Parents' misunderstanding of this principle is a common problem encountered in genetic counseling. If a child is born with an autosomal dominant disease and there is no history of
the disease in the family, the child is probably the product of a new mutation. The gene transmitted by one of the parents has thus undergone a mutation from a normal to a disease-causing allele. The alleles at this locus in most of the parent's other germ cells are still normal. In this situation the recurrence risk for the parent's subsequent offspring is not greater than that of the general population. The offspring of the affected child, however, will have a recurrence risk of one half. Because these diseases often reduce the potential for reproduction, many autosomal dominant diseases result from new mutations. Occasionally, two or more offspring have symptoms of an autosomal dominant
disease when there is no family history of the disease. Because mutation is a rare event, it is unlikely that this disease would be a result of multiple mutations in the same family. The mechanism most likely responsible is termed germline mosaicism. During the embryonic development of one of the parents, a mutation occurred that affected all or part of the germline. Few or none of the somatic cells of the embryo were affected. Thus the parent carries the mutation in his or her germline but does not actually express the disease. As a result, the unaffected parent can transmit the mutation to multiple offspring. This phenomenon, although relatively rare, can have significant effects on recurrence risks.8
Delayed Age of Onset One of the best-known autosomal dominant diseases is Huntington disease, a neurologic disorder whose main features are progressive dementia and increasingly uncontrollable limb movements (chorea; discussed further in Chapter 15). A key feature of this disease is its delayed age of onset: symptoms usually are not seen until 40 years of age or later. Thus those who develop the disease often have borne children before they are aware they have the disease-causing mutation. If the disease was present at birth, nearly all affected persons would die before reaching reproductive age and the occurrence of the disease-causing allele in the population would be much lower. An individual whose parent has the disease has a 50% chance of developing it during middle age. He or she is thus confronted with a torturous question: Should I have children, knowing that there is a 50 : 50 chance that I may have this disease-causing gene and will pass it to half of my children? A DNA test can now be used to determine whether an individual has inherited the trinucleotide repeat mutation that causes Huntington disease.
Penetrance and Expressivity The penetrance of a trait is the percentage of individuals with a specific genotype who also exhibit the expected phenotype. Incomplete penetrance means individuals who have the disease-causing genotype may not exhibit the disease phenotype at all, even though the genotype and the associated disease may be transmitted to the next generation. A pedigree illustrating the transmission of an autosomal dominant mutation with incomplete penetrance is provided in Figure 2-22. Retinoblastoma, the most common malignant eye tumor affecting children, typically exhibits incomplete penetrance. About 10% of the individuals who are obligate carriers of the disease-causing mutation (i.e., those who have an affected parent and affected children and therefore must themselves carry the mutation) do not have the disease. The penetrance of the disease-causing genotype is then said to be 90%.
FIGURE 2-22 Pedigree for Retinoblastoma Showing Incomplete Penetrance. Female with marked arrow in line II must be heterozygous, but she does not express the trait.
The gene responsible for retinoblastoma is a tumor-suppressor gene: the normal function of its protein product is to regulate the cell cycle so cells do not divide uncontrollably. When the protein is altered because of a genetic mutation, its tumor- suppressing capacity is lost and a tumor can form9 (see Chapters 10 and 17). Expressivity is the extent of variation in phenotype associated with a particular
genotype. If the expressivity of a disease is variable, penetrance may be complete but the severity of the disease can vary greatly. A good example of variable expressivity in an autosomal dominant disease is neurofibromatosis type 1, or von Recklinghausen disease. As in retinoblastoma, the mutations that cause neurofibromatosis type 1 occur in a tumor-suppressor gene.10 The expression of this disease varies from a few harmless café-au-lait (light brown) spots on the skin to numerous neurofibromas, scoliosis, seizures, gliomas, neuromas, malignant peripheral nerve sheath tumors, hypertension, and learning disorders (Figure 2-23).
FIGURE 2-23 Neurofibromatosis. Tumors. The most common is sessile or pedunculated. Early tumors are soft, dome-shaped papules or nodules that have a distinctive violaceous hue. Most
are benign. (From Habif et al: Skin disease: diagnosis and treatment, ed 2, St Louis, 2005, Mosby.)
Several factors cause variable expressivity. Genes at other loci sometimes modify the expression of a disease-causing gene. Environmental factors also can influence expression of a disease-causing gene. Finally, different mutations at a locus can cause variation in severity. For example, a mutation that alters only one amino acid of the factor VIII gene usually produces a mild form of hemophilia A, whereas a “stop” codon (premature termination of translation) usually produces a more severe form of this blood coagulation disorder.
Epigenetics and Genomic Imprinting Although this chapter focuses on DNA sequence variation and its consequence for disease, there is increasing evidence that the same DNA sequence can produce dramatically different phenotypes because of chemical modifications altering the expression of genes (these modifications are collectively termed epigenetic, Chapter 3). An important example of such a modification is DNA methylation, the attachment of a methyl group to a cytosine base followed by a guanine base in the DNA sequence (Figure 2-24). These sequences, which are common near many genes, are termed CpG islands. When the CpG islands located near a gene become
heavily methylated, the gene is less likely to be transcribed into mRNA. In other words, the gene becomes transcriptionally inactive. One study showed that identical (monozygotic) twins accumulate different methylation patterns in the DNA sequences of their somatic cells as they age, causing increasing numbers of phenotypic differences.11 Intriguingly, twins with more differences in their lifestyles (e.g., smoking versus nonsmoking) accumulated larger numbers of differences in their methylation patterns. The twins, despite having identical DNA sequences, become more and more different as a result of epigenetic changes, which in turn affect the expression of genes (see Figure 3-5).
FIGURE 2-24 Epigenetic Modifications. Because DNA is a long molecule, it needs packaging to fit in the tiny nucleus. Packaging involves coiling of the DNA in a “left-handed” spiral around
spools, made of four pairs of proteins individually known as histones and collectively termed the histone octamer. The entire spool is called a nucleosome (also see Figure 1-2). Nucleosomes
are organized into chromatin, the repeating building blocks of a chromosome. Histone modifications are correlated with methylation, are reversible, and occur at multiple sites. Methylation occurs at the 5 position of cytosine and provides a “footprint” or signature as a unique epigenetic alteration (red). When genes are expressed, chromatin is open or active; however, when chromatin is condensed because of methylation and histone modification,
genes are inactivated.
Epigenetic alteration of gene activity can have important disease consequences. For example, a major cause of one form of inherited colon cancer (termed hereditary nonpolyposis colorectal cancer [HNPCC]) is the methylation of a gene whose protein product repairs damaged DNA. When this gene becomes inactive,
damaged DNA accumulates, eventually resulting in colon tumors. Epigenetic changes are also discussed in Chapters 3, 10 and 11. Approximately 100 human genes are thought to be methylated differently,
depending on which parent transmits the gene. This epigenetic modification, characterized by methylation and other changes, is termed genomic imprinting. For each of these genes, one of the parents imprints the gene (inactivates it) when it is transmitted to the offspring. An example is the insulin-like growth factor 2 gene (IGF2) on chromosome 11, which is transmitted by both parents, but the copy inherited from the mother is normally methylated and inactivated (imprinted). Thus only one copy of IGF2 is active in normal individuals. However, the maternal imprint is occasionally lost, resulting in two active copies of IGF2. This causes excess fetal growth and contributes to a condition known as Beckwith-Weidemann syndrome (see p. 65). A second example of genomic imprinting is a deletion of part of the long arm of
chromosome 15 (15q11-q13), which, when inherited from the father, causes the offspring to manifest a disease known as Prader-Willi syndrome (short stature, obesity, hypogonadism). When the same deletion is inherited from the mother, the offspring develop Angelman syndrome (intellectual disability, seizures, ataxic gait). The two different phenotypes reflect the fact that different genes are normally active in the maternally and paternally transmitted copies of this region of chromosome 15 (see p. 65).
Autosomal Recessive Inheritance Characteristics of Pedigrees Like autosomal dominant diseases, diseases caused by autosomal recessive genes are rare in populations, although there can be numerous carriers. The most common lethal recessive disease in white children, cystic fibrosis, occurs in about 1 in 2500 births. Approximately 1 in 25 whites carries a copy of a mutation that causes cystic fibrosis (see Chapter 28). Carriers are phenotypically unaffected. Some autosomal recessive diseases are characterized by delayed age of onset, incomplete penetrance, and variable expressivity. Figure 2-25 shows a pedigree for cystic fibrosis. The gene responsible for cystic
fibrosis encodes a chloride ion channel in some epithelial cells. Defective transport of chloride ions leads to a salt imbalance that results in secretions of abnormally thick, dehydrated mucus. Some digestive organs, particularly the pancreas, become obstructed, causing malnutrition, and the lungs become clogged with mucus, making them highly susceptible to bacterial infections. Death from lung disease or heart failure occurs before 40 years of age in about half of persons with cystic
fibrosis.
FIGURE 2-25 Pedigree for Cystic Fibrosis. Cystic fibrosis is an autosomal recessive disorder. The double bar denotes a consanguineous mating. Because cystic fibrosis is relatively common
in European populations, most cases do not involve consanguinity.
The important criteria for discerning autosomal recessive inheritance include the following:
1. Males and females are affected in equal proportions.
2. Consanguinity (marriage between related individuals) is sometimes present, especially for rare recessive diseases.
3. The disease may be seen in siblings of affected individuals but usually not in their parents.
4. On average, one fourth of the offspring of carrier parents will be affected.
Recurrence Risks In most cases of recessive disease, both of the parents of affected individuals are heterozygous carriers. On average, one fourth of their offspring will be normal homozygotes, half will be phenotypically normal carrier heterozygotes, and one fourth will be homozygotes with the disease (Figure 2-26). Thus the recurrence risk for the offspring of carrier parents is 25%. However, in any given family, there are chance fluctuations.
FIGURE 2-26 Punnett Square for the Mating of Heterozygous Carriers Typical of Most Cases of Recessive Disease.
If two parents have a recessive disease, they each must be homozygous for the disease. Therefore all their children also must be affected. This distinguishes recessive from dominant inheritance because two parents both affected by a dominant gene are nearly always both heterozygotes and thus one fourth of their children will be unaffected. Because carrier parents usually are unaware that they both carry the same
recessive allele, they often produce an affected child before becoming aware of their condition. Carrier detection tests can identify heterozygotes by analyzing the DNA sequence to reveal a mutation. Some recessive diseases for which carrier detection tests are routinely used include phenylketonuria (PKU), sickle cell disease, cystic fibrosis, Tay-Sachs disease, hemochromatosis, and galactosemia.
Consanguinity Consanguinity and inbreeding are related concepts. Consanguinity refers to the mating of two related individuals, and the offspring of such matings are said to be inbred. Consanguinity is sometimes an important characteristic of pedigrees for recessive diseases because relatives share a certain proportion of genes received from a common ancestor. The proportion of shared genes depends on the closeness of their biologic relationship. Consanguineous matings produce a significant increase in recessive disorders and are seen most often in pedigrees for rare recessive disorders.
X-Linked Inheritance Some genetic conditions are caused by mutations in genes located on the sex chromosomes, and this mode of inheritance is termed sex linked. Only a few diseases are known to be inherited as X-linked dominant or Y chromosome traits,
so only the more common X-linked recessive diseases are discussed here. Because females receive two X chromosomes, one from the father and one from
the mother, they can be homozygous for a disease allele at a given locus, homozygous for the normal allele at the locus, or heterozygous. Males, having only one X chromosome, are hemizygous for genes on this chromosome. If a male inherits a recessive disease gene on the X chromosome, he will be affected by the disease because the Y chromosome does not carry a normal allele to counteract the effects of the disease gene. Because a single copy of an X-linked recessive gene will cause disease in a male, whereas two copies are required for disease expression in females, more males are affected by X-linked recessive diseases than are females.
X Inactivation In the late 1950s Mary Lyon proposed that one X chromosome in the somatic cells of females is permanently inactivated, a process termed X inactivation.12,13 This proposal, the Lyon hypothesis, explains why most gene products coded by the X chromosome are present in equal amounts in males and females, even though males have only one X chromosome and females have two X chromosomes. This phenomenon is called dosage compensation. The inactivated X chromosomes are observable in many interphase cells as highly condensed intranuclear chromatin bodies, termed Barr bodies (after Barr and Bertram, who discovered them in the late 1940s). Normal females have one Barr body in each somatic cell, whereas normal males have no Barr bodies. X inactivation occurs very early in embryonic development—approximately 7 to
14 days after fertilization. In each somatic cell, one of the two X chromosomes is inactivated. In some cells, the inactivated X chromosome is the one contributed by the father; in other cells it is the one contributed by the mother. Once the X chromosome has been inactivated in a cell, all the descendants of that cell have the same chromosome inactivated (Figure 2-27). Thus inactivation is said to be random but fixed.
FIGURE 2-27 The X Inactivation Process. The maternal (m) and paternal (p) X chromosomes are both active in the zygote and in early embryonic cells. X inactivation then takes place,
resulting in cells having either an active paternal X or an active maternal X. Females are thus X chromosome mosaics, as shown in the tissue sample at the bottom of the page. (From Jorde LB et al:
Medical genetics, ed 4, St Louis, 2010, Mosby.)
Some individuals do not have the normal number of X chromosomes in their somatic cells. For example, males with Klinefelter syndrome typically have two X chromosomes and one Y chromosome. These males do have one Barr body in each cell. Females whose cell nuclei have three X chromosomes have two Barr bodies in each cell, and females whose cell nuclei have four X chromosomes have three Barr bodies in each cell. Females with Turner syndrome have only one X chromosome and no Barr bodies. Thus the number of Barr bodies is always one less than the number of X chromosomes in the cell. All but one X chromosome are always inactivated. Persons with abnormal numbers of X chromosomes, such as those with Turner
syndrome or Klinefelter syndrome, are not physically normal. This situation presents a puzzle because they presumably have only one active X chromosome, the same as individuals with normal numbers of chromosomes. This is probably because the distal tips of the short and long arms of the X chromosome, as well as several other regions on the chromosome arm, are not inactivated. Thus X inactivation is also known to be incomplete. The inactivated X chromosome DNA is heavily methylated. Inactive X
chromosomes can be at least partially reactivated in vitro by administering 5- azacytidine, a demethylating agent.
Sex Determination The process of sexual differentiation, in which the embryonic gonads become either testes or ovaries, begins during the sixth week of gestation. A key principle of mammalian sex determination is that one copy of the Y chromosome is sufficient to initiate the process of gonadal differentiation that produces a male fetus. The number of X chromosomes does not alter this process. For example, an individual with two X chromosomes and one Y chromosome in each cell is still phenotypically a male. Thus the Y chromosome contains a gene that begins the process of male gonadal development. This gene, termed SRY (for “sex-determining region on the Y”), has been located
on the short arm of the Y chromosome.14 The SRY gene lies just outside the pseudoautosomal region (Figure 2-28), which pairs with the distal tip of the short arm of the X chromosome during meiosis and exchanges genetic material with it (crossover), just as autosomes do. The DNA sequences of these regions on the X and Y chromosomes are highly similar. The rest of the X and Y chromosomes, however, do not exchange material and are not similar in DNA sequence.
FIGURE 2-28 Distal Short Arms of the X and Y Chromosomes Exchange Material During Meiosis in the Male. The region of the Y chromosome in which this crossover occurs is called
the pseudoautosomal region. The SRY gene, which triggers the process leading to male gonadal differentiation, is located just outside the pseudoautosomal region. Occasionally, the
crossover occurs on the centromeric side of the SRY gene, causing it to lie on an X chromosome instead of a Y chromosome. An offspring receiving this X chromosome will be an
XX male, and an offspring receiving the Y chromosome will be an XY female.
Other genes that contribute to male differentiation are located on other chromosomes. Thus SRY triggers the action of genes on other chromosomes. This concept is supported by the fact that the SRY protein product is similar to other proteins known to regulate gene expression. Occasionally, the crossover between X and Y occurs closer to the centromere
than it should, placing the SRY gene on the X chromosome after crossover. This variation can result in offspring with an apparently normal XX karyotype but a male phenotype. Such XX males are seen in about 1 in 20,000 live births and resemble males with Klinefelter syndrome. Conversely, it is possible to inherit a Y
chromosome that has lost the SRY gene (the result of either a crossover error or a deletion of the gene). This situation produces an XY female. Such females have gonadal streaks rather than ovaries and have poorly developed secondary sex characteristics.
Quick Check 2-2
1. Why is the influence of environment significant to phenotype?
2. Discuss the differences between a dominant and a recessive allele.
3. Why are the concepts of variable expressivity, incomplete penetrance, and delayed age of onset so important in relation to genetic diseases?
4. What is the recurrence risk for autosomal dominant inheritance and recessive inheritance?
Characteristics of Pedigrees X-linked pedigrees show distinctive modes of inheritance. The most striking characteristic is that females seldom are affected. To express an X-linked recessive trait fully, a female must be homozygous: either both her parents are affected, or her father is affected and her mother is a carrier. Such matings are rare. The following are important principles of X-linked recessive inheritance:
1. The trait is seen much more often in males than in females.
2. Because a father can give a son only a Y chromosome, the trait is never transmitted from father to son.
3. The gene can be transmitted through a series of carrier females, causing the appearance of one or more “skipped generations.”
4. The gene is passed from an affected father to all his daughters, who, as phenotypically normal carriers, transmit it to approximately half their sons, who are affected.
A relatively common X-linked recessive disorder is Duchenne muscular dystrophy (DMD), which affects approximately 1 in 3500 males. As its name suggests, this disorder is characterized by progressive muscle degeneration.
Affected individuals usually are unable to walk by age 10 or 12 years. The disease affects the heart and respiratory muscles, and death caused by respiratory or cardiac failure usually occurs before 20 years of age. Identification of the disease-causing gene (on the short arm of the X chromosome) has greatly increased our understanding of the disorder.15 The DMD gene is the largest gene ever found in humans, spanning more than 2 million DNA bases. It encodes a previously undiscovered muscle protein, termed dystrophin. Extensive study of dystrophin indicates that it plays an essential role in maintaining the structural integrity of muscle cells: it may also help to regulate the activity of membrane proteins. When dystrophin is absent, as in DMD, the cell cannot survive, and muscle deterioration ensues. Most cases of DMD are caused by frameshift deletions of portions of the DMD gene and thus involve alterations of the amino acids encoded by the DNA following the deletion.
Recurrence Risks The most common mating type involving X-linked recessive genes is the combination of a carrier female and a normal male (Figure 2-29, A). On average, the carrier mother will transmit the disease-causing allele to half her sons (who are affected) and half her daughters (who are carriers).
FIGURE 2-29 Punnett Square and X-Linked Recessive Traits. A, Punnett square for the mating of a normal male (XHY) and a female carrier of an X-linked recessive gene (XHXh). B, Punnett square for the mating of a normal female (XHXH) with a male affected by an X-linked recessive disease (XhY). C, Punnett square for the mating of a female who carries an X-linked recessive
gene (XHXh) with a male who is affected with the disease caused by the gene (XhY).
The other common mating type is an affected father and a normal mother (see Figure 2-29, B). In this situation, all the sons will be normal because the father can transmit only his Y chromosome to them. Because all the daughters must receive the father's X chromosome, they will all be heterozygous carriers. Because the sons must receive the Y chromosome and the daughters must receive the X chromosome with the disease gene, these are precise outcomes and not probabilities. None of the children will be affected. The final mating pattern, less common than the other two, involves an affected
father and a carrier mother (see Figure 2-29, C). With this pattern, on average, half the daughters will be heterozygous carriers, and half will be homozygous for the disease allele and thus affected. Half the sons will be normal, and half will be
affected. Some X-linked recessive diseases, such as DMD, are fatal or incapacitating before the affected individual reaches reproductive age, and therefore affected fathers are rare.
Sex-Limited and Sex-Influenced Traits A sex-limited trait can occur in only one sex, often because of anatomic differences. Inherited uterine and testicular defects are two obvious examples. A sex- influenced trait occurs much more often in one sex than the other. For example, male-pattern baldness occurs in both males and females but is much more common in males. Autosomal dominant breast cancer, which is much more commonly expressed in females than males, is another example of a sex-influenced trait.
Linkage Analysis and Gene Mapping Locating genes on specific regions of chromosomes has been one of the most important goals of human genetics. The location and identification of a gene can tell much about the function of the gene, the interaction of the gene with other genes, and the likelihood that certain individuals will develop a genetic disease.
Classic Pedigree Analysis Mendel's second law, the principle of independent assortment, states that an individual's genes will be transmitted to the next generation independently of one another. This law is only partly true, however, because genes located close together on the same chromosome do tend to be transmitted together to the offspring. Thus Mendel's principle of independent assortment holds true for most pairs of genes but not those that occupy the same region of a chromosome. Such loci demonstrate linkage and are said to be linked. During the first meiotic stage, the arms of homologous chromosome pairs
intertwine and sometimes exchange portions of their DNA (Figure 2-30) in a process known as crossover. During crossover, new combinations of alleles can be formed. For example, two loci on a chromosome have alleles A and a and alleles B and b. Alleles A and B are located together on one member of a chromosome pair, and alleles a and b are located on the other member. The genotype of this individual is denoted as AB/ab.
FIGURE 2-30 Genetic Results of Crossing Over. A, No crossing over. B, Crossing over with recombination. C, Double crossing over, resulting in no recombination.
As Figure 2-30, A, shows, the allele pairs AB and ab would be transmitted together when no crossover occurs. However, when crossover occurs (see Figure 2- 30, B), all four possible pairs of alleles can be transmitted to the offspring: AB, aB, Ab, and ab. The process of forming such new arrangements of alleles is called recombination. Crossover does not necessarily lead to recombination, however, because double crossover between two loci can result in no actual recombination of the alleles at the loci (see Figure 2-30, C). Once a close linkage has been established between a disease locus and a “marker”
locus (a DNA sequence that varies among individuals) and once the alleles of the two loci that are inherited together within a family have been determined, reliable predictions can be made as to whether a member of a family will develop the disease. Linkage has been established between several DNA polymorphisms and each of the two major genes that can cause autosomal dominant breast cancer (about 5% of breast cancer cases are caused by these autosomal dominant genes). Determining this kind of linkage means that it is possible for offspring of an individual with autosomal dominant breast cancer to know whether they also carry the gene and thus could pass it on to their own children. In most cases, specific disease-causing mutations can be identified, allowing direct detection and diagnosis.
For some genetic diseases, prophylactic treatment is available if the condition can be diagnosed in time. An example of this is hemochromatosis, a recessive genetic disease in which excess iron is absorbed, causing degeneration of the heart, liver, brain, and other vital organs. Individuals at risk for developing the disease can be determined by testing for a mutation in the hemochromatosis gene and through clinical tests, and preventive therapy (periodic phlebotomy) can be initiated to deplete iron stores and ensure a normal life span.
Complete Human Gene Map: Prospects and Benefits The major goals of the Human Genome Project were to find the locations of all human genes (the “gene map”) and to determine the entire human DNA sequence. These goals have now been accomplished and the genes responsible for more than 4000 mendelian conditions have been identified (Figure 2-31).1,16,17 This has greatly increased our understanding of the mechanisms that underlie many diseases, such as retinoblastoma, cystic fibrosis, neurofibromatosis, and Huntington disease. The project also has led to more accurate diagnosis of these conditions, and in some cases more effective treatment.
FIGURE 2-31 Example of Diseases: A Gene Map. ADA, Adenosine deaminase; ALD, adrenoleukodystrophy; PKU, phenylketonuria.
DNA sequencing has become much less expensive and more efficient in recent years. Consequently, many thousands of individuals have now been completely sequenced, leading in some cases to the identification of disease-causing genes (see Health Alert: Gene Therapy).18
Health Alert Gene Therapy
Thousands of subjects are currently enrolled in more than 1000 gene therapy protocols. Most of these protocols involve the genetic alteration of cells to combat various types of cancer. Others involve the treatment of inherited diseases, such as β-thalassemia, hemophilia B, severe combined immunodeficiency, and retinitis pigmentosa.
Multifactorial Inheritance Not all traits are produced by single genes; some traits result from several genes acting together. These are called polygenic traits. When environmental factors influence the expression of the trait (as is usually the case), the term multifactorial inheritance is used. Many multifactorial and polygenic traits tend to follow a normal distribution in populations (the familiar bell-shaped curve). Figure 2-32 shows how three loci acting together can cause grain color in wheat to vary in a gradual way from white to red, exemplifying multifactorial inheritance. If both alleles at each of the three loci are white alleles, the color is pure white. If most alleles are white but a few are red, the color is somewhat darker; if all are red, the color is dark red.
FIGURE 2-32 Multifactorial Inheritance. Analysis of mode of inheritance for grain color in wheat. The trait is controlled by three independently assorted gene loci.
Other examples of multifactorial traits include height and IQ. Although both height and IQ are determined in part by genes, they are influenced also by environment. For example, the average height of many human populations has increased by 5 to 10 cm in the past 100 years because of improvements in nutrition and health care. Also, IQ scores can be improved by exposing individuals (especially children) to enriched learning environments. Thus both genes and
environment contribute to variation in these traits. A number of diseases do not follow the bell-shaped distribution. Instead they
appear to be either present in or absent from an individual. Yet they do not follow the patterns expected of single-gene diseases. Many of these are probably polygenic or multifactorial, but a certain threshold of liability must be crossed before the disease is expressed. Below the threshold the individual appears normal; above it, the individual is affected by the disease (Figure 2-33).
FIGURE 2-33 Threshold of Liability for Pyloric Stenosis in Males and Females.
A good example of such a threshold trait is pyloric stenosis, a disorder characterized by a narrowing or obstruction of the pylorus, the area between the stomach and small intestine. Chronic vomiting, constipation, weight loss, and electrolyte imbalance can result from the condition, but it is easily corrected by surgery. The prevalence of pyloric stenosis is about 3 in 1000 live births in whites. This disorder is much more common in males than females, affecting 1 in 200
males and 1 in 1000 females. The apparent reason for this difference is the threshold of liability is much lower in males than females, as shown in Figure 2-33. Thus fewer defective alleles are required to generate the disorder in males. This situation also means the offspring of affected females are more likely to have pyloric stenosis because affected females necessarily carry more disease-causing alleles than do most affected males. A number of other common diseases are thought to correspond to a threshold
model. They include cleft lip and cleft palate, neural tube defects (anencephaly, spina bifida), clubfoot (talipes), and some forms of congenital heart disease. Although recurrence risks can be given with confidence for single-gene diseases
(e.g., 50% for autosomal dominants, 25% for autosomal recessives), it is considerably more difficult to do so for multifactorial diseases. The number of genes contributing to the disease is not known, the precise allelic constitution of the biologic parents is not known, and the extent of environmental effects can vary from one population to another. For most multifactorial diseases, empirical risks (i.e., those based on direct observation) have been derived. To determine empirical risks, a large sample of biologic families in which one child has developed the disease is examined. The siblings of each child are then surveyed to calculate the percentage who also develop the disease. Another difficulty is distinguishing polygenic or multifactorial diseases from
single-gene diseases having incomplete penetrance or variable expressivity. Large data sets and good epidemiologic data often are necessary to make the distinction. Box 2-1 lists criteria commonly used to define multifactorial diseases.
Box 2-1 Criteria Used to Define Multifactorial Diseases
1. The recurrence risk becomes higher if more than one family member is affected. For example, the recurrence risk for neural tube defects in a British family increases to 10% if two siblings have been born with the disease. By contrast, the recurrence risk for single-gene diseases remains the same regardless of the number of siblings affected.
2. If the expression of the disease is more severe, the recurrence risk is higher. This is consistent with the liability model; a more severe expression indicates that the individual is at the extreme end of the liability distribution. Relatives of the affected individual are thus at a higher risk for inheriting disease genes. Cleft lip or cleft palate is a condition in which this has been shown to be true.
3. Relatives of probands of the less commonly affected are more likely to develop the disease. As with pyloric stenosis, this occurs because an affected individual of the less susceptible sex is usually at a more extreme position on the liability distribution.
4. Generally, if the population frequency of the disease is f, the risk for offspring and siblings of probands is approximately . This does not usually hold true for single-gene traits.
5. The recurrence risk for the disease decreases rapidly in more remotely related relatives. Although the recurrence risk for single-gene diseases decreases by 50% with each degree of relationship (e.g., an autosomal dominant disease has a 50% recurrence risk for siblings, 25% for uncle-nephew relationship, 12.5% for first cousins), the risk for multifactorial inheritance decreases much more quickly.
The genetics of common disorders such as hypertension, heart disease, and diabetes is complex and often confusing. Nevertheless, the public health impact of these diseases, together with the evidence for hereditary factors in their etiology, demands that genetic studies be pursued. Hundreds of genes contributing to susceptibility for these diseases have been discovered, and the next decade will undoubtedly witness substantial advancements in our understanding of these disorders.
Quick Check 2-3
1. Define linkage analysis; cite an example.
2. Why is “threshold of liability” an important consideration in multifactorial inheritance?
3. Discuss the concept of multifactorial inheritance, and include two examples.
Did You Understand? DNA, RNA, and Proteins: Heredity at the Molecular Level 1. Genes, the basic units of inheritance, are composed of deoxyribonucleic acid (DNA) and are located on chromosomes.
2. DNA is composed of deoxyribose, a phosphate molecule, and four types of nitrogenous bases. The physical structure of DNA is a double helix.
3. The DNA bases code for amino acids, which in turn make up proteins. The amino acids are specified by triplet codons of nitrogenous bases.
4. DNA replication is based on complementary base pairing, in which a single strand of DNA serves as the template for attracting bases that form a new strand of DNA.
5. DNA polymerase is the primary enzyme involved in replication. It adds bases to the new DNA strand and performs “proofreading” functions.
6. A mutation is an inherited alteration of genetic material (i.e., DNA).
7. Substances that cause mutations are called mutagens.
8. The mutation rate in humans varies from locus to locus and ranges from 10−4 to 10−7 per gene per generation.
9. Transcription and translation, the two basic processes in which proteins are specified by DNA, both involve ribonucleic acid (RNA). RNA is chemically similar to DNA, but it is single stranded, has a ribose sugar molecule, and has uracil rather than thymine as one of its four nitrogenous bases.
10. Transcription is the process by which DNA specifies a sequence of messenger RNA (mRNA).
11. Much of the RNA sequence is spliced from the mRNA before the mRNA leaves the nucleus. The excised sequences are called introns, and those that remain to code for proteins are called exons.
12. Translation is the process by which RNA directs the synthesis of polypeptides. This process takes place in the ribosomes, which consist of proteins and ribosomal RNA (rRNA).
13. During translation, mRNA interacts with transfer RNA (tRNA), a molecule that has an attachment site for a specific amino acid.
Chromosomes 1. Human cells consist of diploid somatic cells (body cells) and haploid gametes (sperm and egg cells).
2. Humans have 23 pairs of chromosomes. Twenty-two of these pairs are autosomes. The remaining pair consists of the sex chromosomes. Females have two homologous X chromosomes as their sex chromosomes; males have an X and a Y chromosome.
3. A karyotype is an ordered display of chromosomes arranged according to length and the location of the centromere.
4. Various types of stains can be used to make chromosome bands more visible.
5. About 1 in 150 live births has a major diagnosable chromosome abnormality. Chromosome abnormalities are the leading known cause of mental retardation and miscarriage.
6. Polyploidy is a condition in which a euploid cell has some multiple of the normal number of chromosomes. Humans have been observed to have triploidy (three copies of each chromosome) and tetraploidy (four copies of each chromosome); both conditions are lethal.
7. Somatic cells that do not have a multiple of 23 chromosomes are aneuploid. Aneuploidy is usually the result of nondisjunction.
8. Trisomy is a type of aneuploidy in which one chromosome is present in three copies in somatic cells. A partial trisomy is one in which only part of a chromosome is present in three copies.
9. Monosomy is a type of aneuploidy in which one chromosome is present in only one copy in somatic cells.
10. In general, monosomies cause more severe physical defects than do trisomies, illustrating the principle that the loss of chromosome material has more severe consequences than the duplication of chromosome material.
11. Down syndrome, a trisomy of chromosome 21, is the best-known disease caused by a chromosome aberration. It affects 1 in 800 live births and is much more likely to occur in the offspring of women older than 35 years.
12. Most aneuploidies of the sex chromosomes have less severe consequences than those of the autosomes.
13. The most commonly observed sex chromosome aneuploidies are the 47,XXX karyotype, 45,X karyotype (Turner syndrome), 47,XXY karyotype (Klinefelter syndrome), and 47,XYY karyotype.
14. Abnormalities of chromosome structure include deletions, duplications, inversions, and translocations.
Elements of Formal Genetics 1. Mendelian traits are caused by single genes, each of which occupies a position, or locus, on a chromosome.
2. Alleles are different forms of genes located at the same locus on a chromosome.
3. At any given locus in a somatic cell, an individual has two genes, one from each parent. An individual may be homozygous or heterozygous for a locus.
4. An individual's genotype is his or her genetic makeup, and the phenotype reflects the interaction of genotype and environment.
5. In a heterozygote, a dominant gene's effects mask those of a recessive gene. The recessive gene is expressed only when it is present in two copies.
Transmission of Genetic Diseases 1. Genetic diseases caused by single genes usually follow autosomal dominant, autosomal recessive, or X-linked recessive modes of inheritance.
2. Pedigree charts are important tools in the analysis of modes of inheritance.
3. Recurrence risks specify the probability that future offspring will inherit a genetic disease. For single-gene diseases, recurrence risks remain the same for each offspring, regardless of the number of affected or unaffected offspring.
4. The recurrence risk for autosomal dominant diseases is usually 50%.
5. Germline mosaicism can alter recurrence risks for genetic diseases because unaffected parents can produce multiple affected offspring. This situation occurs because the germline of one parent is affected by a mutation but the parent's somatic cells are unaffected.
6. Skipped generations are not seen in classic autosomal dominant pedigrees.
7. Males and females are equally likely to exhibit autosomal dominant diseases and to pass them on to their offspring.
8. Many genetic diseases have a delayed age of onset.
9. A gene that is not always expressed phenotypically is said to have incomplete penetrance.
10. Variable expressivity is a characteristic of many genetic diseases.
11. Genomic imprinting, which is associated with methylation, results in differing expression of a disease gene, depending on which parent transmitted the gene.
12. Epigenetics involves changes, such as the methylation of DNA bases, that do not alter the DNA sequence but can alter the expression of genes.
13. Most commonly, biologic parents of children with autosomal recessive diseases are both heterozygous carriers of the disease gene.
14. The recurrence risk for autosomal recessive diseases is 25%.
15. Males and females are equally likely to be affected by autosomal recessive diseases.
16. Consanguinity is sometimes present in families with autosomal recessive diseases, and it becomes more prevalent with rarer recessive diseases.
17. Carrier detection tests for an increasing number of autosomal recessive diseases are available.
18. The frequency of genetic diseases approximately doubles in the offspring of first-cousin matings.
19. In each normal female somatic cell, one of the two X chromosomes is inactivated early in embryogenesis.
20. X inactivation is random, fixed, and incomplete (i.e., only part of the chromosome is actually inactivated). It may involve methylation.
21. Gender is determined embryonically by the presence of the SRY gene on the Y chromosome. Embryos that have a Y chromosome (and thus the SRY gene) become males, whereas those lacking the Y chromosome become females. When the Y chromosome lacks the SRY gene, an XY female can be produced. Similarly, an X chromosome that contains the SRY gene can produce an XX male.
22. X-linked genes are those that are located on the X chromosome. Nearly all known X-linked diseases are caused by X-linked recessive genes.
23. Males are hemizygous for genes on the X chromosome.
24. X-linked recessive diseases are seen much more often in males than in females because males need only one copy of the gene to express the disease.
25. Biologic fathers cannot pass X-linked genes to their sons.
26. Skipped generations often are seen in X-linked recessive disease pedigrees because the gene can be transmitted through carrier females.
27. Recurrence risks for X-linked recessive diseases depend on the carrier and affected status of the mother and father.
28. A sex-limited trait is one that occurs only in one sex (gender).
29. A sex-influenced trait is one that occurs more often in one sex than in the other.
Linkage Analysis and Gene Mapping
1. During meiosis I, crossover occurs and can cause recombinations of alleles located on the same chromosome.
2. The frequency of recombinations can be used to infer the map distance between loci on the same chromosome.
3. A marker locus, when closely linked to a disease-gene locus, can be used to predict whether an individual will develop a genetic disease.
4. The major goals of the Human Genome Project were to find the locations of all human genes (the “gene map”) and to determine the entire human DNA sequence. These goals have now been accomplished and the genes responsible for more than 4000 mendelian conditions have been identified.
Multifactorial Inheritance 1. Traits that result from the combined effects of several loci are polygenic. When environmental factors also influence the trait, it is multifactorial.
2. Many multifactorial traits have a threshold of liability. Once the threshold of liability has been crossed, the disease may be expressed.
3. Empirical risks, based on direct observation of large numbers of families, are used to estimate recurrence risks for multifactorial diseases.
4. Recurrence risks for multifactorial diseases become higher if more than one biologic family member is affected or if the expression of the disease in the proband is more severe.
5. Recurrence risks for multifactorial diseases decrease rapidly for more remote relatives.
Key Terms Adenine, 38
Allele, 49
Amino acid, 39
Aneuploid cell, 42
Anticodon, 41
Autosome, 42
Barr body, 54
Base pair substitution, 39
Carrier, 49
Carrier detection test, 54
Chromosomal mosaic, 46
Chromosome, 38
Chromosome band, 42
Chromosome breakage, 47
Chromosome theory of inheritance, 50
Clastogen, 47
Codominance, 49
Codon, 39
Complementary base pairing, 39
Consanguinity, 54
CpG islands, 52
Cri du chat syndrome, 48
Crossover, 56
Cytokinesis, 42
Cytosine, 38
Delayed age of onset, 51
Deletion, 48
Deoxyribonucleic acid (DNA), 38
Diploid cell, 42
DNA methylation, 52
DNA polymerase, 39
Dominant, 49
Dosage compensation, 54
Double-helix model, 38
Down syndrome, 46
Duplication, 48
Dystrophin, 55
Empirical risk, 58
Epigenetic, 52
Euploid cell, 42
Exon, 41
Expressivity, 51
Fragile site, 49
Frameshift mutation, 39
Gamete, 42
Gene, 38
Genomic imprinting, 52
Genotype, 49
Germline mosaicism, 51
Guanine, 38
Haploid cell, 42
Hemizygous, 54
Heterozygote, 49
Heterozygous, 49
Homologous, 42
Homozygote, 49
Homozygous, 49
Inbreeding, 54
Intron, 41
Inversion, 48
Karyotype (karyogram), 42
Klinefelter syndrome, 47
Linkage, 56
Locus, 49
Meiosis, 42
Messenger RNA (mRNA), 39
Metaphase spread, 42
Methylation, 52
Missense, 39
Mitosis, 42
Mode of inheritance, 49
Multifactorial inheritance, 58
Mutagen, 39
Mutation, 39
Mutational hot spot, 39
Nondisjunction, 45
Nonsense, 39
Nucleotide, 39
Obligate carrier, 51
Partial trisomy, 46
Pedigree, 50
Penetrance, 51
Phenotype, 49
Polygenic trait, 57
Polymorphic (polymorphism), 49
Polypeptide, 39
Polyploid cell, 42
Position effect, 48
Principle of independent assortment, 50
Principle of segregation, 50
Proband, 50
Promoter site, 39
Pseudoautosomal, 54
Purine, 38
Pyrimidine, 38
Recessive, 49
Reciprocal translocation, 48
Recombination, 56
Recurrence risk, 50
Ribonucleic acid (RNA), 39
Ribosomal RNA (rRNA), 41
Ribosome, 41
RNA polymerase, 39
Robertsonian translocation, 49
Sex-influenced trait, 55
Sex-limited trait, 55
Sex linked (inheritance), 54
Silent mutation, 39
Somatic cell, 42
Spontaneous mutation, 39
Template, 39
Termination sequence, 41
Tetraploidy, 42
Threshold of liability, 58
Thymine, 38
Transcription, 39
Transfer RNA (tRNA), 41
Translation, 41
Translocation, 48
Triploidy, 42
Trisomy, 42
Tumor-suppressor gene, 51
Turner syndrome, 47
X inactivation, 54
References 1. Jorde LB, et al. Medical genetics. ed 4. Mosby-Elsevier: St Louis; 2010. 2. Gardner RJM, et al. Chromosome abnormalities and genetic counseling. Oxford University Press: Oxford; 2012.
3. Nagaoka SI, et al. Human aneuploidy: mechanisms and new insights into an age-old problem. Nat Rev Genet. 2012;13(7):493–504.
4. Antonarakis SE, Epstein CJ. The challenge of Down syndrome. Trends Mol Med. 2006;12(10):473–479.
5. Gravholt CH. Sex chromosome abnormalities. Rimoin DL, Pyeritz RE, Korf BR. Emery and Rimoin's principles and practice of medical genetics. ed 6. Elsevier: Philadelphia; 2013.
6. Rooms L, Kooy RF. Advances in understanding fragile X syndrome and related disorders. Curr Opin Pediatr. 2011;23(6):601–606.
7. Nelson DL, et al. The unstable repeats—three evolving faces of neurological disease. Neuron. 2013;77(5):825–843.
8. Biesecker LG, Spinner NB. A genomic view of mosaicism and human disease. Nat Rev Genet. 2013;14(5):307–320.
9. Foulkes WD. Inherited susceptibility to common cancers. N Engl J Med. 2008;359(20):2143–2153.
10. Pasmant E, et al. Neurofibromatosis type 1: from genotype to phenotype. J Med Genet. 2012;49(8):483–489.
11. Fraga MF, et al. Epigenetic differences arise during the lifetime of monozygotic twins. Proc Natl Acad Sci U S A. 2005;102(30):10604–10609.
12. Lyon MF. X-chromosome inactivation. Curr Biol. 1999;9(7):R235–R237. 13. Lee JT, Bartolomei MS. X-inactivation, imprinting, and long noncoding
RNAs in health and disease. Cell. 2013;152(6):1308–1323. 14. Larney C, et al. Switching on sex: transcriptional regulation of the testis-
determining gene Sry. Development. 2014;141(11):2195–2205. 15. Flanigan KM. The muscular dystrophies. Semin Neurol. 2012;32(3):255–263. 16. Lander ES. Initial impact of the sequencing of the human genome. Nature.
2011;470(7333):187–197. 17. Yang Y, et al. Clinical whole-exome sequencing for the diagnosis of
mendelian disorders. N Engl J Med. 2013;369(16):1502–1511. 18. Koboldt DC, et al. The next-generation sequencing revolution and its impact
on genomics. Cell. 2013;155(1):27–38.
3
Epigenetics and Disease Diane P. Genereux, Lynn B. Jorde
CHAPTER OUTLINE
Epigenetic Mechanisms, 62
DNA Methylation, 62 Histone Modifications, 63 RNA-Based Mechanisms, 64
Epigenetics and Human Development, 64 Genomic Imprinting, 64
Prader-Willi and Angelman Syndromes, 65 Beckwith-Wiedemann Syndrome, 65 Russell-Silver Syndrome, 66
Long-Term and Multigenerational Persistence of Epigenetic States Induced by Stochastic and Environmental Factors, 66
Epigenetics and Nutrition, 66 Epigenetics and Maternal Care, 66 Epigenetics and Mental Illness, 67 Twin Studies Provide Insights on Epigenetic Modification, 68 Molecular Approaches to Understand Epigenetic Disease, 68
Epigenetics and Cancer, 68
DNA Methylation and Cancer, 68
miRNAs and Cancer, 69 Epigenetic Screening for Cancer, 69 Emerging Strategies for the Treatment of Epigenetic Disease, 69
Future Directions, 70
Human beings exhibit an impressive diversity of physical and behavioral features. Some of this diversity is attributable to genetic variation. Another contributor to human diversity is epigenetic (“upon genetic”) modification (a change in phenotype or gene expression that does not involve DNA mutation or changes in nucleotide sequence). Basically, epigenetics is the study of mechanisms that will switch genes “on,” such that they are expressed, and “off,” such that they are silenced. Epigenetic mechanisms include chemical modifications to DNA and associated histones, and the production of small RNA molecules. Gene regulation by epigenetic processes can occur at the level of either transcription or translation. Epigenetic modification is critical for fundamental processes of human development, including the differentiation of embryonic stem cells into specific cell types, and the inactivation of one of the two X chromosomes in each cell of a genetic female. Some genes are noted to be imprinted, a form of epigenetic regulation where the expression of a gene depends on whether it is inherited from the mother or the father.
Epigenetic Mechanisms A variety of diseases can result from abnormal epigenetic states. Metabolic disease can occur when there is aberrant expression of both copies of a locus that is typically imprinted. Environmental stressors can markedly increase the risk of aberrant epigenetic modification and are strongly associated with some cancers. It is because of their increasing clear role in a wide range of pathologies that abnormal epigenetic states are currently a focus of both preventative efforts and pharmaceutical intervention. Currently known epigenetic mechanisms include DNA methylation, histone modifications, and RNA-based mechanisms (Figure 3-1).
FIGURE 3-1 Three Types of Epigenetic Processes. Investigators are studying three epigenetic mechanisms: (1) DNA methylation, (2) histone modifications, and (3) RNA based-mechanisms.
See text for discussion.
DNA Methylation DNA methylation (see Figure 3-1) occurs through the attachment of a methyl group (CH3) to a cytosine. Dense DNA methylation can be thought of as “insulation” that renders genes silent by blocking access by transcription factors. Dense methylation is typically coincident with hypoacetylation (decrease of the functional group acetyl) of the histone proteins around which the DNA is wound (see Histone Modifications). Together, DNA methylation and histone hypoacetylation can render a gene transcriptionally silent, preventing production of the encoded protein. Methylated cytosines have been found to occur principally at cytosines that are followed by a guanine base (sometimes known as cytosines in “CpG dinucleotides”). In human embryonic stem cells, methylation also can occur at cytosines outside of the CpG context (see Figure 2-24). DNA methylation plays a prominent role in both human health and disease. For
example, in each cell of a normal human female, one of the two X chromosomes is silenced by dense methylation and associated molecular marks, whereas the other X chromosome is transcriptionally active and largely devoid of methylation. During early embryonic development, there is epigenetic inactivation of one of the two X chromosomes in each cell of a human female—either the X chromosome inherited from her mother or the X chromosome inherited from her father. The determination of which chromosome is to be silenced occurs at random and independently in each of the cells present at this stage of development; the silent state of that chromosome is inherited by all subsequent copies. If a woman's two X chromosomes carry different alleles at a given locus, random X inactivation can lead to somatic mosaicism, wherein the alleles active in two different cells can confer two very different traits. Striking examples include the patchy coloration of calico cats and anhidrotic ectodermal dysplasia, a condition characterized by patchy presence and absence of sweat glands in the skin of human females who have one X chromosome bearing a normal allele and one X chromosome bearing a mutant allele at the X-encoded locus. Because of the somatic mosaicism that arises through random inactivation of the X chromosome, females tend to have less severe phenotypes than do males for a variety of X-linked disorders, including color blindness and fragile X syndrome. Aberrant DNA methylation, either the presence of dense methylation where it is
typically absent or the absence of methylation where it is typically present, can lead to misregulation of tumor-suppressor genes and oncogenes. Abnormal DNA methylation states are a common feature of several human cancers, including those of the colon1-3 (see Figures 3-1 and 3-6 [p. 69]; also see Chapter 10).
Histone Modifications Histone modifications (see Figure 3-1) include histone acetylation (adding an acetyl group) and deacetylation (deletion of an acetyl group) to the end of a histone protein. Like DNA methylation, these changes can alter the expression state of chromatin. Histones are proteins that facilitate compaction of genomic DNA into the nucleus of a cell, much as a spool helps to organize a long piece of thread for storage in a small space. When the DNA of the human genome is wound around histones, it is only ≈1/40,000 as long as it would be in its uncondensed state. Chemical modification of histones in a region of DNA can either up-regulate or down-regulate nearby gene expression by increasing or decreasing the tightness of the interaction between DNA and histones, thus modulating the extent to which DNA is accessible to transcription factors. DNA in association with histones is referred to as “chromatin.” At any given time, various regions of chromatin are typically in one of two forms: euchromatin, an open state in which most or all nearby genes are transcriptionally active; or heterochromatin, a closed state in which most or all nearby genes are transcriptionally inactive. Chromatin structure plays a critical role in determining the developmental
potential of a given cell lineage, and can undergo dramatic changes during organismal development. For example, chromatin states differ substantially between embryonic stem cells, which are poised to give rise to all of the different cell types that make up an individual, and terminally differentiated cells, which are committed to a specific developmental path. The fraction of DNA that is in the heterochromatic state increases as cells differentiate, consistent with the reduction in the number of genes that are active as a cell lineage transitions from pluripotency to terminal differentiation.4 Mutations in genes that encode histone-modifying proteins have been implicated in congenital heart disease,5 for example, highlighting histone modification states as critical for normal development. In contrast to the vast majority of other cell types, including oocytes, sperm cells
express not histones but protamines, which are evolutionarily derived from histones.6 Protamines enable sperm DNA to wind into an even more compact state than does the histone-bound DNA in somatic cells. This tight compaction improves the hydrodynamic features of the sperm head, facilitating its movement toward the egg.
RNA-Based Mechanisms Noncoding RNAs (ncRNAs) and other RNA-based mechanisms (see Figure 3-1) play an important role in regulating a wide variety of cellular processes, including
RNA splicing and DNA replication. These ncRNAs have been likened to “sponges” in so far as they can “sop up” complementary RNAs, thus inhibiting their function (see, for example, www.ncbi.nlm.nih.gov/pmc/articles/PMC2957044/). Of particular relevance to gene regulation are the hairpin-shaped microRNAs (miRNAs), which are encoded by DNA sequences of approximately 22 nucleotides, typically within the introns (a segment of a DNA molecule that does not code for proteins) of genes or in noncoding DNA located between genes (see Chapter 2). In contrast to DNA methylation and histone modification, both of which principally affect gene expression at the level of transcription, miRNAs typically modulate the stability and translational efficiency of existing messenger RNAs (mRNAs) encoded at other loci. Interaction between miRNAs and mRNAs target for degradation is typically mediated by regions of partial sequence complementarity. As a result, miRNAs can at once be specific enough so that they do not bind to all of the mRNAs in a cell and general enough to regulate a large number of different mRNA sequences. miRNAs also directly modulate translation by impairing ribosomal function. miRNAs regulate diverse signaling pathways; those that stimulate cancer development and progression are called oncomirs. For example, miRNAs have been linked to carcinogenesis because they alter the activity of oncogenes and tumor-suppressor genes (see Chapter 10).
Epigenetics and Human Development Each of the cells in the very early embryo has the potential to give rise to a somatic cell of any type. These embryonic stem cells are therefore said to be totipotent (“possessing all powers”). A key process in early development then is the differential epigenetic modification of specific DNA nucleotide sequences in these embryonic stem cells, ultimately leading to the differential gene-expression profiles that characterize the various differentiated somatic cell types. These early modifications ensure that specific genes are expressed only in the cells and tissue types in which their gene products typically function (e.g., factor VIII expression primarily in hepatocytes, or dopamine receptor expression in neurons). Epigenetic modifications early in development also highlight a fundamental
feature of genetics as compared to epigenetic information: all of the cells in a given individual contain almost exactly the same genetic information. It is the epigenetic information eventually placed on top of these sequences that enables them to achieve the diverse functions of differentiated somatic cells. A small percentage of genes, termed housekeeping genes, are necessary for the function and maintenance of all cells. These genes escape epigenetic silencing and remain transcriptionally active in all or nearly all cells. Housekeeping genes include encoding histones, DNA and RNA polymerases, and ribosomal RNA genes. How do embryonic stem cells achieve epigenetic states typical of totipotency,
whereby they can give rise to all of the diverse cell types that make up a fully developed organism? One explanation is that early embryogenesis (approximately the 10 days just after fertilization) is characterized by rapid fluctuation in genome- wide DNA methylation densities. Fertilization triggers a global loss of DNA methylation at most loci in both the oocyte-contributed and the sperm-contributed genomes. This loss of methylation is accomplished in part by suppression of the DNA methyltransferases, the enzymes that add methyl groups to DNA. Methylation is not directly copied by the DNA replication process. Instead, immediately following replication, the methyltransferases read the pattern of methylation on the parent DNA strand and use that information to determine which daughter-strand cytosines should be methylated. As embryonic cell division proceeds in the absence of DNA methyltransferases, cell division continues, eventually yielding cells that have nearly all of their loci in unmethylated, transcriptionally active states. Around the time of implantation in the uterus, the DNA methyltransferases become active again, permitting establishment of the cell-lineage–specific marks required for the establishment of organ systems.
Genomic Imprinting A baby inherits two copies of each autosomal gene: one from its mother and one from its father. For a large subset of these genes, expression is biallelic, meaning that both the maternally and the paternally inherited copies contribute to offspring phenotype. For another, smaller subset of these genes, expression is stochastically monoallelic,7 meaning that the maternal copy is randomly chosen for inactivation in some somatic cells and the paternal copy is randomly chosen for inactivation in other somatic cells. For a third and smaller subset of autosomes (about 1%) either the maternal copy or the paternal copy is imprinted, meaning that either the copy inherited through the sperm or the copy inherited through the egg is inactivated and remains in this inactive state in all of the somatic cells of the individual. The subset of genes that are subject to imprinting is highly enriched for loci
relevant to organismal growth. The genetic conflict hypothesis7 was developed as a potential explanation for this pattern. Although both the mother and the father benefit genetically from the birth and survival of offspring, their interests are not entirely aligned. Because a mother makes a large physiologic investment in each child, it is in her evolutionary best interest to limit the flow of energetic resources to any given offspring so as to maintain her physiologic capacity to bear subsequent children. By contrast, except in cases of certain permanent, certain monogamy, it is in the best interest of the father for his child to extract maximal resources from its mother, as his own future fecundity, or fertility, is not contingent on the sustained fecundity of the mother. In general, imprinting of maternally inherited genes tends to reduce offspring size; imprinting of paternally inherited genes tends to increase offspring size. One hallmark of imprinting-associated disease is that the phenotype of affected individuals is critically dependent on whether the mutation is inherited from the mother or from the father. Some examples are included in the following syndromes.
Prader-Willi and Angelman Syndromes A well-known disease example of imprinting is associated with a deletion of about 4 million base (Mb) pairs of the long arm of chromosome 15. When this deletion is inherited from the father, the child manifests Prader-Willi syndrome, with features including short stature, hypotonia, small hands and feet, obesity, mild to moderate intellectual disability, and hypogonadism8 (Figure 3-2, A). The same 4-Mb deletion, when inherited from the mother, causes Angelman syndrome, which is characterized by severe intellectual disability, seizures, and an ataxic gait (Figure 3- 2, B).9 These diseases are each observed in about 1 of every 15,000 live births;
chromosome deletions are responsible for about 70% of cases of both diseases. The deletions that cause Prader-Willi and Angelman syndromes are indistinguishable at the DNA sequence level and affect the same group of genes.
FIGURE 3-2 Prader-Willi and Angelman Syndromes. A, A child with Prader-Willi syndrome (truncal obesity, small hands and feet, inverted V-shaped upper lip). B, A child with Angelman
syndrome (characteristic posture, ataxic gait, bouts of uncontrolled laughter). (From Jorde LB, Carey JC, Bamshad MJ: Medical genetics, ed 4, Philadelphia, 2010, Mosby.)
For several decades, it was unclear how the same deletion could produce such disparate results in different individuals. Further analysis showed that the 4-Mb deletion (the critical region) contains several genes that are normally transcribed only on the copy of chromosome 15 that is inherited from the father.10 These genes are transcriptionally inactive (imprinted) on the copy of chromosome 15 inherited from the mother. Similarly, other genes in the critical region are transcriptionally
active only on the chromosome copy inherited from the mother and are inactive on the chromosome inherited from the father. Thus, several genes in this region are normally active on only one chromosome copy (Figure 3-3). If the single active copy of one of these genes is lost because of a chromosome deletion, then no gene product is produced, resulting in disease.
FIGURE 3-3 Prader-Willi Syndrome Pedigrees. These pedigrees illustrate the inheritance patterns of Prader-Willi syndrome, which can be caused by a 4-Mb deletion of chromosome 15q when inherited from the father. In contrast, Angelman syndrome can be caused by the same deletion but only when it is inherited from the mother. The reason for this difference is that
different genes in this region are normally imprinted (inactivated) in the copies of 15q transmitted by the mother and the father. (From Jorde LB, Carey JC, Bamshad MJ: Medical genetics, ed 4, Philadelphia, 2010,
Mosby.)
Molecular analysis has revealed much about genes in this critical region of chromosome 15.10 The gene responsible for Angelman syndrome encodes a ligase involved in protein degradation during brain development (consistent with the mental retardation and ataxia observed in this disorder). In brain tissue, this gene is active only on the chromosome copy inherited from the mother. Consequently, a maternally transmitted deletion removes the single active copy of this gene. Several genes in the critical region are associated with Prader-Willi syndrome and they are transcribed only on the chromosome transmitted by the father. A paternally transmitted deletion removes the only active copies of these genes producing the features of Prader-Willi syndrome.
Beckwith-Wiedemann Syndrome Another well-known example of imprinting is Beckwith-Wiedemann syndrome, an overgrowth condition accompanied by an increased predisposition to cancer.
Beckwith-Wiedemann syndrome is usually identifiable at birth because of the presence of large size for gestational age, neonatal hypoglycemia, a large tongue, creases on the earlobe, and omphalocele (birth defect of infant intestines).11 Children with Beckwith-Wiedemann syndrome have an increased risk of developing Wilms tumor or hepatoblastoma. Both of these tumors can be treated effectively if they are detected early; thus screening at regular intervals is an important part of management. Some children with Beckwith-Wiedemann syndrome also develop asymmetric overgrowth of a limb or one side of the face or trunk (hemihyperplasia). As with Angelman syndrome, a minority of Beckwith-Wiedemann syndrome
cases (about 20% to 30%) are caused by the inheritance of two copies of a chromosome from the father and no copy of the chromosome from the mother (uniparental disomy, in this case affecting chromosome 11). Several genes on the short arm of chromosome 11 are imprinted on either the paternally or the maternally transmitted chromosome. These genes are found in two separate, differentially methylated regions (DMRs). In DMR1, the gene that encodes insulin- like growth factor 2 (IGF2) is inactive on the maternally transmitted chromosome but active on the paternally transmitted chromosome. Thus, a normal individual has only one active copy of IGF2. When two copies of the paternal chromosome are inherited (i.e., paternal uniparental disomy) or there is loss of imprinting on the maternal copy of IGF2, an active IGF2 gene is present in double dose. These changes produce increased levels of insulin-like growth factor 2 during fetal development, contributing to the overgrowth features of Beckwith-Wiedemann syndrome. Note that, in contrast to Prader-Willi and Angelman syndromes, which are produced by a missing gene product, Beckwith-Wiedemann syndrome is caused, in part, by overexpression of a gene product.
Russell-Silver Syndrome Russell-Silver syndrome is characterized by growth retardation, proportionate short stature, leg length discrepancy, and a small, triangular face. About one third of Russell-Silver syndrome cases are caused by imprinting abnormalities of chromosome 11p15.5 that lead to down-regulation of IGF2 and therefore diminished growth. Another 10% of cases of Russell-Silver syndrome are caused by maternal uniparental disomy. Thus, whereas up-regulation, or extra copies, of active IGF2 causes overgrowth in Beckwith-Wiedemann syndrome, down-regulation of IGF2 causes the diminished growth seen in Russell-Silver syndrome.
Quick Check 3-1
1. Define epigenetics.
2. What are the three kinds of epigenetic mechanisms?
3. What is meant by the genetic conflict hypothesis?
4. Compare and contrast the molecular and phenotypic features of Prader-Willi and Angelman syndromes.
Long-Term and Multigenerational Persistence of Epigenetic States Induced by Stochastic and Environmental Factors It is increasingly clear that imprinted genes are not the only loci for which epigenetic modifications persist over time. Conditions encountered in utero, during childhood, and even during adolescence or later can have long-term impacts on epigenetic states, sometimes with impacts that can be transmitted across generations. A few such examples are listed below.
Epigenetics and Nutrition During the winter of 1943, millions of people in urban areas of the Netherlands suffered starvation conditions as a result of a Nazi blockage that prevented shipments of food from agricultural areas. When researchers sought to investigate how exposure to famine in utero had affected individuals born in a historically prosperous country, they found individuals who suffered nutritional deprivation in utero were more likely to suffer from obesity and diabetes as adults than individuals in the Netherlands who had not experienced nutritional deprivation during gestation. There also seemed to be a transgenerational impact, in that the children of individuals who were in utero during the Dutch Hunger Winter were found to be significantly smaller than the children of those not affected by the blockade. Other data sets reveal elevated risk of cardiovascular and metabolic disease for offspring of individuals exposed during early development to fluctuations in agricultural yields.12 The specific molecular mechanisms that may mediate these apparent relationships
between nutritional deprivation and disease risk on one or more generations are largely unknown. From some animal models, it seems that the insulin-like growth factor 2 gene (IGF2) is a possible target of epigenetic modifications arising through nutritional deprivation. Exposure in utero and through lactation to some chemicals (including bisphenol A, a constituent of plastics sometimes used in food preparation and storage) seems to lead to epigenetic modifications similar to those that arise through nutritional deprivation in early life.13
Epigenetics and Maternal Care It is increasingly clear that parenting style can affect epigenetic states, and that this information can be transmitted from one generation to the next. Mice and other
rodents can exhibit two alternate styles of nursing behavior: frequent arched-back nursing with a high level of licking and grooming behavior, and an alternate style with infrequent arched-back nursing and much reduced licking and grooming behavior. In one especially compelling study,14 pups of mothers that engaged in frequent arched-backed nursing were found to have significantly lower methylation levels and higher transcription activity of a glucocorticoid receptor–encoding locus. Because the glucocorticoid receptor is involved in a pathway that intensifies fearfulness and response to stress, these findings suggest that alteration to methylation states could help explain the finding that exposure to stress early in life can modulate behavior in adulthood. These findings also highlight the concept that epigenetic processes can help store information about the environment, and that the relevant epigenetic modifications can modulate behavior later in life.
Epigenetics and Mental Illness Epigenetics and Ethanol Exposure During Gestation The impact of ethanol exposure in utero on skeletal and neural development was first reported in 197315 and led to broad awareness of fetal alcohol syndrome. It was not until recently, however, that population-based and molecular-level studies began to clarify the epigenetic signals that mediate these impacts. At first, researchers found alcohol exposure in utero can affect the DNA methylation states of various genomic elements but without specific emphasis on loci directly relevant to skeletal and neural development.11 More recently, it has been found that treating cultured neural stem cells with ethanol impairs their ability to differentiate to functional neurons; this impairment seems to be correlated with aberrant, dense methylation at loci that are active in normal neuronal tissue.16 One possible explanation for these effects is that ethanol exposure in utero modulates fetal expression of the DNA methyltransfereases.17
Epigenetic Disease in the Context of Genetic Abnormalities In some diseases, both genetic and epigenetic factors contribute to the origin of abnormal phenotypes. For example, several abnormal phenotypes can arise in individuals with mutations at the fragile X locus FMR1 (Figure 3-4, A). Some of these phenotypes arise in individuals for whom epigenetic changes are coincident with genetic changes. The most common genetic abnormality at FMR1 involves expansion in the number of cytosine-guanine (CG) dinucleotide repeats in the gene promoter. Females who have CG repeats in excess of the approximately 35 that are typical at this locus are at risk for fragile X–associated primary ovarian
insufficiency, characterized by an elevated risk of early menopause.18 Males with moderate expansions are at risk of fragile X tremor ataxia syndrome (FXTAS), characterized by a late-onset intention tremor.19 Both of these conditions seem to arise through accumulation of excess levels of FMR1 mRNAs in nuclear inclusion bodies.18,20 Individuals with 200 repeats are at risk of fragile X syndrome, characterized by reduced IQ and a set of behavioral abnormalities. Remarkably, although possession of a large CG repeat in the FMR1 promoter dramatically increases the probability that an individual will have fragile X syndrome, the disease can be present in males who have the large repeat but be absent in their brothers who have inherited an allele of very similar size.21 This can be explained, at least in part, by the observation that acquisition of methylation-based silencing at FMR1 is stochastic, meaning that the presence of a large repeat increases the probability of the dense promoter methylation that could lead to gene silencing, but does not guarantee it. It remains to be seen whether dietary or environmental features can modulate the probability that dense methylation at FMR1 will accrue in individuals with the full-mutation allele.
FIGURE 3-4 Comparing the Molecular Mechanisms of Fragile X and FSHD. A, FMR1 in normal, expanded permutation, and full-mutation states. B, DUX4 in normal and contracted states.
In another genetic-epigenetic disease, fascioscapulohumeral muscular dystrophy (FSHMD) (see Figure 3-4, B), the disease phenotype arises through loss of normal methylation rather than gain of abnormal methylation. Symptoms of the disease include adverse impacts on skeletal musculature. Though lifespan is not typically reduced by the disease, wheelchair use becomes necessary late in life for a subset of individuals. The primary genetic event in FSHMD is deletion of a nucleotide repeat in the DUX4 gene (see Figure 3-4, A). In normal individuals, the D4Z4 gene promoter has between 11 and 150 copies. This number is typically found to have been reduced by mutation in individuals with FSHMD, who usually have only 1 to 10 such repeats. In healthy individuals with a normal-sized allele, the D4Z4 promoter typically has dense methylation. In individuals with reduced copy-counts, the normally dense methylation is lost (see Figure 3-4, A).22 The disease allele typically also has fewer repressive histone marks than does the normal allele.23
Together, fragile X syndrome and FSHMD highlight that both abnormal gain and abnormal loss of epigenetic modifications can result in disease.
Twin Studies Provide Insights on Epigenetic Modification Identical (monozygotic) twin pairs, whose DNA sequences are essentially the same, offer a unique opportunity to isolate and examine the impacts of epigenetic modifications. A recent study found that as twins age, they exhibit increasingly substantial differences in methylation patterns of the DNA sequences of their somatic cells; these changes are often reflected in increasing numbers of phenotypic differences. Twins with significant lifestyle differences (e.g., smoking versus nonsmoking) tend to accumulate larger numbers of differences in their methylation patterns. These results, along with findings generated in animal studies, suggest that changes in epigenetic patterns may be an important part of the aging process24 (Figure 3-5).
FIGURE 3-5 Twins and Aging. A, Twins as babies look very much alike but, B, as adults, have slight differences in appearance, possibly because of epigenetics. (A, vgm/Shutterstock. B, Stacey
Bates/Shutterstock.)
Molecular Approaches to Understand Epigenetic Disease Because epigenetic information is not encoded by DNA molecules but instead by chemical modifications to those molecules, conventional sequencing approaches
are not sufficient to reveal epigenetic differences between normal individuals and those who have epigenetic modifications associated with disease. To collect information on DNA methylation states of individual nucleotides, DNA is typically subjected to bisulfite conversion before sequencing. Bisulfite treatment does not alter most nucleotides, including methylated cytosines, but deaminates unmethylated cytosines to uracil.25 Because uracil complements adenine, not guanine, methylated and unmethylated cytosines can be distinguished in resulting sequence data, so long as the genetic sequence is known. Histone modification states can be assayed through the use of antibodies specific for histones with various modifications.26
Quick Check 3-2
1. Evaluate the statement: “Epigenetic information is highly dynamic in early development.”
2. How does the epigenetic regulation of imprinted genes compare with that of the rest of the genome?
3. Compare and contrast the molecular mechanisms leading to FX syndrome and to FSHMD.
Epigenetics and Cancer DNA Methylation and Cancer Some of the most extensive evidence for the role of epigenetic modification in human disease comes from studies of cancer (Figure 3-6).27,28 Tumor cells typically exhibit genome-wide hypomethylation (decreased methylation), which can increase the activity of oncogenes (see Chapter 10). Hypomethylation increases as tumors progress from benign neoplasms to malignancy. In addition, the promoter regions of tumor-suppressor genes are often hypermethylated, which decreases their rate of transcription and their ability to inhibit tumor formation. Hypermethylation of the promoter region of the RB1 gene is often seen in retinoblastoma29; hypermethylation of the BRCA1 gene is seen in some cases of inherited breast cancer (Chapter 33).30
FIGURE 3-6 Global Epigenomic Alterations and Cancer. Oncogenesis often occurs through a combination of genetic mutations and epigenetic change. In cancer cells, the promoters of tumor-suppressor genes typically become hypermethylated, leading, in combination with
histone modifications, to abnormal gene silencing. Because tumor-suppressor genes typically help to control cell division, their silencing can result in tumor progression. Global
hypomethylation leads to chromosomal instability and fragility, and increases the risk of additional genetic mutations. Additionally, these modifications create abnormal mRNA and
miRNA expression, which leads to activation of oncogenes and silencing of tumor-suppressor genes. (Adapted from Sandoval J, Esteller M: Cancer epigenomics: beyond genomics, Curr Opin Genet Dev 22:50-55, 2012.)
A major cause of one form of inherited colon cancer (hereditary nonpolyposis colorectal cancer [HNPCC]) is the methylation of the promoter region of a gene, MLH1, whose protein product repairs damaged DNA. When MLH1 becomes inactive, DNA damage accumulates, eventually resulting in colon tumors31,32. Abnormal methylation of tumor-suppressor genes also is common in the progression of Barrett esophagus, a condition in which the lining of the esophagus is replaced by cells that have features associated with the lower intestinal tract, and to adenocarcinoma possibly through up-regulation of one of the enzymes that adds methyl groups to DNA.33
miRNAs and Cancer Hypermethylation also is seen in microRNA genes, which encode small (22 base pair) RNA molecules that bind to the ends of mRNAs, degrading them and preventing their translation. More than 1000 microRNA sequences have been identified in humans, and hypermethylation of specific subgroups of microRNAs is associated with tumorigenesis. When microRNA genes are methylated, their mRNA targets are overexpressed, and this overexpression has been associated with metastasis.27
Epigenetic Screening for Cancer The common finding of epigenetic alteration in cancerous tissue raises the possibility that epigenetic screening approaches could complement or even replace existing early-detection methods. In some cases, epigenetic screening could be done using bodily fluids, such as urine or sputum, eliminating the need for the more invasive, costly, and risky strategies currently in place. Monitoring for misregulation of miRNAs has shown promise as a tool for early diagnosis of cancers of the colon,34 breast,35 and prostate.36 Other epigenetics-based screening approaches have shown promise for detection of cancers of the bladder,37 lung,38 and prostate.39
Emerging Strategies for the Treatment of Epigenetic Disease Epigenetic modifications are potentially reversible: DNA can be demethylated, histones can be modified to change the transcriptional state of nearby DNA, and miRNA-encoding loci can be up-regulated or down-regulated. This raises the prospect for treating epigenetic disease with pharmaceutical agents that directly
reverse the changes associated with the disease phenotype. In recent years, interventions involving all three types of epigenetic modulators (DNA methylation, histone modification, and miRNAs) have shown considerable promise for the treatment of disease.
DNA Demethylating Agents 5-Azacytidine (Figure 3-7) has been used as a therapeutic drug in the treatment of leukemia and myelodysplastic syndrome.40 A cytosine analog, 5-azacytidine, is incorporated into DNA opposite its complementary nucleotide, guanine. 5- Azacytidine differs from cytosine in that it has a nitrogen, rather than a carbon, in the 5th position of its cytidine ring. As result, the DNMTs cannot add methyl groups to 5-azacytidine, and DNAs that contain 5-azacytidine decline in their methylation density over successive rounds of DNA replication.41 Administration of 5- azacytidine is associated with various side effects, including digestive disturbance, but has shown promise in the treatment of diseases, including pancreatic cancer42 and myelodysplastic syndromes.43,44
FIGURE 3-7 5-Azacytosine as Demethylating Agent. A, Unmethylated cytosines in DNA are typically subject to the addition of methyl groups by DNMT1, a DNA methyltransferase, using methyl groups supplied by the methyl donor S-adenosylmethionine. B, In 5-Azacytosine, the 5′ carbon of cytosine is replaced with a nitrogen. This chemical difference is sufficient both to
block the addition of a methyl group and to confer irreversible binding to DNMT1. Incorporation of 5-Azacytosine into DNA is therefore sufficient to drive passive loss of methylation from
replicating DNA, and thus to reactivate hypermethylated loci. 5-Azacytosine, bound to a sugar, can be integrated into DNA, and has been administered with some success in treating
epigenetic diseases that arise through hypermethylation of individual loci.
Histone Deacetylase Inhibitors The activity of the histone deacetylases (HDACs) increases chromatin compaction, decreasing transcriptional activity (see Figure 3-7). In many cases, excessive activity of HDACs results in transcriptional inactivation of tumor-suppressor genes, leading ultimately to the development of tumors. Treatment with HDAC inhibitors, either alone or in combination with other drugs, has shown promise in the treatment of cancers of the breast45 and prostate,46 but only very limited success in the treatment of pancreatic cancer.47
miRNA Coding A major challenge in developing drugs that modify epigenetic alterations is to target only the genes responsible for a specific cancer. Therapeutic approaches that use microRNA offer a potential solution to this problem as treatment can be targeted to individual loci using sequence characteristics of relevant RNA molecules.
Quick Check 3-3
1. Assess the statement that cancer is, in many cases, an epigenetic disease.
2. Discuss the role of miRNAs in cancer.
3. Describe a potential strategy for the treatment of epigenetic disease.
Future Directions Robust experimental observations are clarifying the roles of epigenetic states in determining cell fates and disease phenotypes. The well-documented involvement of epigenetic abnormalities in carcinogenesis and the mounting evidence for these epigenetic changes in other common diseases (discussed in other chapters) will likely elucidate possibilities for reversing the epigenetic abnormalities and possibly preventing their establishment in utero.
Did You Understand? Overview 1. Why are pairs of identical twins especially useful in the study of epigenetic phenomena?
2. Describe some of the challenges of developing pharmaceutical approaches to remedy abnormal epigenetic states.
Epigenetics and Human Development 1. Epigenetics modification alters gene expression without changes to DNA sequence.
2. Investigators are studying three major types of epigenetic processes: (1) DNA methylation, which results from attachment of a methyl group to a cytosine; in the somatic cells, all or nearly all methylation occurs at cytosines that are followed by guanines (“CpG dinucleotides”); (2) histone modification, through the addition of various chemical groups, including methyl and acetyl; and (3) noncoding RNAs (ncRNAs or miRNAs), short nucleotides derived from introns of protein coding genes or transcribed as independent genes from regions of the genome whose functions, if any, remain poorly understood. MicroRNAs regulate diverse signaling pathways.
3. DNA methylation is, at present, the best-studied epigenetic process. When a gene becomes heavily methylated the DNA is less likely to be transcribed into mRNA.
4. Methylation, along with histone hypoacetylation and condensation of chromatin, inhibits the binding of proteins that promote transcription, such that the gene becomes transcriptionally inactive.
5. Environmental factors, such as diet and exposure to certain chemicals, may cause epigenetic modifications.
6. The heritable transmission to future generations of epigenetic modifications is called transgenerational inheritance.
7. As twins age, they demonstrate increasing differences in methylation patterns of
their DNA sequences, causing increasing numbers of phenotypic differences.
8. In studies of twins with significant lifestyle differences (e.g., smoking versus nonsmoking) large numbers of differences in their methylation patterns are observed to accrue over time.
Genomic Imprinting 1. Gregor Mendel's experiments with garden peas demonstrated that the phenotype is the same whether a given allele is inherited from the mother or the father. This principle, which has long been part of the central dogma of genetics, does not always hold. For some human genes, a given gene is transcriptionally active on only one copy of a chromosome (e.g., the copy inherited from the father). On the other copy of the chromosome (the one inherited from the mother) the gene is transcriptionally inactive. This process of gene silencing, in which genes are silenced depending on which parent transmits them, is known as imprinting; the transcriptionally silenced genes are said to be “imprinted.”
2. When an allele is imprinted, it typically has heavy methylation. By contrast, the nonimprinted allele is typically not methylated.
3. A well-known disease example of imprinting is associated with a deletion of about 4 million base pairs (Mb) of the long arm of chromosome 15. When this deletion is inherited from the father, the child manifests Prader-Willi syndrome.
4. The same 4 Mb deletion, when inherited from the mother, causes Angelman syndrome.
5. Another well-known example of imprinting is Beckwith-Wiedemann syndrome, an overgrowth condition accompanied by an increased predisposition to cancer.
6. Whereas up-regulation, or extra copies, of active IGF2 causes overgrowth in Beckwith-Wiedemann syndrome, down-regulation of IGF2 causes the diminished growth seen in Russell-Silver syndrome.
Long-Term and Multigenerational Persistence of Epigenetic States Induced by Stochastic and Environmental Factors
1. Events encountered in utero, in childhood, and in adolescence can result in specific epigenetic changes that yield a wide range of phenotypic abnormalities, including metabolic syndromes.
2. Fetal alcohol syndrome, which results from ethanol exposure in utero, may be mediated by the repressive impact of ethanol on the DNA methyltransferases.
3. Both abnormal gain of methylation, as in the case of fragile X syndrome, and abnormal loss of methylation, as in the case of FSHMD, can produce disease phenotypes.
Epigenetics and Cancer 1. The best evidence for epigenetic effects on disease risk comes from studies of human cancer.
2. Methylation densities decline as tumors progress, which can increase the activity of oncogenes, causing tumors to progress from benign neoplasms to malignancy. Additionally, the promoter regions of tumor-suppressor genes are often hypermethylated. These elevated methylation levels decreases their rate of transcription at these critical genes, thus reducing the ability to inhibit tumor formation.
3. Hypermethylation also is seen in microRNA genes and is associated with tumorigenesis.
4. Unlike DNA sequence mutations, epigenetic modifications can be reversed through pharmaceutical intervention. For example, 5-azacytidine, a demethylating agent, has been used as a therapeutic drug in the treatment of leukemia and myelodysplastic syndrome.
Future Directions 1. Robust experimental observations are defining the roles of epigenetic states in shaping cell fates.
2. The well-documented involvement of epigenetic abnormalities in carcinogenesis and the mounting evidence for these epigenetic changes in other common diseases (discussed throughout the text) will likely elucidate new therapies with the
possibilities of reversing the epigenetic abnormalities.
Key Terms 5-Azacytidine, 70
Angelman syndrome, 65
Beckwith-Wiedemann syndrome, 65
Biallelic, 64
DNA methylation, 62
Embryonic stem cell, 64
Epigenetics, 62
Fascioscapulohumeral muscular dystrophy (FSHMD), 68
Fragile X, 67
Histone, 63
Histone modification, 63
Housekeeping genes, 64
Imprinted, 64
MicroRNA (miRNA), 64
Monoallelic, 64
Noncoding RNA (ncRNA), 64
Prader-Willi syndrome, 65
Russell-Silver syndrome, 66
References 1. King WD, et al. A cross-sectional study of global DNA methylation and risk of colorectal adenoma. BMC Cancer. 2014;14(1):488.
2. Dhimolea E, et al. Prenatal exposure to BPA alters the epigenome of the rat mammary gland and increases the propensity to neoplastic development. PLoS One. 2014;9(7):e99800.
3. Ashour N, et al. A DNA hypermethylation profile reveals new potential biomarkers for prostate cancer diagnosis and prognosis. Prostate. 2014;74(12):1171–1182.
4. Meshorer E, Misteli T. Chromatin in pluripotent embryonic stem cells and differentiation. Nat Rev Mol Cell Biol. 2006;7(7):540–546.
5. Zaidi S, et al. De novo mutations in histone-modifying genes in congenital heart disease. Nature. 2013;498(7453):220–223.
6. Balhorn R. The protamine family of sperm nuclear proteins. Genome Biol. 2007;8(9):227.
7. Deng O, et al. Single-cell RNA-seq reveals dynamic, random monoallelic gene expression in mammalian cells. Science. 2014;343(6167):193–196.
8. Cassidy SB, et al. Prader-Willi syndrome. Genet Med. 2012;14:10–26. 9. Williams CA, et al. Angelman syndrome 2005: updated consensus for diagnostic criteria. Am J Med Genet A. 2006;140:413–418.
10. Horsthemke B, Wagstaff J. Mechanisms of imprinting of the Prader- Willi/Angelman region. Am J Med Genet A. 2008;146A:2041–2052.
11. Kaminen-Ahola N, et al. Maternal ethanol consumption alters the epigenotype and the phenotype of offspring in a mouse model. PLoS Genet. 2010;6(1):e1000811.
12. Bygren LO, et al. Change in paternal grandmothers' early food supply influenced cardiovascular mortality of the female grandchildren. BMC Genet. 2014;15:12.
13. van Esterik JC, et al. Programming of metabolic effects in C57BL/6JxFVB mice by exposure to bisphenol A during gestation and lactation. Toxicology. 2014;321:40–52.
14. Weaver IC, et al. Epigenetic programming by maternal behavior. Nat Neurosci. 2004;7(8):847–854.
15. Jones KL, et al. Pattern of malformation in offspring of chronic alcoholic mothers. Lancet. 1973;1(7815):1267–1271.
16. Zhou FC, et al. Alcohol alters DNA methylation patterns and inhibits neural stem cell differentiation. Alcohol Clin Exp Res. 2011;35(4):735–746.
17. Mukhopadhyay P, et al. Alcohol modulates expression of DNA
methyltransferases and methyl CpG-/CpG domain-binding proteins in murine embryonic fibroblasts. Reprod Toxicol. 2013;37:40–48.
18. Lu C, et al. Fragile X premutation RNA is sufficient to cause primary ovarian insufficiency in mice. Hum Mol Genet. 2012;21(23):5039–5047.
19. Jacquemot S. Penetrance of the fragile X–associated tremor/ataxia syndrome in a premutation carrier population. J Am Med Assoc. 2004;291(4):460–469.
20. Tassone F, et al. Intranuclear inclusions in neural cells with premutation alleles in fragile X associated tremor/ataxia syndrome. J Med Genet. 2004;41(4):e43.
21. Stöger R, et al. Epigenetic variation illustrated by DNA methylation patterns of the fragile-X gene FMR1. Hum Mol Genet. 1997;6(11):1791–1801.
22. Cabianca DS, Gabellini D. The cell biology of disease: FSHD: copy number variations on the theme of muscular dystrophy. J Cell Biol. 2010;191(6):1049–1060.
23. Bodega B, et al. Remodeling of the chromatin structure of the facioscapulohumeral muscular dystrophy (FSHD) locus and upregulation of FSHD-related gene 1 (FRG1) expression during human myogenic differentiation. BMC Biol. 2009;7:41.
24. Fraga MF, et al. Epigenetic differences arise during the lifetime of monozygotic twins. Proc Natl Acad Sci U S A. 2005;102:10604–10609.
25. Frommer M, et al. A genomic sequencing protocol that yields a positive display of 5-methylcytosine residues in individual DNA strands. Proc Natl Acad Sci U S A. 1992;89(5):1827–1831.
26. Peters AH, et al. Partitioning and plasticity of repressive histone methylation states in mammalian chromatin. Mol Cell. 2003;12(6):1577–1589.
27. Esteller M. Epigenetics in cancer. N Engl J Med. 2008;358:1148–1159. 28. Sandoval J, Esteller M. Cancer epigenomics: beyond genomics. Curr Opin
Genet Dev. 2012;22:50–55. 29. Giacinti C, Giordano A. RB and cell cycle progression. Oncogene.
2006;25(38):5220–5227. 30. Hansmann T, et al. Constitutive promoter methylation of BRCA1 and
RAD51C in patients with familial ovarian cancer and early-onset sporadic breast cancer. Hum Mol Genet. 2012;21(21):4669–4679.
31. Lynch HT, de la Chapelle A. Hereditary colorectal cancer. N Engl J Med. 2003;348:919–932.
32. Pino MS, Chung DC. Microsatellite instability in the management of colorectal cancer. Expert Rev Gastroenterol Hepatol. 2011;5(3):385–399.
33. Hong J, et al. Role of NADPH oxidase NOX5-S, NF-κB, and DNMT1 in
acid-induced p16 hypermethylation in Barrett's cells. Am J Physiol Cell Physiol. 2013;305(10):C1069–C1079.
34. Tao K, et al. Prognostic value of miR-221-3p, miR-342-3p and miR-491-5p expression in colon cancer. Am J Transl Res. 2014;6(4):391–401.
35. Ahmad A, et al. Up-regulation of microRNA-10b is associated with the development of breast cancer brain metastasis. Am J Transl Res. 2014;6(4):384–390.
36. Ren Q, et al. Epithelial and stromal expression of miRNAs during prostate cancer progression. Am J Transl Res. 2014;6(4):329–339.
37. Dulaimi E, et al. Detection of bladder cancer in urine by a tumor suppressor gene hypermethylation panel. Clin Cancer Res. 2004;10(6):1887–1893.
38. Guzmán L, et al. Analysis of aberrant methylation on promoter sequences of tumor suppressor genes and total DNA in sputum samples: a promising tool for early detection of COPD and lung cancer in smokers. Diagn Pathol. 2012;7:87.
39. Henrique R, Jerónimo C. Molecular detection of prostate cancer: a role for GSTP1 hypermethylation. Eur Urol. 2004;46(5):660–669 [discussion 669].
40. Di Costanzo A, et al. Epigenetic drugs against cancer: an evolving landscape. Toxicology. 2014;88(9):1651–1668.
41. Christman JK. 5-Azacytidine and 5-aza-2′-deoxycytidine as inhibitors of DNA methylation: mechanistic studies and their implications for cancer therapy. Oncogene. 2002;21(35):5483–5495.
42. Zhang H, et al. 5-Azacytidine suppresses the proliferation of pancreatic cancer cells by inhibiting the Wnt/β-catenin signaling pathway. Genet Mol Res. 2014;13(3):5064–5072.
43. Jabbour E, Garcia-Manero G. Deacetylase inhibitors for the treatment of myelodysplastic syndromes. Leuk Lymphoma. 2015 Feb 24;1–8 [Epub ahead of print].
44. Müller-Thomas C, et al. Response to azacitidine is independent of p53 expression in higher-risk myelodysplastic syndromes and secondary acute myeloid leukemia. Haematologica. 2014;99(10):e179–e181.
45. Tate CR, et al. Targeting triple-negative breast cancer cells with the histone deacetylase inhibitor panobinostat. Breast Cancer Res. 2012;14(3):R79.
46. Chen CS, et al. Histone deacetylase inhibitors sensitize prostate cancer cells to agents that produce DNA double-strand breaks by targeting Ku70 acetylation. Cancer Res. 2007;67(11):5318–5327.
47. Koutsounas I, et al. Histone deacetylase inhibitors and pancreatic cancer: are there any promising clinical trials? World J Gastroenterol. 2013;19(8):1173–1181.
4
Altered Cellular and Tissue Biology Kathryn L. McCance, Todd Cameron Grey
CHAPTER OUTLINE
Cellular Adaptation, 74
Atrophy, 74 Hypertrophy, 75 Hyperplasia, 76 Dysplasia: Not a True Adaptive Change, 77 Metaplasia, 77
Cellular Injury, 77
General Mechanisms of Cell Injury, 78 Hypoxic Injury, 78 Free Radicals and Reactive Oxygen Species— Oxidative Stress, 81 Chemical or Toxic Injury, 84 Unintentional and Intentional Injuries, 93 Infectious Injury, 96 Immunologic and Inflammatory Injury, 96
Manifestations of Cellular Injury: Accumulations, 96
Water, 97 Lipids and Carbohydrates, 98 Glycogen, 98
Proteins, 98 Pigments, 99 Calcium, 100 Urate, 101 Systemic Manifestations, 101
Cellular Death, 101
Necrosis, 102 Apoptosis, 104 Autophagy, 105
Aging and Altered Cellular and Tissue Biology, 107
Normal Life Span, Life Expectancy, and Quality- Adjusted Life Year, 108 Degenerative Extracellular Changes, 108 Cellular Aging, 108 Tissue and Systemic Aging, 109 Frailty, 109
Somatic Death, 109
The majority of diseases are caused by many factors acting together (i.e., multifactorial) or interacting with a genetically susceptible person. Injury to cells and their surrounding environment, called the extracellular matrix, leads to tissue and organ injury. Although the normal cell is restricted by a narrow range of structure and functions, including metabolism and specialization, it can adapt to physiologic demands or stress to maintain a steady state called homeostasis. Adaptation is a reversible, structural, or functional response both to normal or physiologic conditions and to adverse or pathologic conditions. For example, the uterus adapts to pregnancy—a normal physiologic state—by enlarging. Enlargement occurs because of an increase in the size and number of uterine cells. In an adverse condition, such as high blood pressure, myocardial cells are stimulated to enlarge by the increased work of pumping. Like most of the body's adaptive mechanisms, however, cellular adaptations to adverse conditions are usually only temporarily successful. Severe or long-term stressors overwhelm adaptive processes and cellular injury or death ensues. Altered cellular and tissue biology can result from adaptation, injury, neoplasia, accumulations, aging, or death. (Neoplasia is discussed in Chapters 10 and 11.) Knowledge of the structural and functional reactions of cells and tissues to
injurious agents, including genetic defects, is vital to understanding disease processes. Cellular injury can be caused by any factor that disrupts cellular structures or deprives the cell of oxygen and nutrients required for survival. Injury may be reversible (sublethal) or irreversible (lethal) and is classified broadly as chemical, hypoxic (lack of sufficient oxygen), free radical, intentional, unintentional, immunologic, infection, and inflammatory. Cellular injuries from various causes have different clinical and pathophysiologic manifestations. Stresses from metabolic derangements may be associated with intracellular accumulations and include carbohydrates, proteins, and lipids. Sites of cellular death can cause accumulations of calcium resulting in pathologic calcification. Cellular death is
confirmed by structural changes seen when cells are stained and examined under a microscope. The two main types of cell death include necrosis and apoptosis and nutrient deprivation can initiate autophagy that results in cell death. All of these pathways of cellular death are discussed later in this chapter. Cellular aging causes structural and functional changes that eventually may lead
to cellular death or a decreased capacity to recover from injury. Mechanisms explaining how and why cells age are not known, and distinguishing between pathologic changes and physiologic changes that occur with aging is often difficult. Aging clearly causes alterations in cellular structure and function, yet senescence, growing old, is both inevitable and normal.
Cellular Adaptation Cells adapt to their environment to escape and protect themselves from injury. An adapted cell is neither normal nor injured—its condition lies somewhere between these two states. Adaptations are reversible changes in cell size, number, phenotype, metabolic activity, or functions of cells.1 Adaptive responses have limits, however, and additional cell stresses can affect essential cell function leading to cell injury. Cellular adaptations also can be a common and central part of many disease states. In the early stages of a successful adaptive response, cells may have enhanced function; thus, it is hard to distinguish a pathologic response from an extreme adaptation to an excessive functional demand. The most significant adaptive changes in cells include atrophy (decrease in cell size), hypertrophy (increase in cell size), hyperplasia (increase in cell number), and metaplasia (reversible replacement of one mature cell type by another less mature cell type or a change in the phenotype). Dysplasia (deranged cellular growth) is not considered a true cellular adaptation but rather an atypical hyperplasia. These changes are shown in Figure 4-1.
FIGURE 4-1 Adaptive and Dysplastic Alterations in Simple Cuboidal Epithelial Cells.
Atrophy Atrophy is a decrease or shrinkage in cellular size. If atrophy occurs in a sufficient number of an organ's cells, the entire organ shrinks or becomes atrophic. Atrophy can affect any organ, but it is most common in skeletal muscle, the heart, secondary sex organs, and the brain. Atrophy can be classified as physiologic or pathologic. Physiologic atrophy occurs with early development. For example, the thymus gland undergoes physiologic atrophy during childhood. Pathologic atrophy
occurs as a result of decreases in workload, pressure, use, blood supply, nutrition, hormonal stimulation, and nervous system stimulation (Figure 4-2). Individuals immobilized in bed for a prolonged time exhibit a type of skeletal muscle atrophy called disuse atrophy. Aging causes brain cells to become atrophic and endocrine- dependent organs, such as the gonads, to shrink as hormonal stimulation decreases. Whether atrophy is caused by normal physiologic conditions or by pathologic conditions, atrophic cells exhibit the same basic changes.
FIGURE 4-2 Atrophy. A, Normal brain of a young adult. B, Atrophy of the brain in an 82-year-old male with atherosclerotic cerebrovascular disease, resulting in reduced blood supply. Note that
loss of brain substance narrows the gyri and widens the sulci. The meninges have been stripped from the right half of each specimen to reveal the surface of the brain. (From Kumar V et al,
editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
The atrophic muscle cell contains less endoplasmic reticulum (ER) and fewer mitochondria and myofilaments (part of the muscle fiber that controls contraction) than found in the normal cell. In muscular atrophy caused by nerve loss, oxygen consumption and amino acid uptake are immediately reduced. The mechanisms of atrophy include decreased protein synthesis, increased protein catabolism, or both. A new hypothesis includes ribosome function and its role as translation machinery or the conversion of mRNA into protein called ribosome biogenesis. Ribosome biogenesis has an important role in the regulation of skeletal muscle mass.2 The primary pathway of protein catabolism is the ubiquitin-proteasome pathway and catabolism involves proteasomes (protein-degrading complexes. Proteins degraded in this pathway are first conjugated to ubiquitin (another small protein) and then
degraded by proteasomes. An increase in proteasome activity is characteristic of atrophic muscle changes. Deregulation of this pathway often leads to abnormal cell growth and is associated with cancer and other diseases. Atrophy as a result of chronic malnutrition is often accompanied by a “self-
eating” process called autophagy that creates autophagic vacuoles (see p. 105). These vacuoles are membrane-bound vesicles within the cell that contain cellular debris and hydrolytic enzymes, which function to break down substances to the simplest units of fat, carbohydrate, or protein. The levels of hydrolytic enzymes rise rapidly in atrophy. The enzymes are isolated in autophagic vacuoles to prevent uncontrolled cellular destruction. Thus the vacuoles form as needed to protect uninjured organelles from the injured organelles and are eventually engulfed and destroyed by lysosomes. Certain contents of the autophagic vacuole may resist destruction by lysosomal enzymes and persist in membrane-bound residual bodies. An example of this is granules that contain lipofuscin, the yellow-brown age pigment. Lipofuscin accumulates primarily in liver cells, myocardial cells, and atrophic cells.
Hypertrophy Hypertrophy is a compensatory increase in the size of cells in response to mechanical stimuli (also called mechanical load or stress, such as from repetitive stretching, chronic pressure, or volume overload) and consequently increases the size of the affected organ (Figures 4-3 and 4-4). The cells of the heart and kidneys are particularly prone to enlargement. Hypertrophy, as an adaptive response (muscular enlargement), occurs in the striated muscle cells of both the heart and skeletal muscles. Initial cardiac enlargement is caused by dilation of the cardiac chambers, is short lived, and is followed by increased synthesis of cardiac muscle proteins, allowing muscle fibers to do more work. The increase in cellular size is associated with an increased accumulation of protein in the cellular components (plasma membrane, ER, myofilaments, mitochondria) and not with an increase in cellular fluid. Yet, individual protein pools may expand or shrink.3 Cardiac hypertrophy involves changes in signaling and transcription factor pathways resulting in increased protein synthesis leading to left ventricular hypertrophy (LVH). Emerging evidence suggests that the ubiquitin-proteasome system (UPS) not only attends to damaged, misfolded, or mutant proteins by protein breakdown but also may attend to cell growth eventually leading to LVH.4 With time, cardiac hypertrophy is characterized by extracellular matrix remodeling and increased growth of adult myocytes. The myocytes progressively increase in size and reach a limit beyond which no further hypertrophy can occur.5,6
FIGURE 4-3 Hypertrophy of Cardiac Muscle in Response to Valve Disease. A, Transverse slices of a normal heart and a heart with hypertrophy of the left ventricle (L, normal thickness of left ventricular wall; T, thickened wall from heart in which severe narrowing of aortic valve caused resistance to systolic ventricular emptying). B, Histology of cardiac muscle from the normal heart. C, Histology of cardiac muscle from a hypertrophied heart. (From Stevens A, Lowe J: Pathology:
illustrated review in color, ed 2, Edinburgh, 2000, Mosby.)
FIGURE 4-4 Mechanisms of Myocardial Hypertrophy. Mechanical sensors appear to be the main stimulators for physiologic hypertrophy. Other stimuli possibly more important for
pathologic hypertrophy include agonists (initiators) and growth factors. These factors then signal transcription pathways whereby transcription factors then bind to DNA sequences, activating muscle proteins that are responsible for hypertrophy. These pathways include
induction of embryonic/fetal genes, increased synthesis of contractile proteins, and production of growth factors. (Adapted from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
Although hypertrophy can be classified as physiologic or pathologic, time may be the critical factor or determinant of the transition from physiologic to pathologic cardiac hypertrophy. With physiologic hypertrophy, preservation of myocardial structure characterizes postnatal development, moderate endurance exercise training, pregnancy, and the early phases of increased pressure and volume loading on the adult human heart. This physiologic response is temporary; however, aging, strenuous exercise, and sustained workload or stress lead to pathologic hypertrophy with structural and functional manifestations. Pathologic hypertrophy in the heart is secondary to hypertension, coronary heart disease, or problem valves and is presumably a key risk factor for heart failure. Additionally, it is associated with increased interstitial fibrosis, cell death, and abnormal cardiac function (see Figure 4-3). Historically, the progression of pathologic cardiac hypertrophy has been considered irreversible. Emerging data, however, from experimental studies and clinical observations show in certain cases reversal of pathologic cardiac hypertrophy. Cardiac hypertrophy can be reversed when the increased wall stress is
normalized, a process termed regression.7 For example, unloading of hemodynamic stress by a left ventricular assist device (used in individuals with heart failure for bridging to heart transplantation) induces regression of cardiac hypertrophy and improvement of left ventricular (LV) function in those with end-stage heart failure.8 Regression of cardiac hypertrophy is accompanied by activation of unique sets of genes, including fetal-type genes and those involved in protein degradation.9,10 However, the signaling mechanisms mediating regression of cardiac hypertrophy have been poorly understood. Improvement in new blood vessel development (angiogenesis) in the hypertrophic heart can lead to regression of the hypertrophy and prevention of heart failure.11,12 In mice, dietary supplementation of physiologically relevant levels of copper can reverse pathologic cardiac hypertrophy.12,13 When a diseased kidney is removed, the remaining kidney adapts to the increased
workload with an increase in both the size and the number of cells. The major contributing factor to this renal enlargement is hypertrophy. Another example of normal or physiologic hypertrophy is the increased growth of the uterus and mammary glands in response to pregnancy.
Hyperplasia Hyperplasia is an increase in the number of cells, resulting from an increased rate of cellular division. Hyperplasia, as a response to injury, occurs when the injury has been severe and prolonged enough to have caused cell death. Loss of epithelial cells and cells of the liver and kidney triggers deoxyribonucleic acid (DNA) synthesis and mitotic division. Increased cell growth is a multistep process involving the production of growth factors, which stimulate the remaining cells to synthesize new cell components and, ultimately, to divide. Hyperplasia and hypertrophy often occur together, and both take place if the cells can synthesize DNA. Two types of normal, or physiologic, hyperplasia are compensatory hyperplasia
and hormonal hyperplasia. Compensatory hyperplasia is an adaptive mechanism that enables certain organs to regenerate. For example, removal of part of the liver leads to hyperplasia of the remaining liver cells (hepatocytes) to compensate for the loss. Even with removal of 70% of the liver, regeneration is complete in about 2 weeks. Several growth factors and cytokines (chemical messengers) are induced and play critical roles in liver regeneration. Not all types of mature cells have the same capacity for compensatory
hyperplastic growth. Nondividing tissues contain cells that can no longer (i.e., postnatally) go through the cell cycle and undergo mitotic division. These highly specialized cells, for example, neurons and skeletal muscle cells, never divide again
once they have differentiated—that is, they are terminally differentiated.14 In human cells, cell growth and cell division depend on signals from other cells; but cell growth, unlike cell division, does not depend on the cell-cycle control system.14 Nerve cells and most muscle cells do most of their growing after they have terminally differentiated and permanently ceased dividing.14 Significant compensatory hyperplasia occurs in epidermal and intestinal epithelia, hepatocytes, bone marrow cells, and fibroblasts; and some hyperplasia is noted in bone, cartilage, and smooth muscle cells. Another example of compensatory hyperplasia is the callus, or thickening, of the skin as a result of hyperplasia of epidermal cells in response to a mechanical stimulus. Hormonal hyperplasia occurs chiefly in estrogen-dependent organs, such as the
uterus and breast. After ovulation, for example, estrogen stimulates the endometrium to grow and thicken in preparation for receiving the fertilized ovum. If pregnancy occurs, hormonal hyperplasia, as well as hypertrophy, enables the uterus to enlarge. (Hormone function is described in Chapters 19 and 33.) Pathologic hyperplasia is the abnormal proliferation of normal cells, usually in
response to excessive hormonal stimulation or growth factors on target cells (Figure 4-5). The most common example is pathologic hyperplasia of the endometrium (caused by an imbalance between estrogen and progesterone secretion, with oversecretion of estrogen) (see Chapter 33). Pathologic endometrial hyperplasia, which causes excessive menstrual bleeding, is under the influence of regular growth-inhibition controls. If these controls fail, hyperplastic endometrial cells can undergo malignant transformation. Benign prostatic hyperplasia is another example of pathologic hyperplasia and results from changes in hormone balance. In both of these examples, if the hormonal imbalance is corrected, hyperplasia regresses.1
FIGURE 4-5 Hyperplasia of the Prostate with Secondary Thickening of the Obstructed Urinary Bladder (Bladder Cross Section). The enlarged prostate is seen protruding into the lumen of the
bladder, which appears trabeculated. These “trabeculae” result from hypertrophy and hyperplasia of smooth muscle cells that occur in response to increased intravesical pressure caused by urinary obstruction. (From Damjanov I: Pathology for the health professions, ed 4, St Louis, 2012, Saunders.)
Dysplasia: Not a True Adaptive Change Dysplasia refers to abnormal changes in the size, shape, and organization of mature cells (Figure 4-6). Dysplasia is not considered a true adaptive process but is related to hyperplasia and is often called atypical hyperplasia. Dysplastic changes often are encountered in epithelial tissue of the cervix and respiratory tract, where they are strongly associated with common neoplastic growths and often are found adjacent to cancerous cells. Importantly, however, the term dysplasia does not indicate cancer and may not progress to cancer. Dysplasia is often classified as mild, moderate, or severe; yet, because this classification scheme is somewhat subjective, it has prompted some to recommend the use of either “low grade” or “high grade” instead. If the inciting stimulus is removed, dysplastic changes often are reversible. (Dysplasia is discussed further in Chapter 10.)
FIGURE 4-6 Dysplasia of the Uterine Cervix. A, Mild dysplasia. B, Severe dysplasia. (From Damjanov I, Linder J: Anderson's pathology, ed 10, St Louis, 1996, Mosby.)
Metaplasia Metaplasia is the reversible replacement of one mature cell type (epithelial or mesenchymal) by another, sometimes less differentiated, cell type. It is thought to develop, as an adaptive response better suited to withstand the adverse environment, from a reprogramming of stem cells that exist on most epithelia or of undifferentiated mesenchymal (tissue from embryonic mesoderm) cells present in connective tissue. These precursor cells mature along a new pathway because of
signals generated by growth factors in the cell's environment. The best example of metaplasia is replacement of normal columnar ciliated epithelial cells of the bronchial (airway) lining by stratified squamous epithelial cells (Figure 4-7). The newly formed cells do not secrete mucus or have cilia, causing loss of a vital protective mechanism. Bronchial metaplasia can be reversed if the inducing stimulus, usually cigarette smoking, is removed. With prolonged exposure to the inducing stimulus, however, dysplasia and cancerous transformation can occur.
FIGURE 4-7 Reversible Changes in Cells Lining the Bronchi.
Cellular Injury Injury to cells and to the extracellular matrix (ECM) leads to injury of tissues and organs, ultimately determining the structural patterns of disease. Loss of function is derived from cell and ECM injury and cell death. Cellular injury occurs if the cell is unable to maintain homeostasis—a normal or adaptive steady state—in the face of injurious stimuli or stress. Injured cells may recover (reversible injury) or die (irreversible injury). Injurious stimuli include chemical agents, lack of sufficient oxygen (hypoxia), free radicals, infectious agents, physical and mechanical factors, immunologic reactions, genetic factors, and nutritional imbalances. Types of injuries and their responses are summarized in Table 4-1 and Figure 4-8.
TABLE 4-1 Types of Progressive Cell Injury and Responses
Type Responses Adaptation Atrophy, hypertrophy, hyperplasia, metaplasia Active cell injury Immediate response of “entire” cell Reversible Loss of ATP, cellular swelling, detachment of ribosomes, autophagy of lysosomes Irreversible “Point of no return” structurally when severe vacuolization of mitochondria occurs and Ca++ moves into cell Necrosis Common type of cell death with severe cell swelling and breakdown of organelles Apoptosis, or programmed cell death Cellular self-destruction for elimination of unwanted cell populations Autophagy Eating of self, cytoplasmic vesicles engulf cytoplasm and organelles, recycling factory Chronic cell injury (subcellular alterations) Persistent stimuli response may involve only specific organelles or cytoskeleton (e.g., phagocytosis of bacteria) Accumulations or infiltrations Water, pigments, lipids, glycogen, proteins Pathologic calcification Dystrophic and metastatic calcification
ATP, Adenosine triphosphate; Ca++, calcium.
FIGURE 4-8 Stages of Cellular Adaptation, Injury, and Death. The normal cell responds to physiologic and pathologic stresses by adapting (atrophy, hypertrophy, hyperplasia, metaplasia). Cell injury occurs if the adaptive responses are exceeded or compromised by injurious agents,
stress, and mutations. The injury is reversible if it is mild or transient, but if the stimulus persists the cell suffers irreversible injury and eventually death.
The extent of cellular injury depends on the type, state (including level of cell differentiation and increased susceptibility to fully differentiated cells), and adaptive processes of the cell, as well as the type, severity, and duration of the injurious stimulus. Two individuals exposed to an identical stimulus may incur varying degrees of cellular injury. Modifying factors, such as nutritional status, can profoundly influence the extent of injury. The precise “point of no return” that leads to cellular death is a biochemical puzzle, but once changes to the nucleus occur and cell membranes are disrupted, the cell moves to irreversible injury and death.
General Mechanisms of Cell Injury Common biochemical themes are important to understanding cell injury and cell death regardless of the injuring agent. These include adenosine triphosphate (ATP) depletion, mitochondrial damage, oxygen and oxygen-derived free radical membrane damage (depletion of ATP), protein folding defects, DNA damage defects, and calcium level alterations (Table 4-2). Examples of common forms of
cell injury are (1) hypoxic injury, (2) free radicals and reactive oxygen species injury, and (3) chemical injury.
TABLE 4-2 Common Themes in Cell Injury and Cell Death
Theme Comments ATP depletion Loss of mitochondrial ATP and decreased ATP synthesis; results include cellular swelling, decreased protein synthesis, decreased membrane
transport, and lipogenesis, all changes that contribute to loss of integrity of plasma membrane Reactive oxygen species (↑ROS)
Lack of oxygen is key in progression of cell injury in ischemia (reduced blood supply); activated oxygen species (ROS, , H 2
O 2
, OH•)
cause destruction of cell membranes and cell structure Ca++ entry Normally intracellular cytosolic calcium concentrations are very low; ischemia and certain chemicals cause an increase in cytosolic Ca++
concentrations; sustained levels of Ca++ continue to increase with damage to plasma membrane; Ca++ causes intracellular damage by activating a number of enzymes
Mitochondrial damage
Can be damaged by increases in cytosolic Ca++, ROS; two outcomes of mitochondrial damage are loss of membrane potential, which causes depletion of ATP and eventual death or necrosis of cell, and activation of another type of cell death (apoptosis) (see p. 104)
Membrane damage
Early loss of selective membrane permeability found in all forms of cell injury, lysosomal membrane damage with release of enzymes causing cellular digestion
Protein misfolding, DNA damage
Proteins may misfold, triggering unfolded protein response that activates corrective responses; if overwhelmed, response activates cell suicide program or apoptosis; DNA damage (genotoxic stress) also can activate apoptosis (see p. 104)
ATP, Adenosine triphosphate; Ca++, calcium.
Hypoxic Injury Hypoxia, or lack of sufficient oxygen within cells, is the single most common cause of cellular injury (Figure 4-9). Hypoxia can result from a reduced amount of oxygen in the air, loss of hemoglobin or decreased efficacy of hemoglobin, decreased production of red blood cells, diseases of the respiratory and cardiovascular systems, and poisoning of the oxidative enzymes (cytochromes) within the cells. Hypoxia plays a role in physiologic processes including cell differentiation, angiogenesis, proliferation, erythropoiesis, and overall cell viability.15 The main consumers of oxygen are mitochondria and the cellular responses to hypoxia are reported to be mediated by the production of reactive oxygen species (ROS) at the mitochondrial complex III.15 Investigators are studying the role of ROS as hypoxia signaling molecules. More commonly, hypoxia is associated with the pathophysiologic conditions such as inflammation, ischemia, and cancer. Hypoxia can induce inflammation and inflamed lesions can become hypoxic (Figure 4-10).16 The cellular mechanisms involved in hypoxia and inflammation are emerging and include activation of immune responses and oxygen-sensing compounds called prolyl hydroxylases (PHDs) and hypoxia- inducible transcription factor (HIF). The hypoxia-inducible factor (HIF) is a family of transcription regulators that coordinate the expression of many genes in response to oxygen deprivation. Mammalian development occurs in a hypoxic
environment.17 Hypoxia-induced signaling involves complicated crosstalk between hypoxia and inflammation, linking hypoxia and inflammation to inflammatory bowel disease, certain cancers, and infections.16 Research is ongoing to understand the mechanisms of how tumors adapt to low oxygen levels by inducing angiogenesis, increasing glucose consumption, and promoting the metabolic state of glycolysis.18
FIGURE 4-9 Hypoxic Injury Induced by Ischemia. A, Consequences of decreased oxygen delivery or ischemia with decreased ATP. The structural and physiologic changes are reversible
if oxygen is delivered quickly. Significant decreases in ATP result in cell death, mostly by necrosis. B, Mitochondrial damage can result in changes in membrane permeability, loss of
membrane potential, and decrease in ATP concentration. Between the outer and inner membranes of the mitochondria are proteins that can activate the cell's suicide pathways,
called apoptosis. C, Calcium ions are critical mediators of cell injury. Calcium ions are usually maintained at low concentrations in the cell's cytoplasm; thus ischemia and certain toxins can initially cause an increase in the release of Ca++ from intracellular stores and later an increased movement (influx) across the plasma membrane. (Adapted from Kumar V et al, editors: Pathology, St Louis, 2014,
Elsevier.)
FIGURE 4-10 Hypoxia and Inflammation. Shown is a simplified drawing of clinical conditions characterized by tissue hypoxia that causes inflammatory changes (left) and inflammatory
diseases that ultimately lead to hypoxia (right). These diseases and conditions are discussed in more detail in their respective chapters. (Adapted from Eltzschig HK, Carmeliet P: Hypoxia and inflammation, N Engl J Med
364:656-665, 2011.)
The most common cause of hypoxia is ischemia (reduced blood supply). Ischemic injury often is caused by the gradual narrowing of arteries (arteriosclerosis) or complete blockage by blood clots (thrombosis), or both. Progressive hypoxia caused by gradual arterial obstruction is better tolerated than the acute anoxia (total lack of oxygen) caused by a sudden obstruction, as with an embolus (a blood clot or other blockage in the circulation). An acute obstruction in
a coronary artery can cause myocardial cell death (infarction) within minutes if the blood supply is not restored, whereas the gradual onset of ischemia usually results in myocardial adaptation. Myocardial infarction and stroke, which are common causes of death in the United States, generally result from atherosclerosis (a type of arteriosclerosis) and consequent ischemic injury. (Vascular obstruction is discussed in Chapter 24.) Cellular responses to hypoxic injury caused by ischemia have been demonstrated
in studies of the heart muscle. Within 1 minute after blood supply to the myocardium is interrupted, the heart becomes pale and has difficulty contracting normally. Within 3 to 5 minutes, the ischemic portion of the myocardium ceases to contract because of a rapid decrease in mitochondrial phosphorylation, causing insufficient ATP production. Lack of ATP leads to increased anaerobic metabolism, which generates ATP from glycogen when there is insufficient oxygen. When glycogen stores are depleted, even anaerobic metabolism ceases. A reduction in ATP levels causes the plasma membrane's sodium-potassium (Na+-
K+) pump and sodium-calcium exchange mechanism to fail, which leads to an intracellular accumulation of sodium and calcium and diffusion of potassium out of the cell. Sodium and water then can enter the cell freely, and cellular swelling, as well as early dilation of the endoplasmic reticulum (ER), results. Dilation causes the ribosomes to detach from the rough ER, reducing protein synthesis. With continued hypoxia, the entire cell becomes markedly swollen, with increased concentrations of sodium, water, and chloride and decreased concentrations of potassium. These disruptions are reversible if oxygen is restored. If oxygen is not restored, however, vacuolation (formation of vacuoles) occurs within the cytoplasm and swelling of lysosomes and marked mitochondrial swelling result from damage to the outer membrane. Continued hypoxic injury with accumulation of calcium subsequently activates multiple enzyme systems resulting in membrane damage, cytoskeleton disruption, DNA and chromatin degradation, ATP depletion, and eventual cell death (see Figures 4-9, C, and 4-27). Structurally, with plasma membrane damage, extracellular calcium readily moves into the cell and intracellular calcium stores are released. Increased intracellular calcium levels activate cell enzymes (caspases) that promote cell death by apoptosis (see Figures 4-29 and 4-33). Persistent ischemia is associated with irreversible injury and necrosis. Irreversible injury is associated structurally with severe swelling of the mitochondria, severe damage to plasma membranes, and swelling of lysosomes. Overall, death is mainly by necrosis but apoptosis also contributes.1 Restoration of blood flow and oxygen, however, can cause additional injury
called ischemia-reperfusion injury (Figure 4-11). Ischemia-reperfusion injury is very important clinically because it is associated with tissue damage during
myocardial and cerebral infarction. Several mechanisms are now proposed for ischemia-reperfusion injury and include: • Oxidative stress—Reoxygenation causes the increased generation of reactive oxygen species (ROS) and nitrogen species.1 Highly reactive oxygen intermediates (oxidative stress) generated include hydroxyl radical (OH−), superoxide radical (
), and hydrogen peroxide (H2O2) (see pp. 82-83). The nitrogen species include nitric oxide (NO) generated by endothelial cells, macrophages, neurons, and other cells. These radicals can all cause further membrane damage and mitochondrial calcium overload. The white blood cells (neutrophils) are especially affected with reperfusion injury, including neutrophil adhesion to the endothelium. Antioxidant treatment not only reverses neutrophil adhesion but also can reverse neutrophil- mediated heart injury. In one study of individuals undergoing elective percutaneous coronary intervention (PCI), pretreatment with vitamin C was associated with less myocardial injury.19 The PREVEC Trial (Prevention of reperfusion damage associated with percutaneous coronary angioplasty following acute myocardial infarction) seeks to evaluate whether vitamins C and E reduce infarct size in patients subjected to percutaneous coronary angioplasty after acute myocardial infarction.20
• Increased intracellular calcium concentration—Intracellular and mitochondrial calcium overload the cell; this process begins during acute ischemia. Reperfusion causes even more calcium influx because of cell membrane damage and ROS- induced injury to the sarcoplasmic reticulum. The increased calcium increases mitochondrial permeability, eventually leading to depletion of ATP and further cell injury.
• Inflammation—Ischemic injury increases inflammation because danger signals (from cytokines) are released by resident immune cells when cells die and this signaling initiates inflammation.
• Complement activation—The activation of complement may increase the tissue damage from reperfusion-ischemia injury.1
Quick Check 4-1
1. When does a cell become irreversibly injured?
2. Discuss the pathogenesis of hypoxic injury?
3. What are the mechanisms of ischemia-reperfusion injury?
FIGURE 4-11 Reperfusion Injury. Without oxygen, or anoxia, the cells display hypoxic injury and become swollen. With reoxygenation, reperfusion injury increases because of the formation of
reactive oxygen radicals that can cause cell necrosis. (Redrawn from Damjanov I: Pathology for the health professions, ed 3, St Louis, 2006, Saunders.)
Free Radicals and Reactive Oxygen Species— Oxidative Stress An important mechanism of cellular injury is injury induced by free radicals, especially by reactive oxygen species (ROS); this form of injury is called oxidative stress. Oxidative stress occurs when excess ROS overwhelm endogenous antioxidant systems. A free radical is an electrically uncharged atom or group of atoms that has an unpaired electron. Having one unpaired electron makes the molecule unstable; the molecule becomes stabilized either by donating or by accepting an electron from another molecule. When the attacked molecule loses its electron, it becomes a free radical. Therefore it is capable of injurious chemical bond formation with proteins, lipids, and carbohydrates—key molecules in membranes and nucleic acids. Free radicals are difficult to control and initiate chain reactions. They are highly reactive because they have low chemical specificity, meaning they can react with most molecules in their proximity. Oxidative stress can activate several intracellular signaling pathways because ROS can modulate enzymes and transcription factors. This is an important mechanism of cell damage in many conditions including chemical and radiation injury, ischemia-reperfusion
injury, cellular aging, and microbial killing by phagocytes, particularly neutrophils and macrophages.1 Free radicals may be generated within cells, first by the reduction-oxidation
reactions (redox reactions) in normal metabolic processes such as respiration. Under normal physiologic conditions ROS serve as “redox messengers” in the regulation of intracellular signaling; however, excess ROS may produce irreversible damage to cellular components. All biologic membranes contain redox systems, which also are important for cell defense (e.g., inflammation, iron uptake, growth and proliferation, and signal transduction) (Figure 4-12). Second, absorption of extreme energy sources (e.g., ultraviolet light, radiation) produces free radicals. Third, enzymatic metabolism of exogenous chemicals or drugs (e.g.,
, a product of carbon tetrachloride [CCl4]) results in the formation of free radicals. Fourth, transition metals (i.e., iron and copper) donate or accept free electrons during intracellular reactions and activate the formation of free radicals such as in the Fenton reaction (see Figure 4-12). Finally, nitric oxide (NO) is an important colorless gas that is an intermediate in many reactions generated by endothelial cells, neurons, macrophages, and other cell types. NO can act as a free radical and can be converted to highly reactive peroxynitrite anion (ONOO−), NO2, and . Table 4-3 describes the most significant free radicals.
FIGURE 4-12 Generation of Reactive Oxygen Species and Antioxidant Mechanisms in Biologic Systems. Free radicals are generated within cells in several ways, including from normal respiration; absorption of radiant energy; activation of leukocytes during inflammation;
metabolism of chemicals or drugs; transition metals, such as iron (Fe+++) or copper (Cu+), where the metals donate or accept electrons as in the Fenton reaction; nitric oxide (NO) generated by endothelial cells (not shown); and reperfusion injury. Ubiquinone (coenzyme Q), a lipophilic
molecule, transfers electrons in the inner membrane of mitochondria, ultimately enabling their interaction with oxygen (O2) and hydrogen (H2) to yield water (H2O). In so doing, the transport allows free energy change and the synthesis of 1 mole of adenosine triphosphate (ATP). With
the transport of electrons, free radicals are generated within the mitochondria. Reactive oxygen
species ( , H2O2, OH•) act as physiologic modulators of some mitochondrial functions but
may also cause cell damage. O2 is converted to superoxide ( ) by oxidative enzymes in the mitochondria, endoplasmic reticulum (ER), plasma membrane, peroxisomes, and cytosol. O2 is
converted to H2O2 by superoxide dismutase (SOD) and further to OH• by the Cu/Fe Fenton reaction. Superoxide catalyzes the reduction of Fe++ to Fe+++, thus increasing OH• formation by the Fenton reaction. H2O2 is also derived from oxidases in peroxisomes. The three reactive
oxygen species (H2O2, OH•, and ) cause free radical damage to lipids (peroxidation of the membrane), proteins (ion pump damage), and DNA (impaired protein synthesis). The major
antioxidant enzymes include SOD, catalase, and glutathione peroxidase.
TABLE 4-3 Biologically Relevant Free Radicals
Reactive oxygen species (ROS) Superoxide
Generated either (1) directly during autoxidation in mitochondria or (2) enzymatically by enzymes in cytoplasm, such as xanthine oxidase or cytochrome P-450; once produced, it can be inactivated spontaneously or more rapidly by enzyme superoxide dismutase (SOD):
Hydrogen peroxide (H2O2)
Or Oxidases present in peroxisomes O 2
peroxisome
Generated by SOD or directly by oxidases in intracellular peroxisomes; NOTE: SOD is considered an antioxidant because it converts superoxide to H2O2; catalase (another antioxidant) can then decompose H2O2 to O2 + H2O.)
Hydroxyl radicals (OH−)
Or
Or
Generated by hydrolysis of water caused by ionizing radiation or by interaction with metals—especially iron (Fe) and copper (Cu); iron is important in toxic oxygen injury because it is required for maximal oxidative cell damage
Nitric oxide (NO) NO by itself is an important mediator that can act as a free radical; it can be converted to another radical— peroxynitrite anion (ONOO
− ), as well as and
Data from Cotran RS et al: Robbins pathologic basis of disease, ed 6, Philadelphia, 1999, Saunders.
Free radicals cause several damaging effects by (1) lipid peroxidation, which is the destruction of polyunsaturated lipids (the same process by which fats become rancid), leading to membrane damage and increased permeability; (2) protein alterations, causing fragmentation of polypeptide chains that can lead to loss and protein misfolding; and (3) DNA damage, causing mutations (Figure 4-13; also see p. 39). Because of the increased understanding of free radicals, a growing number of diseases and disorders have been linked either directly or indirectly to these reactive species (Box 4-1).
FIGURE 4-13 The Role of Reactive Oxygen Species (ROS) in Cell Injury. The production of ROS can be initiated by many cell stressors, such as radiation, toxins, and reperfusion of oxygen. Free radicals are removed by normal decay and enzymatic systems. ROS accumulates in cells
because of insufficient removal or excess production leading to cell injury, including lipid peroxidation, protein modifications, and DNA damage or mutations. (Adapted from Kumar V et al, editors:
Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
Box 4-1 Diseases and Disorders Linked to Oxygen- Derived Free Radicals
Deterioration noted in aging
Atherosclerosis
Ischemic brain injury
Alzheimer disease
Neurotoxins
Cancer
Cardiac myopathy
Chronic granulomatous disease
Diabetes mellitus
Eye disorders
Macular degeneration
Cataracts
Inflammatory disorders
Iron overload
Lung disorders
Asbestosis
Oxygen toxicity
Emphysema
Nutritional deficiencies
Radiation injury
Reperfusion injury
Rheumatoid arthritis
Skin disorders
Toxic states
Xenobiotics (CCl4, paraquat, cigarette smoke, etc.)
Metal irons (Ni, Cu, Fe, etc.)
The body can eliminate free radicals. The oxygen free radical superoxide may spontaneously decay into oxygen and hydrogen peroxide. Table 4-4 summarizes other methods that contribute to inactivation or termination of free radicals. The toxicity of certain drugs and chemicals can be attributed either to conversion of these chemicals to free radicals or to the formation of oxygen-derived metabolites (see the following discussion).
TABLE 4-4 Methods Contributing to Inactivation or Termination of Free Radicals
Method Process Antioxidants Endogenous or exogenous; either blocks synthesis or inactivates (e.g., scavenges) free radicals; includes vitamin E, vitamin C, cysteine,
glutathione, albumin, ceruloplasmin, transferrin, γ-lipoacid, others Enzymes Superoxide dismutase,* which converts superoxide to H2O2; catalase* (in peroxisomes) decomposes H2O2; glutathione peroxidase* decomposes
OH• and H2O2
*These enzymes are important in modulating the cellular destructive effects of free radicals, also released in inflammation.
Mitochondrial Effects Mitochondria are key players in cell injury and cell death because they produce ATP or life-sustaining energy. Mitochondria can be damaged by ROS and by increases of cytosolic Ca++ concentration (see Figure 4-9). Box 4-2 summarizes the three major types and consequences of mitochondrial damage. Currently, investigators are trying to identify the polypeptides (i.e., proteomes) directly involved in diseases associated with mitochondrial dysfunction. ROS not only damage proteins and mitochondria but also can promote damage in neighboring cells. An important area of research emphasis is that protein aggregates can increase mitochondrial damage and damaged mitochondria can further induce protein damage, thus resulting in neurodegeneration. An emerging area of research concerns mitochondrial DNA that escapes from autophagy, which may be a mechanism of tissue inflammation.21
Box 4-2 Three Major Types and Consequences of Mitochondrial Damage
1. Damage to the mitochondria results in the formation of the mitochondrial
permeability transition pore, a high-conductance channel or pore. The opening of this channel results in the loss of mitochondrial membrane potential, causing failure of oxidative phosphorylation, depletion of ATP, and damage to mitochondrial DNA (mtDNA), leading to necrosis of the cell.
2. Altered oxidative phosphorylation leads to the formation of ROS that can damage cellular components.
3. Because mitochondria store several proteins between their membranes, increased permeability of the outer membrane may result in leakage of pro-apoptotic proteins and cause cell death by apoptosis.
Data from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.
Chemical or Toxic Injury Mechanisms Humans are exposed to thousands of chemicals that have inadequate toxicologic data.22 The given societal considerations of time, cost, and reduced animal use have increased the need to develop new methods for toxicity testing. To meet this public health need, many agencies have partnered to investigate how chemicals interact with biologic systems. Advances in molecular and systems biology, computational toxicology, and bioinformatics have increased the development of powerful new tools. The systems biology approach includes delineation of toxicity pathways that may
be defined as cellular response pathways, which when disturbed are expected to result in adverse health effects. Using this model of testing, investigators proposed screening and classifying compounds using a “cellular stress response pathway.” Components or mechanisms of these pathways include oxidative stress, heat shock response, DNA damage response, hypoxia, ER stress (see Chapter 1), mental stress, inflammation, and osmotic stress. Many chemicals have already been classified under these mechanisms. Humans are constantly exposed to a variety of compounds termed xenobiotics
(Greek xenos, “foreign”; bios, “life”) that include toxic, mutagenic, and carcinogenic chemicals (Figure 4-14). Some of these chemicals are found in the human diet, for example, fungal mycotoxins such as aflatoxin B1. Many xenobiotics are toxic to the liver (hepatotoxic). The liver is the initial site of contact for many ingested xenobiotics, drugs, and alcohol, making this organ most susceptible to
chemically induced injury. The toxicity of many chemicals results from absorption through the gastrointestinal tract after oral ingestion. A main cause for withdrawing medications from the market is hepatotoxicity. Dietary supplements, for example, chaparral and ma huang, are potent hepatotoxins.23 Other common routes of exposure for xenobiotics are absorption through the skin and inhalation. The severity of chemically induced liver injury varies from minor liver injury to acute liver failure, cirrhosis, and liver cancer.24
FIGURE 4-14 Human Exposure to Pollutants. Pollutants contained in air, water, and soil are absorbed through the lungs, gastrointestinal tract, and skin. In the body, the pollutants may act
at the site of absorption but are generally transported through the bloodstream to various organs where they can be stored or metabolized. Metabolism of xenobiotics may result in the formation of water-soluble compounds that are excreted, or a toxic metabolite may be created by activation of the agent. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
The liver as the principal site for xenobiotic metabolism, called biotransformation, converts the lipophilic xenobiotics to more hydrophilic forms for efficient excretion. Biotransformation, however, also can produce short-lived unstable highly reactive chemical intermediates that can lead to adverse effects.25 These harmful intermediates, classified and cataloged, are called toxicophores. The intermediates include electrophiles, nucleophiles, free radicals, and redox-active reactants. Electrophiles (electron lovers) are an atom or molecule attracted to electrons and accepts a pair of electrons to make a covalent bond. This process creates a partially or fully charged center in electrophilic molecules.25 A nucleophile is an atom or molecule that donates an electron pair to an electrophile to make a chemical bond. All chemical species with a free pair of electrons can act as nucleophiles. Nucleophiles are strongly attracted to positively charged regions in other chemicals and can be oxidized to free radicals and electrophiles.25 In general, the majority of all reactive chemical species are electrophilic because the formation of nucleophiles is rare25 (for a discussion on free radicals, see p. 81). The generation of these excess reactive chemical species leads to molecular damage in liver cells (Figure 4-15). These reactive intermediates can interact with cellular macromolecules (such as proteins and DNA), can covalently bind to proteins and form protein adducts (chemical bound to protein) and DNA adducts, or can react directly with cell structures to cause cell damage.26 Adduct formation can lead to adverse conditions including disruption in protein function, excess formation of fibrous connective tissue (fibrogenesis), and activation of immune responses.25 The identity of proteins modified by xenobiotics can be found in the resource known as the reactive metabolite target protein database.27 The body has two major defense systems for counteracting these effects: (1) detoxification enzymes and their cofactors and (2) antioxidant systems (see p. 82). Phases of detoxification include phase I enzymes, such as cytochrome P-450 (CYP) oxidases, which are the most important oxidative reactions. Other phase I detoxification enzymes include those for reduction and hydrolysis. In phase II detoxification, conjugation enzymes, such as glutathione (GSH), detoxify reactive electrophiles and produce polar metabolites that cannot diffuse across membranes. Most conjugation enzymes are located in the cytosol. Phase III detoxification is often called the efflux transporter system because enzymes remove the parent drugs, metabolites, and xenobiotics from cells. The liver has the highest supply of biotransformation enzymes of all organs and, therefore, has the key role in protection from chemical toxicity.25 Figure 4-16 is a summary of chemically induced liver injury.
FIGURE 4-15 Liver Toxicants: Chemical Injury.
FIGURE 4-16 Chemical Liver Injury. Liver injury is a result of genetic, environmental, biologic, and dietary factors. Certain chemicals can form toxic or chemically reactive metabolites. The risk of liver injury also can increase with increasing doses of a toxicant. Xenobiotic enzyme induction can lead to altered metabolism of chemicals, and drugs can either inhibit or induce drug-metabolizing enzymes. These changes can lead to greater toxicity. The dose at the site of action is controlled by the Phase I to III xenobiotic metabolites and metabolizing enzymes are encoded by numerous different genes. Therefore, the metabolism and toxicity outcomes can vary greatly among individuals. Additionally, all aspects of xenobiotic metabolism are regulated by certain transcription factors (cellular mediators of gene regulation). Overall, the extent of cell damage depends on the balance between reactive chemical species and protective responses aimed at decreasing oxidative stress, repairing macromolecular damage, or preserving cell health by inducing apoptosis or cell death. Significant clinical outcomes of chemical-induced
liver injury occur with necrosis and the immune response. Covalent binding of reactive metabolites to cellular proteins can produce new antigens (haptens) that initiate autoantibody production and cytotoxic T-cell responses. Necrosis, a form of cell death (see p. 102), can result
from extensive damage to the plasma membrane with altered ion transport, changes of membrane potential, cell swelling, and eventual dissolution. Altogether the pathogenesis of chemically induced liver injury is determined by genetics, environmental factors, and other underlying pathologic conditions. Green arrows are pathways leading to cell recovery; red arrows indicate pathways to cell damage or death; black arrows are pathways leading to
chemically induced liver injury. (Adapted from Gu X, Manautou JE: Molecular mechanisms underlying chemical liver injury, Exp Rev Mol Med 14:e4, 2013.)
The consequence of self-propagating chain reactions of free radicals is lipid peroxidation (also see p. 82). Free radicals react mainly with polyunsaturated fatty
acids in membranes and can initiate lipid peroxidation. The breakdown of membrane lipids results in altered function of the mitochondria, ER, plasma membranes, and Golgi apparatus, and therefore has a role in acute liver cell death (necrosis) and progression of liver injury (Figure 4-17).25
FIGURE 4-17 Chemical Injury of Liver Cells Induced by Carbon Tetrachloride (CCl4) Poisoning. Light blue boxes are mechanisms unique to chemical injury, purple boxes involve
hypoxic injury, and green boxes are clinical manifestations.
Chemical Agents Including Drugs
Numerous chemical agents cause cellular injury. Because chemical injury remains a constant problem in clinical settings, it is a major limitation to drug therapy. Over- the-counter and prescribed drugs can cause cellular injury, sometimes leading to death. The leading cause of child poisoning is medications (see Health Alert: The Percentage of Child Medication–Related Poisoning Deaths Is Increasing). The site of injury is frequently the liver, where many chemicals and drugs are metabolized (see Figure 4-17). Long-term exposure to air pollutants, insecticides, and herbicides can cause cellular injury (see Health Alert: Air Pollution Reported as Largest Single Environmental Health Risk).
Health alert The Percentage of Child Medication–Related Poisoning Deaths Is Increasing
Today, the leading cause of child poisoning is medications. Each year, more than 500,000 children, ages 5 and younger, experience a potential poisoning related to medications. More than 60,000 children are treated in emergency departments because of accidental medication exposure or overdose. Of every 150 2-year-old children, one is being sent to the emergency department for an unintentional medication overdose. Among children younger than age 5, 95% of emergency department visits are caused by unsupervised accidental ingestions and about 5% from dosing errors made by clinicians. Importantly, investigators analyzed records from the American Association of
Poison Control Centers' National Poison Data System (NPDS), an electronic database of all calls to the 61 poison control centers across the United States. Their analysis included all calls for children age 5 years or younger who were seen in a hospital emergency department between 2001 and 2008 for either unintentional self-exposure to a single drug (prescription or over-the-counter [OTC]) or unintentional therapeutic error for a single drug (prescription or OTC). The number of such calls during this 8-year period totaled 453,559. Medication-related poisoning deaths among children 5 years and younger now most frequently involve exposures to opioid analgesics and cardiovascular medications. About half of all poisoning-related deaths involve analgesics, antihistamines, and sedatives. Development of new medications also has led to more of them being available in
American homes. With aging, more adults are taking OTC and prescription medications as well as multiple medications. Oxycodone, morphine, and methadone prescriptions have increased between 159% and 559% between 2000 and 2009,
depending on the drug; the number of prescribed cardiovascular drugs (e.g., meto- prolol) has increased about fivefold. Additionally, more medications, such as those utilized for attention-deficit disorder and diabetes, are being prescribed to younger adults and children. Prescription pain killer overdose is a growing epidemic, especially among women. How can we increase the safety of children exposed to so many medications?
Safe storage is the most important solution and safe dosing from clinicians will reduce dosing errors. Additionally, improvements are continuing through improved packaging and labeling of medications as well as education of parents and consumers on dosing information.
Data from Bond GR et al: J Pediatr 160(2):265-270, 2011; Bronstein AC et al: Clin Toxicol 49:910-941, 2011; Budnitz DS, Lovegrove MC: J Pediatr 160(2):190-192, 2012; Bunitz DS, Salis S: Pediatrics 127(6):e1597- e1599, 2011; Centers for Disease Control and Prevention: Available at www.cdc/gov/features/medicationstorage/. Accessed February 9, 2010.
Health Alert Air Pollution Reported as Largest Single Environmental Health Risk
The World Health Organization (WHO) reports that about 7 million people died in 2012 as a result of air pollution exposure. Improved measurements and better technology have enabled scientists to make more detailed analyses of health risks. These findings confirm that air pollution is now the world's largest single environmental health risk and reducing air pollution could save millions of lives. New data show a stronger link between indoor and outdoor air pollution exposure and cardiovascular diseases, for example, strokes and ischemic heart disease, as well as the link between air pollution and cancer. These data are in addition to the role of air pollution and the development of respiratory diseases including infections and chronic obstructive pulmonary diseases. Using these 2012 data for low- and middle-income countries, Southeast Asia and Western Pacific regions had the largest air pollution burden. Included in the analysis is a breakdown of deaths for adults and children attributed to specific diseases:
Outdoor Air Pollution–Caused Deaths—Breakdown by Disease:
• 40% ischemic heart disease
• 40% stroke
• 11% chronic obstructive pulmonary disease (COPD)
• 6% lung cancer
• 3% acute lower respiratory tract infections in children
Indoor Air Pollution–Caused Deaths—Breakdown by Disease:
• 34% stroke
• 26% ischemic heart disease
• 22% COPD
• 12% acute lower respiratory tract infections in children
• 6% lung cancer
The WHO estimates that indoor air pollution was linked to 4.3 million deaths in 2012 from cooking over coal, wood, dung, and biomass stoves. Outdoor air pollution estimates were 3.7 million deaths in 2012 from urban and rural sources.
Data from World Health Organization (WHO): 7 million premature deaths annually linked to air pollution. Available from www.who.int/mediacentre/news/releases/2014/air-pollution/en/#.
Another way to classify mechanisms by which drug actions, chemicals, and toxins produce injury includes (1) direct damage, also called on-target toxicity; (2) exaggerated response at the target, including overdose; (3) biologic activation to toxic metabolites, including free radicals; (4) hypersensitivity and related immunologic reactions; and (5) rare toxicities.28 These mechanisms are not mutually exclusive; thus several may be operating concurrently. Direct damage is when chemicals and drugs injure cells by combining directly
with critical molecular substances. For example, cyanide is highly toxic (e.g., poisonous) because it inhibits mitochondrial cytochrome oxidase and hence blocks electron transport. Many chemotherapeutic drugs, known as antineoplastic agents, induce cell damage by direct cytotoxic effects. Exaggerated pharmacologic responses at the target include tumors caused by industrial chemicals and the birth defects attributed to thalidomide.28 Importantly, another example includes common drugs of abuse (Table 4-5). Drug abuse can involve mind-altering substances beyond therapeutic or social norms (Table 4-6). Drug addiction and overdose are serious public health issues.
TABLE 4-5 Common Drugs of Abuse
Class Molecular Target Example Opioid narcotics Mu opioid receptor (agonist) Heroin, hydromorphone (Dilaudid)
Oxycodone (Percodan, Percocet, OxyContin) Methadone (Dolophine) Meperidine (Demerol)
Sedative-hypnotics GABAA receptor (agonist) Barbiturates Ethanol Methaqualone (Quaalude) Glutethimide (Doriden) Ethchlorvynol (Placidyl)
Psychomotor stimulants Dopamine transporter (antagonist) Serotonin receptors (toxicity)
Cocaine Amphetamines 3,4-Methylenedioxymethamphetamine (MDMA, ecstasy)
Phencyclidine-like drugs NMDA glutamate receptor channel (antagonist) Phencyclidine (PCP, angel dust) Ketamine
Cannabinoids CB1 cannabinoid receptors (agonist) Marijuana Hashish
Hallucinogens Serotonin 5-HT2 receptors (agonist) Lysergic acid diethylamide (LSD) Mescaline Psilocybin
CB1, Cannabinoid receptor type 1; GABA, γ-aminobutyric acid; 5-HT2, 5-hydroxytryptamine; NMDA, N- methyl-D-aspartate.
From Kumar V et al: Cellular responses to stress and toxic insults: adaptation, injury, and death. In Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, St Louis, 2014, Saunders; Hyman SE: JAMA 286:2586, 2001.
TABLE 4-6 Social or Street Drugs and Their Effects
Type of Drug Description and Effects Marijuana (pot) Active substance: Δ9-Tetrahydrocannabinol (THC), found in resin of Cannabis sativa plant
With smoking (e.g., “joints”), about 5% to 10% is absorbed through lungs; with heavy use the following adverse effects have been reported: alterations of sensory perception; cognitive and psychomotor impairment (e.g., inability to judge time, speed, distance); it increases heart rate and blood pressure; increases susceptibility to laryngitis, pharyngitis, bronchitis; causes cough and hoarseness; may contribute to lung cancer (different dosages need study; contains large number of carcinogens); data from animal studies only indicate reproductive changes include reduced fertility, decreased sperm motility, and decreased levels of circulatory testosterone; fetal abnormalities include low birth weight; increased frequency of infectious illness is thought to be result of depressed cell-mediated and humoral immunity; beneficial effects include decreased nausea secondary to cancer chemotherapy and decreased pain in certain chronic conditions
Methamphetamine (meth)
An amine derivation of amphetamine (C10H15N) used as crystalline hydrochloride CNS stimulant; in large doses causes irritability, aggressive (violent) behavior, anxiety, excitement, auditory hallucinations, and paranoia (delusions and psychosis); mood changes are common and abuser can swiftly change from friendly to hostile; paranoiac swings can result in suspiciousness, hyperactive behavior, and dramatic mood swings Appeals to abusers because body's metabolism is increased and produces euphoria, alertness, and perception of increased energy Stages: Low intensity: User is not psychologically addicted and uses methamphetamine by swallowing or snorting Binge and high intensity: User has psychologic addiction and smokes or injects to achieve a faster, stronger high Tweaking: Most dangerous stage; user is continually under the influence, not sleeping for 3-15 days, extremely irritated, and paranoid
Cocaine and crack Extracted from leaves of cocoa plant and sold as a water-soluble powder (cocaine hydrochloride) liberally diluted with talcum powder or other white powders; extraction of pure alkaloid from cocaine hydrochloride is “free-base” called crack because it “cracks” when heated Crack is more potent than cocaine; cocaine is widely used as an anesthetic, usually in procedures involving oral cavity; it is a potent CNS stimulant, blocking reuptake of neurotransmitters norepinephrine, dopamine, and serotonin; also increases synthesis of norepinephrine and dopamine; dopamine induces sense of euphoria, and norepinephrine causes adrenergic potentiation, including hypertension, tachycardia, and vasoconstriction; cocaine can therefore cause severe coronary artery narrowing and ischemia; reason cocaine increases thrombus formation is unclear; other cardiovascular effects include dysrhythmias, sudden death, dilated cardiomyopathy, rupture of descending aorta (i.e., secondary to hypertension); effects on fetus include premature labor, retarded fetal development, stillbirth, hyperirritability
Heroin Opiate closely related to morphine, methadone, and codeine Highly addictive, and withdrawal causes intense fear (“I'll die without it”); sold “cut” with similar-looking white powder; dissolved in water it is often highly contaminated; feeling of tranquility and sedation lasts only a few hours and thus encourages repeated intravenous or subcutaneous injections; acts on the receptors enkephalins, endorphins, and dynorphins, which are widely distributed throughout body with high affinity to CNS; effects can include infectious complications, especially Staphylococcus aureus, granulomas of lung, septic embolism, and pulmonary edema—in addition, viral infections from casual exchange of needles and HIV; sudden death is related to overdosage secondary to respiratory depression, decreased cardiac output, and severe pulmonary edema
CNS, Central nervous system; HIV, human immunodeficiency virus.
Data from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier; Nahas G et al: N Engl J Med 343(7):514, 2000.
Most toxic chemicals are not biologically active in their parent (native) form but must be converted to reactive metabolites, which then act on target molecules. This conversion is usually performed by the cytochrome P-450 oxidase enzymes in the smooth ER of the liver and other organs. These toxic metabolites cause membrane damage and cell injury mostly from formation of free radicals and subsequent membrane damage from lipid peroxidation (see Figure 4-17). For example, acetaminophen (paracetamol) is converted to a toxic metabolite in the liver, causing cell injury (Figure 4-18). Acetaminophen is one of the most common causes of poisoning worldwide.29 Many investigators are studying hepatoprotective strategies.30
FIGURE 4-18 Acetaminophen Metabolism and Toxicity. CYP2E1, A cytochrome; GSH, glutathione; NAPQI, toxic byproduct.
Hypersensitivity reactions are a common drug toxicity and range from mild skin rashes to immune-mediated organ failure.28 One type of hypersensitivity reaction is the delayed-onset reaction, which occurs after multiple doses of a drug are administered. Some protein drugs and large polypeptide drugs (e.g., insulin) can directly stimulate antibody production (see Chapter 8). Most drugs, however, act as haptens and bind covalently to serum or cell-bound proteins. The binding makes the protein immunogenic, stimulating antidrug antibody production, T-cell responses against the drug, or both. For example, penicillin itself is not antigenic but its metabolic degradation products can become antigenic and cause an allergic reaction. Rare toxicities simply mean infrequent occurrences as described previously by the other four mechanisms. These toxicities reflect individual genetic predispositions that affect drug or chemical metabolism, disposition, and immune responses. Carbon monoxide, carbon tetrachloride, and social drugs, such as alcohol, can
significantly alter cellular function and injure cellular structures. Accidental or
suicidal poisonings by chemical agents cause numerous deaths. The injurious effects of some agents—lead, carbon monoxide, ethyl alcohol, mercury—are common cellular injuries.
Lead. Lead (Pb) is a heavy toxic metal that persists in older homes, the environment, and the workplace. Lead may be found in hazardous concentrations in food, water, and air and it is one of the most common overexposures found in industry.31 Despite efforts to reduce exposure through government regulation, exposure still persists for many people and toxicity is still a primary hazard for children32 (see Health Alert: Low-Level Lead Exposure Harms Children: A Renewed Call for Primary Prevention). Although Pb was removed from paint in Europe in 1922 and removed in the United States in 1978, many homes in the United States still contain leaded paint and chipped and peeling leaded paint constitutes a major source of current childhood exposure.33-36 The chipped paint can disintegrate at friction surfaces to form Pb dust.36 Another source of contamination is Pb dust dispersed along roadways from previous leaded gasoline emissions.36 When Pb was removed from gasoline, blood lead levels (BLLs) dropped significantly.37-39 Previous emissions of leaded fuel created large dispersions of lead dust in the environment. Particulate lead (2 to 10 µm) does not degrade and persists in the environment, making it a notable source of human exposure.40 Other airborne sources include smelters and piston-engine airplanes.41 Drinking water exposed to Pb occurs from outdated fixtures, plumbing without corrosion control, and solders.36 Because well water is not subject to EPA regulation it may not be tested for Pb.36 Although the average blood levels of Pb in children in the United States have dropped since the 1970s, there are at-risk populations with higher than average BLLs.36 Children of lower social economic status or racial minority status are still at higher risk of Pb poisoning and some regions in the United States have an increased prevalence of higher BLLs in children.36 Importantly, the CDC reports “no safe blood lead level in children has been identified.”42 Common sources of Pb are included in Table 4-7.
Health Alert Low-Level Lead Exposure Harms Children: A Renewed Call for Primary Prevention
An advisory committee of the CDC recently suggested that the current threshold for harmful lead exposure in children should be cut in half because even lower levels
cause irreversible harm. The report noted that studies have found reduced intelligence quotients (IQs) and behavioral problems in children with exposure levels less than 10 mcg/dl and that such low levels have effects on cardiovascular, endocrine, and immunologic systems. Based on these data, the panel recommended reducing the threshold for harmful levels of lead in the blood to 5 mcg/dl. Despite progress in reducing blood lead levels (BLLs), racial and income disparities persist. An internal review process from both the Centers for Disease Control and Prevention and the U.S. Department of Health and Human Services will determine how to implement any accepted recommendations. This is a very important process because BLLs appear to be irreversible, underscoring the need for primary prevention.
Data from Advisory Committee for Childhood Lead Poisoning Prevention of the Centers for Disease Control and Prevention: Low level lead exposure harms children: a renewed call for primary prevention, 2012. Available at www.cdc.gov/nceh/lead/ACCLPP/FinalDocument030712.pdf. Accessed September 24, 2012.
TABLE 4-7 Common Sources of Lead Exposure
Exposure Source Environmental Lead paint, soil, or dust near roadways or lead-painted homes; plastic window blinds; plumbing materials (from pipes or solder); pottery
glazes and ceramic ware; lead-core candle wicks; leaded gasoline; water (pipes) Occupational Lead mining and refining, plumbing and pipe fitting, auto repair, glass manufacturing, battery manufacturing and recycling, printing shop,
construction work, plastic manufacturing, gas station attendant, firing-range attendant Hobbies Glazed pottery making, target shooting at firing ranges, lead soldering, preparing fishing sinkers, stained-glass making, painting, car or boat
repair Other Gasoline sniffing, costume jewelry, cosmetics, contaminated herbal products
Data from Sanborn MD et al: CMAJ 166(10):1287-1292, 2002.
Children are more susceptible to the effects of Pb than adults for several reasons, including (1) children have increased hand-to-mouth behavior and exposure from the ingestion of Pb dust; (2) the blood-brain barrier in children is immature during fetal development, contributing to greater accumulation in the developing brain; and (3) infant absorption of Pb is greater than that in adults and bone turnover (in adults the body burden of lead is found in bone) in children from skeletal growth results in continuous leaching of Pb into blood, causing constant body exposure.36,42 If nutrition is compromised, especially if dietary intake of iron and calcium is insufficient, children are more likely to have elevated BLLs.36 Particularly worrisome is lead exposure during pregnancy because the developing fetal nervous system is especially vulnerable; lead exposure can result in lower IQs, learning disorders, hyperactivity, and attention problems.32 The organ systems primarily affected by lead ingestion include the nervous
system, the hematopoietic system (tissues that produce blood cells), and the kidneys
of the urologic system. The neurologic effect of Pb in exposed children is the driving factor for reducing Pb levels in the environment.36 Elevated BLLs not only are linked to cognitive deficits but also are associated with behavioral changes including antisocial behavior, acting out in school, and difficulty paying attention.36 The cognitive and behavioral changes of Pb-exposed children persist after complete cessation of Pb exposure.36 In 1991 the CDC lowered the definition of Pb intoxication to 10 µm/dl BLL because several studies reported that children with BLLs of at least 10 µm/dl had impaired intellectual functioning36 (Figure 4-19). Studies in animals have led to the hypothesis that Pb targets the learning and memory processes by inhibiting the N-methyl-D-aspartate receptor (NMDAR), which is necessary for hippocampus-mediated learning and memory.36,43 Similar changes also have been found in cultured neuron systems.36 Inhibition of either voltage-gated calcium channels or NMDARs by Pb results in reduction of Ca++ entry into the cell, thereby disrupting the necessary Ca++ signaling for neurotransmission.44,45 Lead induces cellular damage by increasing oxidative stress.46 Lead toxicity involves the direct formation of ROS (singlet oxygen, hydrogen peroxides, hydroperoxides) and depletion of antioxidants.46 Pb exposure leads to lowered levels of glutathione; and because glutathione is important for the metabolism of specific drugs and other toxins, low Pb levels can increase their toxicity, as well as the levels of other metals.46 From animal studies and human population studies, low-level lead exposure may cause hypertension.47 Lead interferes with the normal remodeling of cartilage and bone in children. From radiologic studies of bone, “lead lines” are detectable and lead also can be found in the gums as a result of hyperpigmentation. Lead inhibits several enzymes involved in hemoglobin synthesis and causes anemia (most obvious is a microcytic hypochromic anemia). Renal lesions can cause tubular dysfunction resulting in glycosuria (glucose in the urine), aminoaciduria (amino acids in the urine), and hyperphosphaturia (excess phosphate in the urine). Gastrointestinal symptoms are less severe and include nausea, loss of appetite, weight loss, and abdominal cramping.
FIGURE 4-19 Lead Poisoning in Children Related to Blood Levels. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
Carbon monoxide. Gaseous substances can be classified according to their ability to asphyxiate (interrupt respiration) or irritate. Toxic asphyxiants, such as carbon monoxide, hydrogen cyanide, and hydrogen sulfide, directly interfere with cellular respiration. Carbon monoxide (CO) is an odorless, colorless, nonirritating, and undetectable
gas unless it is mixed with a visible or odorous pollutant. CO is produced by the incomplete combustion of fuels such as gasoline. Although CO is a chemical agent, the ultimate injury it produces is a hypoxic injury—namely, oxygen deprivation. As a systemic asphyxiant, CO causes death by inducing central nervous system (CNS)
depression. Normally, oxygen molecules are carried to tissues bound to hemoglobin in red blood cells (see Chapter 27). Because CO's affinity for hemoglobin is 300 times greater than that of oxygen, CO quickly binds with the hemoglobin, preventing the oxygen molecules' ability to bind to the hemoglobin. Minute amounts of CO can produce a significant percentage of carboxyhemoglobin (carbon monoxide bound with hemoglobin). With increasing levels of carboxyhemoglobin, hypoxia occurs insidiously, evoking widespread ischemic changes in the CNS, and individuals are often unaware of their plight. The diagnosis is made from measurement of carboxyhemoglobin levels in the blood. Symptoms related to CO poisoning include headache, giddiness, tinnitus (ringing
in the ears), chest pain, confusion, nausea, weakness, and vomiting. CO is an air pollutant found in combustion fumes produced by cars and trucks, small gasoline engines, stoves, gas ranges, gas refrigerators, heating systems, lanterns, burning charcoal or wood, and cigarette smoke. Chronic exposure can occur in people working in confined spaces, such as underground garages and tunnels. Fumes can accumulate in enclosed or semi-enclosed spaces, and poisoning from breathing CO can occur in humans and animals. High levels of CO can cause loss of consciousness and death. Death can occur in individuals sleeping or intoxicated before experiencing any symptoms. Although all people and animals are at risk, those most susceptible to poisoning include unborn babies, infants, and people with chronic heart disease, respiratory problems, and anemia. For information on preventing CO poisoning from home appliances and proper venting, see the Centers for Disease Control and Prevention (CDC) website at www.cdc.gov/co/faqs.htm.
Ethanol. Alcohol (ethanol) is the primary choice among mood-altering drugs available in the United States. It is estimated there are more than 10 million chronic alcoholics in the United States. Alcohol contributes to more than 100,000 deaths annually with 50% of these deaths from drunk driving accidents, alcohol-related homicides, and suicides.48 A blood concentration of 80 mg/dl is the legal definition for drunk driving in the United States. This level of alcohol in an average person may be reached after consumption of three drinks (three 12-ounce bottles of beer, 15 ounces of wine, and 4 to 5 ounces of distilled liquor). The effects of alcohol vary by age, gender, and percent body fat; the rate of metabolism affects the blood alcohol level. Because alcohol is not only a psychoactive drug but also a food, it is considered part of the basic food supply in many societies. A large intake of alcohol has enormous effects on nutritional status. Liver and
nutritional disorders are the most serious consequences of alcohol abuse. Major
nutritional deficiencies include magnesium, vitamin B6, thiamine, and phosphorus. Folic acid deficiency is a common problem in chronic alcoholic populations. Ethanol alters folic acid (folate) homeostasis by decreasing intestinal absorption of folate, increasing liver retention of folate, and increasing the loss of folate through urinary and fecal excretion.49 Folic acid deficiency becomes especially serious in pregnant women who consume alcohol and may contribute to fetal alcohol syndrome (see p. 92). Most of the alcohol in blood is metabolized to acetaldehyde in the liver by three
enzyme systems: alcohol dehydrogenase (ADH), the microsomal ethanol-oxidizing system (MEOS; CYP2E1), and catalase (Figure 4-20). The major pathway involves ADH, an enzyme located in the cytosol of hepatocytes. The microsomal ethanol oxidizing system (MEOS) depends on cytochrome P-450 (CYP2E1), an enzyme needed for cellular oxidation. Activation of CYP2E1 requires a high ethanol concentration and thus is thought to be important in the accelerated ethanol metabolism (i.e., tolerance) noted in persons with chronic alcoholism. Acetaldehyde has many toxic tissue effects and is responsible for some of the acute effects of alcohol and for development of head and neck cancer (HNC).48 A recent and first study showed that head and neck cancer risk may be influenced by alcohol- metabolizing genes (ADH1B and ALDH2) and oral hygiene.50
FIGURE 4-20 Ethanol Metabolism Pathway. Ethanol is metabolized into acetaldehyde through the cytosolic enzyme alcohol dehydrogenase (ADH), the microsomal enzyme cytochrome P-450 2E1 (CYP2E1), and the peroxisomal enzyme catalase. The ADH enzyme reaction is the main
ethanol metabolic pathway involving an intermediate carrier of electrons, namely, nicotinamide adenine dinucleotide (NAD+), which is reduced by two electrons to form NADH. Acetaldehyde is metabolized mainly by aldehyde dehydrogenase 2 (ALDH2) in the mitochondria to acetate and
NADH before being cleared into the systemic circulation. (Adapted from Zhang Y, Ren J: Pharmacol Ther 132[1]:86-92, 2011.)
The major effects of acute alcoholism involve the central nervous system (CNS). After alcohol is ingested, it is absorbed, unaltered, in the stomach and small intestine. Fatty foods and milk slow absorption. Alcohol then is distributed to all tissues and fluids of the body in direct proportion to the blood concentration. Individuals differ in their capability to metabolize alcohol. Genetic differences in the metabolism of liver alcohol, including levels of aldehyde dehydrogenases, have been identified.51 These genetic polymorphisms may account for ethnic and gender differences in ethanol metabolism. Persons with chronic alcoholism develop tolerance because of production of enzymes, leading to an increased rate of metabolism (e.g., P-450). Numerous studies have validated the so-called J- or U-shaped inverse association
between alcohol and overall or cardiovascular mortality, such as from myocardial infarction and ischemic stroke. These studies have found that light to moderate (nonbinge) drinkers tend to have lower mortality than nondrinkers and heavy drinkers have higher mortality.52 For both men and women, former drinkers and regular heavy drinkers had higher mortality.52 Light to moderate drinkers in the United States may have reduced mortality but this may be confounded by medical
care and social relationships, especially among women.52,53 These relationships need further study. The suggested mechanisms for cardioprotection for light to moderate drinkers include increase in levels of high-density lipoprotein–cholesterol (HDL-C), decrease in levels of low-density lipoprotein (LDL), prevention of clot formation, reduction in platelet aggregation, decrease in blood pressure, increase in coronary vessel vasodilation, increase in coronary blood flow, decrease in coronary inflammation, decrease in atherosclerosis, limited ischemia-reperfusion injury (I/R injury), and a decrease in diabetic vessel pathology.54 The American Heart Association recommends no more than two drinks per day for men and one drink per day for women (one 12-oz beer, 4 oz of wine, 1.5 oz of 80-proof spirits, or 1 oz of 100-proof spirits). Drinking more alcohol can increase the risks of alcoholism, high blood pressure, obesity, stroke, breast cancer, suicide, and accidents.55 Individuals who do not consume alcohol should not be encouraged to start drinking.56 Acute alcoholism (drunkenness) affects the CNS (see Health Alert: Alcohol:
Global Burden, Adolescent Onset, Chronic or Binge Drinking). Alcohol intoxication causes CNS depression. Depending on the amount consumed, CNS depression is associated with sedation, drowsiness, loss of motor coordination, delirium, altered behavior, and loss of consciousness. Toxic amounts (300 to 400 mg/dl) result in a lethal coma or respiratory arrest because of medullary center depression. Investigators studied the effects of snoring and multiple variables including alcohol. They found that a low level of self-reported physical activity is a risk factor for future habitual snoring complaints in women independent of alcohol dependence, smoking, current weight, and weight gain. Furthermore, increased physical activity can modify the risk.57 Acute alcoholism may induce reversible hepatic and gastric changes.48 Acute alcoholism contributes significantly to motor vehicle fatalities.
Health Alert Alcohol: Global Burden, Adolescent Onset, Chronic or Binge Drinking
Alcohol is widely consumed worldwide, and in the United States 50% of the adult population (18 years and older) consumes alcohol regularly. Alcohol continues to be the drug of choice among teens and young adults with one third of twelfth graders and 40% of college students reporting “binge drinking” (four standard alcohol drinks on one occasion in females and five in males). Alcohol abuse is the
leading cause of liver-related morbidity and mortality. Chronic and binge drinking causes alcoholic liver disease (ALD) with a spectrum from hepatic steatosis (fatty change) to steatohepatitis (fatty change and inflammation) and cirrhosis (see Chapter 36). These alterations can eventually lead to hepatocellular carcinoma. The pathogenesis of ALD is not fully characterized and recent studies reveal a major role of mitochondria. Animal studies have shown that alcohol causes mitochondrial DNA damage, lipid accumulation, and oxidative stress. Understanding the role of the mitochondria may help identify therapeutic targets. Investigations of adolescent drinking behaviors, especially binge drinking, is
providing evidence of neurocognitive changes, including changes in both gray and white matter. These studies are examining risk-taking behaviors that begin in adolescence and coincide with vulnerable and significant neurodevelopmental changes.
Data from Adams PF et al: Vital Health Stat 10(255), 2012; available from www.cdc.gov/nchs/data/series/sr_10/sr10_255.pdf; Hicks BM et al: Addiction 107:540-548, 2012; Johnston LD et al: Monitoring the future national results on adolescent drug use: overview of key findings, Bethesda, Md, 2009, National Institute on Drug Abuse; Lisdahl KM et al: Front Psychiatry 4:53, 2013; Mathews S et al: Am J Physiol Gastrointest Liver Physiol 2014 Apr 3 [Epub ahead of print]; Nassir F, Ibdah JA: World J Gastroenterol 20(9):2136-2142, 2014; White HR et al: Alcohol Clin Exp Res 35:295-303, 2010.
Chronic alcoholism causes structural alterations in practically all organs and tissues in the body because most tissues contain enzymes capable of ethanol oxidation or nonoxidative metabolism. The most significant activity, however, occurs in the liver. Alcohol is the leading cause of liver-related morbidity and mortality.58 In general, hepatic changes, initiated by acetaldehyde, include inflammation, deposition of fat, enlargement of the liver, interruption of microtubular transport of proteins and their secretion, increase in intracellular water, depression of fatty acid oxidation in the mitochondria, increase in membrane rigidity, and acute liver cell necrosis (see Chapter 36). Specifically, chronic or binge alcohol consumption causes alcoholic liver disease (ALD) with a spectrum ranging from simple fatty liver (steatosis), to steatohepatitis (fatty with inflammation), to cirrhosis (Figure 4-21) (see Chapter 36). Cirrhosis is associated with portal hypertension and an increased risk for hepatocellular carcinoma. Cellular damage is increased by reactive oxygen species (ROS) and oxidative stress (see p. 81). Activation of proinflammatory cytokines from neutrophils and lymphocytes mediates liver damage.59 Oxidative stress is associated with cell membrane phospholipid depletion, which alters the fluidity and function of cell membranes as well as intercellular transport. Chronic alcoholism is related to several disorders, including injury to the myocardium (alcoholic cardiomyopathy);
increased tendency to hypertension, acute gastritis, and acute and chronic pancreatitis; and regressive changes in skeletal muscle. Chronic alcohol consumption is associated with an increased incidence of cancer of the oral cavity, liver, esophagus, and breast (see Health Alert: Alcohol: Global Burden, Adolescent Onset, Chronic or Binge Drinking).
FIGURE 4-21 Alcoholic Hepatitis. Chicken-wire fibrosis extending between hepatocytes (Mallory trichrome stain). (From Damjanov I, Linder J, editors: Anderson's pathology, ed 10, St Louis, 1996, Mosby.)
Ethanol is implicated in the onset of a variety of immune defects, including effects on the production of cytokines involved in inflammatory responses. Alcohol can induce epigenetic variations in the developmental pathways of many types of immune cells (e.g., granulocytes, macrophages, and T-lymphocytes) that promote increased inflammation.60 Alcohol increases the development of serious medical conditions related to immune system dysfunction, including acute respiratory distress syndrome (ARDS) as well as liver cancer and alcoholic liver disease (ALD).60 Binge and chronic drinking increases susceptibility to many infectious microorganisms and can enhance the progression of human immunodeficiency virus (HIV) by affecting innate and adaptive immunity.60 The deleterious effects of prenatal alcohol exposure can cause mental deficiency
and neurobehavioral disorders, as well as fetal alcohol syndrome. Fetal alcohol syndrome includes growth retardation, facial anomalies, cognitive impairment, and ocular malformations (Figure 4-22). It is among the common causes of mental deficiency.61 Evidence of epigenetic alterations has led to the hypothesis that alcohol
effects on fetal development may be caused not only by maternal alcohol consumption but also by the father's exposure as well.61 Epigenetic alterations may be carried through the male germline for generations.62 Alcohol crosses the placenta, reaching the fetus, and blood levels of the fetus may reach equivalent levels to maternal levels in 1 to 2 hours.63 Research has demonstrated an unimpeded bidirectional movement of alcohol between the fetus and the mother. The fetus may completely depend on maternal hepatic detoxification because the activity of alcohol dehydrogenase (ADH) in fetal liver is less than 10% of that in the adult liver.63 Additionally, the amniotic fluid acts as a reservoir for alcohol, prolonging fetal exposure.63 The specific mechanisms of injury are unknown; however, acetaldehyde can alter fetal development by disrupting differentiation and growth; DNA and protein synthesis; modification of carbohydrates, proteins, and fats; flow of nutrients across the placenta; and neuro-circuitry dysfunction that may be long- lasting.61,63
FIGURE 4-22 Fetal Alcohol Syndrome. When alcohol enters the fetal blood, the potential result can cause tragic congenital abnormalities, such as microcephaly (“small head”), low birth
weight, and cardiovascular defects, as well as developmental disabilities, such as physical and intellectual disability, and even death. Note the small head, thinned upper lip, small eye openings
(palpebral fissures), epicanthal folds, and receded upper jaw (retrognathia) typical of fetal alcohol syndrome. (From Fortinash KM, Holoday W orret PA: Psychiatric mental health nursing, ed 5, St Louis, 2012, Mosby.)
Mercury. Mercury is a global threat to human and environmental health. A recent report
presents an overview of the Global Mercury Assessment 2013.64 This report provides the most recent information on worldwide atmospheric mercury emissions, releases to the aquatic environment, and the fate of mercury in the global environment. Causes from human activity, called anthropogenic, are responsible for about 30% of annual emissions of mercury to air, another 10% arise from natural geologic sources, and the remainder (60%) occurs from re-emissions or earlier released mercury that has increased over decades and centuries in surface soil and water.64 The major sources of anthropogenic mercury emissions to air are artisanal and small-scale gold mining (ASGM) and coal burning. The next major sources are the production of ferrous and nonferrous metals, and cement production. Importantly, investigators report that emissions from industrial sectors have increased since 2005.64 Types of aquatic releases of mercury include industrial sites (power plants, factories), old mines, landfills, and waste disposal locations. Artisanal and small-scale gold mining are significant producers of aquatic mercury release. It is estimated that more than 90% of mercury in marine animals is from anthropogenic emissions.64 Large amounts of inorganic mercury have accumulated in surface soils and in the oceans. Climate change, with thawing of enormous areas of frozen lands, may release even more long-stored mercury and organic matter into lakes, rivers, and oceans.64 Dental amalgams, or “silver fillings,” are made of two almost equal parts of
liquid mercury and a powder containing silver, tin, copper, zinc, and other metals.41 When amalgams are placed or removed they can release a small amount of mercury vapor. Chewing can release a small amount of vapor and people absorb the vapor by inhalation or ingestion.41 Researchers are studying the effects of exposure to magnetic fields, such as from mobile phone use, and the release of mercury from amalgams.65 Susceptibility to mercury toxicity varies in a dose-dependent fashion, and among individuals based on multiple genes, not all have been identified.66,67 Worldwide efforts are under way to phase down or eliminate the use of mercury dental amalgam.67 Thimerosal, a mercury-containing preservative, was removed from all vaccines in 2001, with the exception of inactivated influenza vaccines.68
Quick Check 4-2
1. Why are children more susceptible to the toxic effects of lead exposure?
2. Discuss the sources of lead exposure?
3. Discuss the mechanisms of cell injury related to chronic alcoholism?
4. What are the sources of mercury exposure?
Unintentional and Intentional Injuries Unintentional and intentional injuries are an important health problem in the United States. In 2012 there were 192,945 deaths, an injury death rate of 60.2/100,000.69 The number of deaths because of poisoning was 48,545 with 15.4 deaths per 100,000. Motor vehicle traffic deaths were 33,804 with a rate of 10.7 deaths per 100,000. Deaths from all firearms were 33,636 with a rate of 10.6 deaths per 100,000. From data reporting in 2010, drug poisoning deaths were 12.4 per 100,000.69 Death from injury is significantly more common for men than for women; the overall rate for men is 83.46/100,000 versus 39.28/100,000 for women. Significant racial differences are noted in the death rate, with whites at 64.85/100,000, blacks at 56.20/100,000, and other racial groups at a combined rate of 28.96/100,000. There also is a bimodal age distribution for injury-related deaths, with peaks in the young adult and elderly groups. Unintentional injury is the leading cause of death for people between the ages of 1 and 34 years; intentional injury (suicide, homicide) ranks between the second and fourth leading cause of death in this age group. The 1999 report published by the Institute of Medicine (IOM) indicated that between 44,000 and 98,000 unnecessary deaths per year occurred in hospitals alone as a result of errors by healthcare professionals (see Health Alert: Unintentional Injury Errors in Health Care and Patient Safety). Statistics on nonfatal injuries are harder to document accurately, but they are known to be a significant cause of morbidity and disability and to cost society billions of dollars annually. The more common terms used to describe and classify unintentional and intentional injuries and brief descriptions of important features of these injuries are discussed in Table 4-8.
Health Alert Unintentional Injury Errors in Health Care and Patient Safety
According to a US Senate subcommittee hearing (July 17, 2014), despite more than a decade of national efforts to improve patient safety, hospitals and ambulatory care centers remain problematic for patients. This assessment follows the 15-year anniversary of the release of the IOM report on patient safety. Testimony from the senate hearings challenged the IOM report that patient harms were likely underestimated. A more recent estimate suggests the number of U.S. deaths as a result of medical error may be greater than 400,000 per year with more than 1000 each day.
Progress has been made in certain areas including the reduction of bloodstream infections from central lines. Success with this program has been expanded nationwide. Checklists are a very useful tool for improving patient safety. They have become more widely implemented and their success depends on appropriately targeting the intervention and utilizing a careful implementation strategy. Besides checklists, other examples of patient safety primers include adverse events after hospital discharge, computerized provider order entry, detection of safety hazards, diagnostic errors, disruptive and unprofessional behavior, error disclosure, handoffs and signouts, health care–associated infections, nursing and patient safety, and medication errors. In a testimony at the hearings it was stated “that one of the biggest barriers to improved patient safety is the lack of a robust national system for tracking patient safety data.” Additionally, speakers testified that better systems of care are needed in understanding that a complex set of factors—complexity of hospital systems, time pressures, growing use of technology, financial incentives that reward hospitals by paying them to care for patients' complications, CEO compensation not tied to quality of care—all contribute to poor patient outcomes. The entrenched challenges of the U.S. health care system demand a transformed approach. Left unchanged, health care will continue to underperform; cause unnecessary harm; and strain national, state, and family budgets. The actions required to reverse this trend will be notable, substantial, sometimes disruptive— and absolutely necessary.” (IOM Best Care at Lower Cost; The Path to Continously Learning Health Care in America Institute of Medicine Report Brief Washington DC, 2012)
Data from Agency for Healthcare Research and Quality: Patient safety primers, Rockville, MD, 2014, U.S. Department of Health and Human Services; James JT: J Patient Saf 9(3):122-128, 2013; Kohn LT et al, editors: To err is human: building a safer health system, Washington DC, 1999, National Academy Press; Kuehn BM: J Am Med Assoc 312(9):879-880, 2014.
TABLE 4-8 Unintentional and Intentional Injuries
Type of Injury Description BLUNT-FORCE INJURIES Mechanical injury to body
resulting in tearing, shearing, or crushing; most common type of injury seen in healthcare settings; caused by blows or impacts; motor vehicle accidents and falls most common cause (see photo, A) Contusion (bruise): Bleeding into skin or underlying tissues; initial color will be red-purple, then
blue-black, then yellow- brown or green (see Figure 4-26); duration of bruise depends on extent, location, and degree of vascularization; bruising of soft tissue may be confined to deeper structures; hematoma is collection of blood in soft tissue; subdural hematoma is blood between inner surface of dura mater and surface of brain; can result from blows, falls, or sudden acceleration/deceleration of head as occurs in shaken baby syndrome; epidural hematoma is collection of blood between inner surface of skull and dura; is most often associated with a skull fracture Laceration: Tear or rip resulting when tensile strength of skin or tissue is exceeded; is ragged and irregular with abraded edges; an extreme example is avulsion, where a wide area of tissue is pulled away; lacerations of internal organs are common in blunt-force injuries; lacerations of liver, spleen, kidneys, and bowel occur from blows to abdomen; thoracic aorta may be lacerated in sudden deceleration accidents; severe blows or impacts to chest may rupture heart with lacerations of atria or ventricles Fracture: Blunt-force blows or impacts can cause bone to break or shatter (see Chapter 39)
SHARP-FORCE INJURIES Cutting and piercing injuries accounted for 2734 deaths in 2007; men have a higher rate (1.37/100,000) than women (0.44/100,000); differences by race are whites 0.71/100,000, blacks 2.12/100,000, and other groups 0.80/100,000 Incised wound: A wound that is longer than it is deep; wound can be straight or jagged with sharp, distinct edges without abrasion; usually produces significant external bleeding with little internal hemorrhage; these wounds are noted in sharp- force injury suicides; in addition to a deep, lethal cut, there will be superficial
incisions in same area called hesitation marks (see photo, B) Stab wound: A penetrating sharp-force injury that is deeper than it is long; if a sharp instrument is used, depths of wound are clean and distinct but can be abraded if object is inserted deeply and wider portion (e.g., hilt of a knife) impacts skin; depending on size and location of wound, external bleeding may be surprisingly small; after an initial spurt of blood, even if a major vessel or heart is struck, wound may be almost completely closed by tissue pressure, thus allowing only a trickle of visible blood despite copious internal bleeding Puncture wound: Instruments or objects with sharp points but without sharp edges produce puncture wounds; classic example is wound of foot after stepping on a nail; wounds are prone to infection, have abrasion of edges, and can be very deep Chopping wound: Heavy, edged instruments (axes, hatchets, propeller blades) produce wounds with a combination of sharp- and blunt-force characteristics
GUNSHOT WOUNDS Accounted for more than 33,636 deaths in the United States in 2015; men more likely to die than women (18.16 vs. 2.73/100,000); black men between ages of 15 and 24 have greatest death rate (86.95/100,000); gunshot wounds are either penetrating (bullet remains in body) or perforating (bullet exits body); bullet also can fragment; most important factors or appearances are whether it is an entrance or exit wound and range of fire
Entrance wound: All wounds share some common features; overall appearance is most affected by range of fire Contact range entrance wound: Distinctive type of wound when gun is held so muzzle rests on or presses into skin surface; there is searing of edges of wound from flame and soot or smoke on edges of wound in addition to hole; hard contact wounds of head cause severe tearing and disruption of tissue (because of thin layer of skin and muscle overlying bone); wound is gaping and jagged, known as blow back; can produce a patterned abrasion that mirrors weapon used (see photo, C)
Intermediate (distance) range entrance wound: Surrounded by gunpowder tattooing or stippling; tattooing results from fragments of burning or unburned pieces of gunpowder exiting barrel and forcefully striking skin; stippling results when gunpowder abrades but does not penetrate skin (see photo, D) Indeterminate range entrance wound: Occurs when flame, soot, or gunpowder does not reach skin surface but bullet does; indeterminate is used rather than distant because appearance may be same regardless of distance; for example, if an individual is shot at close range through multiple layers of clothing the wound may look the same as if the shooting occurred at a distance Exit wound: Has the same appearance regardless of range of fire; most important factors are speed of projectile and degree of
deformation; size cannot be used to determine if hole is an exit or entrance wound; usually has clean edges that can often be reapproximated to cover defect; skin is one of toughest structures for a bullet to penetrate; thus it is not uncommon for a bullet to pass entirely through body but stopped just beneath skin on “exit” side Wounding potential of bullets: Most damage done by a bullet is a result of amount of energy transferred to tissue impacted; speed of bullet has much greater effect than increased size; some bullets are designed to expand or fragment when striking an object, for example, hollow- point ammunition; lethality of a wound depends on what structures are damaged; wounds of brain may not be lethal; however, they are usually immediately incapacitating and lead to significant long-term disability; a person with a “lethal” injury (wound of heart or aorta) also may not be immediately incapacitated
Asphyxial Injuries Asphyxial injuries are caused by a failure of cells to receive or use oxygen. Deprivation of oxygen may be partial (hypoxia) or total (anoxia). Asphyxial injuries can be grouped into four general categories: suffocation, strangulation, chemical asphyxiants, and drowning.
Suffocation. Suffocation, or oxygen failing to reach the blood, can result from a lack of oxygen in the environment (entrapment in an enclosed space or filling of the environment with a suffocating gas) or blockage of the external airways. Classic examples of these types of asphyxial injuries are a child who is trapped in an abandoned refrigerator or a person who commits suicide by putting a plastic bag over his or her head. A reduction in the ambient oxygen level to 16% (normal is 21%) is immediately dangerous. If the level is below 5%, death can ensue within a matter of minutes. The diagnosis of these types of asphyxial injuries depends on obtaining an accurate and thorough history because there will be no specific physical findings. Diagnosis and treatment in choking asphyxiation (obstruction of the internal
airways) depend on locating and removing the obstructing material. Injury or disease also may cause swelling of the soft tissues of the airway, leading to partial or complete obstruction and subsequent asphyxiation. Suffocation also may result from compression of the chest or abdomen (mechanical or compressional asphyxia), preventing normal respiratory movements. Usual signs and symptoms include florid facial congestion and petechiae (pinpoint hemorrhages) of the eyes and face.
Strangulation. Strangulation is caused by compression and closure of the blood vessels and air passages resulting from external pressure on the neck. This causes cerebral hypoxia or anoxia secondary to the alteration or cessation of blood flow to and from the brain. It is important to remember that the amount of force needed to close the jugular veins (2 kg [4.5 lb]) or carotid arteries (5 kg [11 lb]) is significantly less than that required to crush the trachea (15 kg [33 lb]). It is the alteration of cerebral blood flow in most types of strangulation that causes injury or death—not the lack of airflow. With complete blockage of the carotid arteries, unconsciousness can occur within 10 to 15 seconds. A noose is placed around the neck, and the weight of the body is used to cause
constriction of the noose and compression of the neck in hanging strangulations. The body does not need to be completely suspended to produce severe injury or death. Depending on the type of ligature used, there usually is a distinct mark on the neck—an inverted V with the base of the V pointing toward the point of suspension. Internal injuries of the neck are actually quite rare in hangings, and only in judicial hangings, in which the body is weighted and dropped, is significant soft tissue or cervical spinal trauma seen. Petechiae of the eyes or face may be seen, but they are
rare. In ligature strangulation, the mark on the neck is horizontal without the inverted
V pattern seen in hangings. Petechiae may be more common because intermittent opening and closure of the blood vessels may occur as a result of the victim's struggles. Internal injuries of the neck are rare. Variable amounts of external trauma on the neck are found with contusions and
abrasions in manual strangulation caused either by the assailant or by the victim clawing at his or her own neck in an attempt to remove the assailant's hands. Internal damage can be quite severe, with bruising of deep structures and even fractures of the hyoid bone and tracheal and cricoid cartilages. Petechiae are common.
Chemical asphyxiants. Chemical asphyxiants either prevent the delivery of oxygen to the tissues or block its utilization. Carbon monoxide is the most common chemical asphyxiant (see p. 90). Cyanide acts as an asphyxiant by combining with the ferric iron atom in cytochrome oxidase, thereby blocking the intracellular use of oxygen. A victim of cyanide poisoning will have the same cherry-red appearance as a carbon monoxide intoxication victim because cyanide blocks the use of circulating oxyhemoglobin. An odor of bitter almonds also may be detected. (The ability to smell cyanide is a genetic trait that is absent in a significant portion of the general population.) Hydrogen sulfide (sewer gas) is a chemical asphyxiant in which victims of hydrogen cyanide poisoning may have brown-tinged blood in addition to the nonspecific signs of asphyxiation.
Drowning. Drowning is an alteration of oxygen delivery to tissues resulting from the inhalation of fluid, usually water. In 2012 there were 3391 drowning deaths in the United States. Although research in the 1940s and 1950s indicated that changes in blood electrolyte levels and volume as a result of absorption of fluid from the lungs may be an important factor in some drownings, the major mechanism of injury is hypoxemia (low blood oxygen levels). Even in freshwater drownings, where large amounts of water can pass through the alveolar-capillary interface, there is no evidence that increases in blood volume cause significant electrolyte disturbances or hemolysis, or that the amount of fluid loading is beyond the compensatory capabilities of the kidneys and heart. Airway obstruction is the more important pathologic abnormality, underscored by the fact that in as many as 15% of drownings little or no water enters the lungs because of vagal nerve–mediated laryngospasms. This phenomenon is called dry-lung drowning.
No matter what mechanism is involved, cerebral hypoxia leads to unconsciousness in a matter of minutes. Whether this progresses to death depends on a number of factors, including the age and the health of the individual. One of the most important factors is the temperature of the water. Irreversible injury develops much more rapidly in warm water than it does in cold water. Submersion times of up to 1 hour with subsequent survival have been reported in children who were submerged in very cold water. Complete submersion is not necessary for a person to drown. An incapacitated or helpless individual (epileptic, alcoholic, infant) may drown in water that is only a few inches deep. It is important to remember that no specific or diagnostic findings prove that a
person recovered from the water is actually a drowning victim. In cases where water has entered the lung, there may be large amounts of foam exiting the nose and mouth, although this also can be seen in certain types of drug overdoses. A body recovered from water with signs of prolonged immersion could just as easily be a victim of some other type of injury with the immersion acting to obscure the actual cause of death. When working with a living victim recovered from water, it is essential to keep in mind that an underlying condition may have led to the person's becoming incapacitated and submerged—a condition that also may need to be treated or corrected while correcting hypoxemia and dealing with its sequelae.
Quick Check 4-3
1. Give examples of intentional and unintentional injury in the United States..
2. Discuss unintentional injury as a form of injury with health care delivery in the United States.
3. What is the major mechanism of injury with drowning?
Infectious Injury The pathogenicity (virulence) of microorganisms lies in their ability to survive and proliferate in the human body, where they injure cells and tissues. The disease- producing potential of a microorganism depends on its ability to (1) invade and destroy cells, (2) produce toxins, and (3) produce damaging hypersensitivity reactions. (See Chapter 8 for a description of infection and infectious organisms.)
Immunologic and Inflammatory Injury
Cellular membranes are injured by direct contact with cellular and chemical components of the immune and inflammatory responses, such as phagocytic cells (lymphocytes, macrophages) and substances such as histamine, antibodies, lymphokines, complement, and proteases (see Chapter 6). Complement is responsible for many of the membrane alterations that occur during immunologic injury. Membrane alterations are associated with a rapid leakage of potassium (K+) out
of the cell and a rapid influx of water. Antibodies can interfere with membrane function by binding with and occupying receptor molecules on the plasma membrane. Antibodies also can block or destroy cellular junctions, interfering with intercellular communication. Other mechanisms of cellular injury are genetic and epigenetic factors, nutritional imbalances, and physical agents. These are summarized in Table 4-9.
TABLE 4-9 Mechanisms of Cellular Injury
Mechanism Characteristics Examples Genetic Factors
Alter cell's nucleus and plasma membrane's structure, shape, receptors, or transport mechanisms
Sickle cell anemia, Huntington disease, muscular dystrophy, abetalipoproteinemia, familial hypercholesterolemia
Epigenetic Factors
Induction of mitotically heritable alterations in gene expression without changing DNA Gene silencing in cancer
Nutritional Imbalances
Pathophysiologic cellular effects develop when nutrients are not consumed in diet and transported to body's cells or when excessive amounts of nutrients are consumed and transported
Protein deficiency, protein-calorie malnutrition, glucose deficiency, lipid deficiency (hypolipidemia), hyperlipidemia (increased lipoproteins in blood causing deposits of fat in heart, liver, and muscle), vitamin deficiencies
Physical Agents Temperature extremes
Hypothermic injury results from chilling or freezing of cells, creating high intracellular sodium concentrations; abrupt drops in temperature lead to vasoconstriction and increased viscosity of blood, causing ischemic injury, infarction, and necrosis; reactive oxygen species (ROS) are important in this process
Frostbite
Hyperthermic injury is caused by excessive heat and varies in severity according to nature, intensity, and extent of heat
Burns, burn blisters, heat cramps usually from vigorous exercise with water and salt loss; heat exhaustion with salt and water loss causes heme contraction; heat stroke is life-threatening with a clinical rectal temperature of 106° F
Tissue injury caused by compressive waves of air or fluid impinging on body, followed by sudden wave of decreased pressure; changes may collapse thorax, rupture internal solid organs, and cause widespread hemorrhage: carbon dioxide and nitrogen that are normally dissolved in blood precipitate from solution and form small bubbles (gas emboli), causing hypoxic injury and pain
Blast injury (air or immersion), decompression sickness (caisson disease or “the bends”); recently reported in a few individuals with subdural hematomas after riding high-speed roller coasters
Ionizing radiation
Refers to any form of radiation that can remove orbital electrons from atoms; source is usually environment and medical use; damage is to DNA molecule, causing chromosomal aberrations, chromosomal instability, and damage to membranes and enzymes; also induces growth factors and extracellular matrix remodeling; uncertainty exists regarding effects of low levels of radiation
X-rays, γ-rays, and α- and β-particles cause skin redness, skin damage, chromosomal damage, cancer
Illumination Fluorescent lighting and halogen lamps create harmful oxidative stresses; ultraviolet light has been linked to skin cancer
Eyestrain, obscured vision, cataracts, headaches, melanoma
Mechanical stresses
Injury is caused by physical impact or irritation; they may be overt or cumulative Faulty occupational biomechanics, leading to overexertion disorders
Noise Can be caused by acute loud noise or cumulative effects of various intensities, frequencies, and duration of noise; considered a public health threat
Hearing impairment or loss; tinnitus, temporary threshold shift (TTS), or loss can occur as a complication of critical illness, from mechanical trauma, ototoxic medications, infections, vascular disorders, and noise
Manifestations of Cellular Injury: Accumulations An important manifestation of cell injury is the intracellular accumulation of abnormal amounts of various substances and the resultant metabolic disturbances. Cellular accumulations, also known as infiltrations, not only result from sublethal, sustained injury by cells, but also result from normal (but inefficient) cell function. Two categories of substances can produce accumulations: (1) a normal cellular substance (such as excess water, proteins, lipids, and carbohydrates) or (2) an abnormal substance, either endogenous (such as a product of abnormal metabolism or synthesis) or exogenous (such as infectious agents or a mineral). These products can accumulate transiently or permanently and can be toxic or harmless. Most accumulations are attributed to four types of mechanisms, all abnormal (Figure 4- 23). Abnormal accumulations of these substances can occur in the cytoplasm (often in the lysosomes) or in the nucleus if (1) there is insufficient removal of the normal substance because of altered packaging and transport, for example, fatty change in the liver called steatosis; (2) an abnormal substance, often the result of a mutated gene, accumulates because of defects in protein folding, transport, or abnormal degradation; (3) an endogenous substance (normal or abnormal) is not effectively catabolized, usually because of lack of a vital lysosomal enzyme, called storage diseases; or (4) harmful exogenous materials, such as heavy metals, mineral dusts, or microorganisms, accumulate because of inhalation, ingestion, or infection.
FIGURE 4-23 Mechanisms of Intracellular Accumulations. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
In all storage diseases, the cells attempt to digest, or catabolize, the “stored” substances. As a result, excessive amounts of metabolites (products of catabolism) accumulate in the cells and are expelled into the extracellular matrix, where they are consumed by phagocytic cells called macrophages (see Chapter 6). Some of these scavenger cells circulate throughout the body, whereas others remain fixed in certain tissues, such as the liver or spleen. As more and more macrophages and other phagocytes migrate to tissues that are producing excessive metabolites, the affected tissues begin to swell. This is the mechanism that causes enlargement of the liver (hepatomegaly) or the spleen (splenomegaly) as a clinical manifestation of many storage diseases.
Water Cellular swelling, the most common degenerative change, is caused by the shift of extracellular water into the cells. In hypoxic injury, movement of fluid and ions into the cell is associated with acute failure of metabolism and loss of ATP production. Normally, the pump that transports sodium ions (Na+) out of the cell is maintained by the presence of ATP and adenosinetriphosphatase (ATPase), the active transport enzyme. In metabolic failure caused by hypoxia, reduced levels of ATP and ATPase permit sodium to accumulate in the cell while potassium (K+) diffuses outward. The increased intracellular sodium concentration increases osmotic pressure, drawing more water into the cell. The cisternae of the ER become distended, rupture, and then unite to form large vacuoles that isolate the water from the cytoplasm, a process called vacuolation. Progressive vacuolation results in cytoplasmic swelling called oncosis (which has replaced the old term hydropic [water] degeneration) or vacuolar degeneration (Figure 4-24). If cellular swelling affects all the cells in an organ, the organ increases in weight and becomes distended and pale.
FIGURE 4-24 The Process of Oncosis (Formerly Referred to as “Hydropic Degeneration”). ATP, Adenosine triphosphate.
Cellular swelling is reversible and is considered sublethal. It is, in fact, an early manifestation of almost all types of cellular injury, including severe or lethal cell injury. It is also associated with high fever, hypokalemia (abnormally low concentrations of potassium in the blood; see Chapter 5), and certain infections.
Lipids and Carbohydrates Certain metabolic disorders result in the abnormal intracellular accumulation of carbohydrates and lipids. These substances may accumulate throughout the body but are found primarily in the spleen, liver, and CNS. Accumulations in cells of the CNS can cause neurologic dysfunction and severe intellectual disability. Lipids accumulate in Tay-Sachs disease, Niemann-Pick disease, and Gaucher disease; whereas in the diseases known as mucopolysaccharidoses, carbohydrates are in excess. The mucopolysaccharidoses are progressive disorders that usually involve multiple organs, including liver, spleen, heart, and blood vessels. The accumulated mucopolysaccharides are found in reticuloendothelial cells, endothelial cells, intimal smooth muscle cells, and fibroblasts throughout the body. These carbohydrate accumulations can cause clouding of the cornea, joint stiffness, and intellectual disability. Although lipids sometimes accumulate in heart, muscle, and kidney cells, the
most common site of intracellular lipid accumulation, or fatty change (steatosis),
is liver cells (Figure 4-25). Because hepatic metabolism and secretion of lipids are crucial to proper body function, imbalances and deficiencies in these processes lead to major pathologic changes. In developed countries the most common cause of fatty change in the liver is alcohol abuse. Other causes of fatty change include diabetes mellitus, protein malnutrition, toxins, anoxia, and obesity. As lipids fill the cells, vacuolation pushes the nucleus and other organelles aside. The liver's outward appearance is yellow and greasy. Alcohol abuse is one of the most common causes of fatty liver (see Chapter 36).
FIGURE 4-25 Fatty Liver. The liver appears yellow. (From Damjanov I, Linder J: Pathology: a color atlas, St Louis, 2000, Mosby.)
Lipid accumulation in liver cells occurs after cellular injury instigates one or more of the following mechanisms:
1. Increased movement of free fatty acids into the liver (starvation, for example, increases the metabolism of triglycerides in adipose tissue, releasing fatty acids that subsequently enter liver cells)
2. Failure of the metabolic process that converts fatty acids to phospholipids, resulting in the preferential conversion of fatty acids to triglycerides
3. Increased synthesis of triglycerides from fatty acids (increased levels of the enzyme α-glycerophosphatase can accelerate triglyceride synthesis)
4. Decreased synthesis of apoproteins (lipid-acceptor proteins)
5. Failure of lipids to bind with apoproteins and form lipoproteins
6. Failure of mechanisms that transport lipoproteins out of the cell
7. Direct damage to the ER by free radicals released by alcohol's toxic effects
Many pathologic states show accumulation of cholesterol and cholesterol esters. These states include atherosclerosis, in which atherosclerotic plaques, smooth muscle cells, and macrophages within the intimal layer of the aorta and large arteries are filled with lipid-rich vacuoles of cholesterol and cholesterol esters. Other states include cholesterol-rich deposits in the gallbladder and Niemann-Pick disease (type C), which involve genetic mutations of an enzyme affecting cholesterol transport.
Glycogen Glycogen storage is important as a readily available energy source in the cytoplasm of normal cells. Intracellular accumulations of glycogen are seen in genetic disorders called glycogen storage diseases and in disorders of glucose and glycogen metabolism. As with water and lipid accumulation, glycogen accumulation results in excessive vacuolation of the cytoplasm. The most common cause of glycogen accumulation is the disorder of glucose metabolism (i.e., diabetes mellitus) (see Chapter 19).
Proteins Proteins provide cellular structure and constitute most of the cell's dry weight. The proteins are synthesized on ribosomes in the cytoplasm from the essential amino acids lysine, threonine, leucine, isoleucine, methionine, tryptophan, valine, phenylalanine, and histidine. The accumulation of protein probably damages cells in two ways. First, metabolites, produced when the cell attempts to digest some proteins, are enzymes that when released from lysosomes can damage cellular organelles. Second, excessive amounts of protein in the cytoplasm push against cellular organelles, disrupting organelle function and intracellular communication. Protein excess accumulates primarily in the epithelial cells of the renal
convoluted tubules of the nephron unit and in the antibody-forming plasma cells (B lymphocytes) of the immune system. Several types of renal disorders cause excessive excretion of protein molecules in the urine (proteinuria). Normally, little or no protein is present in the urine, and its presence in significant amounts indicates cellular injury and altered cellular function.
Accumulations of protein in B lymphocytes can occur during active synthesis of antibodies during the immune response. The excess aggregates of protein are called Russell bodies (see Chapter 6). Russell bodies have been identified in multiple myeloma (plasma cell tumor) (see Chapter 21). Mutations in protein can slow protein folding, resulting in the accumulation of
partially folded intermediates. An example is α1-antitrypsin deficiency, which can cause emphysema. Certain types of cell injury are associated with the accumulation of cytoskeleton proteins. For example, the neurofibrillary tangle found in the brain in Alzheimer disease contains these types of proteins.
Pigments Pigment accumulations may be normal or abnormal, endogenous (produced within the body) or exogenous (produced outside the body). Endogenous pigments are derived, for example, from amino acids (e.g., tyrosine, tryptophan). They include melanin and the blood proteins porphyrins, hemoglobin, and hemosiderin. Lipid- rich pigments, such as lipofuscin (the aging pigment), give a yellow-brown color to cells undergoing slow, regressive, and often atrophic changes. The most common exogenous pigment is carbon (coal dust), a pervasive air pollutant in urban areas. Inhaled carbon interacts with lung macrophages and is transported by lymphatic vessels to regional lymph nodes. This accumulation blackens lung tissues and involved lymph nodes. Other exogenous pigments include mineral dusts containing silica and iron particles, lead, silver salts, and dyes for tattoos.
Melanin Melanin accumulates in epithelial cells (keratinocytes) of the skin and retina. It is an extremely important pigment because it protects the skin against long exposure to sunlight and is considered an essential factor in the prevention of skin cancer (see Chapters 11 and 41). Ultraviolet light (e.g., sunlight) stimulates the synthesis of melanin, which probably absorbs ultraviolet rays during subsequent exposure. Melanin also may protect the skin by trapping the injurious free radicals produced by the action of ultraviolet light on skin. Melanin is a brown-black pigment derived from the amino acid tyrosine. It is
synthesized by epidermal cells called melanocytes and is stored in membrane-bound cytoplasmic vesicles called melanosomes. Melanin also can accumulate in melanophores (melanin-containing pigment
cells), macrophages, or other phagocytic cells in the dermis. Presumably these cells acquire the melanin from nearby melanocytes or from pigment that has been
extruded from dying epidermal cells. This is the mechanism that causes freckles. Melanin also occurs in the benign form of pigmented moles called nevi (see Chapter 41). Malignant melanoma is a cancerous skin tumor that contains melanin. A decrease in melanin production occurs in the inherited disorder of melanin
metabolism called albinism. Albinism is often diffuse, involving all the skin, the eyes, and the hair. Albinism is also related to phenylalanine metabolism. In classic types, the person with albinism is unable to convert tyrosine to DOPA (3,4- dihydroxyphenylalanine), an intermediate in melanin biosynthesis. Melanocytes are present in normal numbers, but they are unable to make melanin. Individuals with albinism are very sensitive to sunlight and quickly become sunburned. They are also at high risk for skin cancer.
Hemoproteins Hemoproteins are among the most essential of the normal endogenous pigments. They include hemoglobin and the oxidative enzymes, the cytochromes. Central to an understanding of disorders involving these pigments is knowledge of iron uptake, metabolism, excretion, and storage (see Chapter 20). Hemoprotein accumulations in cells are caused by excessive storage of iron, which is transferred to the cells from the bloodstream. Iron enters the blood from three primary sources: (1) tissue stores, (2) the intestinal mucosa, and (3) macrophages that remove and destroy dead or defective red blood cells. The amount of iron in blood plasma depends also on the metabolism of the major iron transport protein, transferrin. Iron is stored in tissue cells in two forms: as ferritin and, when increased levels
of iron are present, as hemosiderin. Hemosiderin is a yellow-brown pigment derived from hemoglobin. With pathologic states, excesses of iron cause hemosiderin to accumulate within cells, often in areas of bruising and hemorrhage and in the lungs and spleen after congestion caused by heart failure. With local hemorrhage, the skin first appears red-blue and then lysis of the escaped red blood cells occurs, causing the hemoglobin to be transformed to hemosiderin. The color changes noted in bruising reflect this transformation (Figure 4-26).
FIGURE 4-26 Hemosiderin Accumulation Is Noted as the Color Changes in a “Black Eye.”
Hemosiderosis is a condition in which excess iron is stored as hemosiderin in the cells of many organs and tissues. This condition is common in individuals who have received repeated blood transfusions or prolonged parenteral administration of iron. Hemosiderosis is associated also with increased absorption of dietary iron, conditions in which iron storage and transport are impaired, and hemolytic anemia. Excessive alcohol (wine) ingestion also can lead to hemosiderosis. Normally, absorption of excessive dietary iron is prevented by an iron absorption process in the intestines. Failure of this process can lead to total body iron accumulations in the range of 60 to 80 g, compared with normal iron stores of 4.5 to 5 g. Excessive accumulations of iron, such as occur in hemochromatosis (a genetic disorder of iron metabolism and the most severe example of iron overload), are associated with liver and pancreatic cell damage. Bilirubin is a normal, yellow-to-green pigment of bile derived from the
porphyrin structure of hemoglobin. Excess bilirubin within cells and tissues causes jaundice (icterus), or yellowing of the skin. Jaundice occurs when the bilirubin level exceeds 1.5 to 2 mg/dl of plasma, compared with the normal values of 0.4 to 1 mg/dl. Hyperbilirubinemia occurs with (1) destruction of red blood cells (erythrocytes), such as in hemolytic jaundice; (2) diseases affecting the metabolism and excretion of bilirubin in the liver; and (3) diseases that cause obstruction of the common bile duct, such as gallstones or pancreatic tumors. Certain drugs (specifically chlorpromazine and other phenothiazine derivatives), estrogenic hormones, and halothane (an anesthetic) can cause the obstruction of normal bile flow through the liver.
Because unconjugated bilirubin is lipid soluble, it can injure the lipid components of the plasma membrane. Albumin, a plasma protein, provides significant protection by binding unconjugated bilirubin in plasma. Unconjugated bilirubin causes two cellular outcomes: uncoupling of oxidative phosphorylation and a loss of cellular proteins. These two changes could cause structural injury to the various membranes of the cell.
Calcium Calcium salts accumulate in both injured and dead tissues (Figure 4-27). An important mechanism of cellular calcification is the influx of extracellular calcium in injured mitochondria. Another mechanism that causes calcium accumulation in alveoli (gas-exchange airways of the lungs), gastric epithelium, and renal tubules is the excretion of acid at these sites, leading to the local production of hydroxyl ions. Hydroxyl ions result in precipitation of calcium hydroxide, Ca(OH)2, and hydroxyapatite, (Ca3[PO4]2)3•Ca(OH)2, a mixed salt. Damage occurs when calcium salts cluster and harden, interfering with normal cellular structure and function.
FIGURE 4-27 Free Cytosolic Calcium: A Destructive Agent. Normally, calcium is removed from the cytosol by adenosine triphosphate (ATP)–dependent calcium pumps. In normal cells,
calcium is bound to buffering proteins, such as calbindin or parvalbumin, and is contained in the endoplasmic reticulum and the mitochondria. If there is abnormal permeability of calcium-ion channels, direct damage to membranes, or depletion of ATP (i.e., hypoxic injury), calcium increases in the cytosol. If the free calcium cannot be buffered or pumped out of cells,
uncontrolled enzyme activation takes place, causing further damage. Uncontrolled entry of calcium into the cytosol is an important final common pathway in many causes of cell death.
Pathologic calcification can be dystrophic or metastatic. Dystrophic calcification occurs in dying and dead tissues in areas of necrosis (see also the types of necrosis: coagulative, liquefactive, caseous, and fatty). It is present in chronic tuberculosis of the lungs and lymph nodes, advanced atherosclerosis (narrowing of the arteries as a result of plaque accumulation), and heart valve injury (Figure 4-28). Calcification of the heart valves interferes with their opening and closing, causing heart murmurs (see Chapter 24). Calcification of the coronary arteries predisposes them to severe narrowing and thrombosis, which can lead to myocardial infarction. Another site of dystrophic calcification is the center of tumors. Over time, the center is deprived of its oxygen supply, dies, and becomes calcified. The calcium salts appear as gritty, clumped granules that can become hard as stone. When several layers clump together, they resemble grains of sand and are called psammoma bodies.
FIGURE 4-28 Aortic Valve Calcification. A, This calcified aortic valve is an example of dystrophic calcification. B, This algorithm shows the dystrophic mechanism of calcification. (A
from Damjanov I: Pathology for the health professions, ed 4, St Louis, 2012, Saunders.)
Metastatic calcification consists of mineral deposits that occur in undamaged normal tissues as the result of hypercalcemia (excess calcium in the blood; see
Chapter 5). Conditions that cause hypercalcemia include hyperparathyroidism, toxic levels of vitamin D, hyperthyroidism, idiopathic hypercalcemia of infancy, Addison disease (adrenocortical insufficiency), systemic sarcoidosis, milk-alkali syndrome, and the increased bone demineralization that results from bone tumors, leukemia, and disseminated cancers. Hypercalcemia also may occur in advanced renal failure with phosphate retention. As phosphate levels increase, the activity of the parathyroid gland increases, causing higher levels of circulating calcium.
Urate In humans, uric acid (urate) is the major end product of purine catabolism because of the absence of the enzyme urate oxidase. Serum urate concentration is, in general, stable: approximately 5 mg/dl in postpubertal males and 4.1 mg/dl in postpubertal females. Disturbances in maintaining serum urate levels result in hyperuricemia and the deposition of sodium urate crystals in the tissues, leading to painful disorders collectively called gout. These disorders include acute arthritis, chronic gouty arthritis, tophi (firm, nodular, subcutaneous deposits of urate crystals surrounded by fibrosis), and nephritis (inflammation of the nephron). Chronic hyperuricemia results in the deposition of urate in tissues, cell injury, and inflammation. Because urate crystals are not degraded by lysosomal enzymes, they persist in dead cells.
Systemic Manifestations Systemic manifestations of cellular injury include a general sense of fatigue and malaise, a loss of well-being, and altered appetite. Fever is often present because of biochemicals produced during the inflammatory response. Table 4-10 summarizes the most significant systemic manifestations of cellular injury.
TABLE 4-10 Systemic Manifestations of Cellular Injury
Manifestation Cause Fever Release of endogenous pyrogens (interleukin-1, tumor necrosis factor-alpha, prostaglandins) from bacteria or
macrophages; acute inflammatory response Increased heart rate Increase in oxidative metabolic processes resulting from fever Increase in leukocytes (leukocytosis) Increase in total number of white blood cells because of infection; normal is 5000-9000/mm3 (increase is directly
related to severity of infection) Pain Various mechanisms, such as release of bradykinins, obstruction, pressure Presence of cellular enzymes Release of enzymes from cells of tissue* in extracellular fluid Lactate dehydrogenase (LDH) (LDH isoenzymes)
Release from red blood cells, liver, kidney, skeletal muscle
Creatine kinase (CK) (CK isoenzymes)
Release from skeletal muscle, brain, heart
Aspartate aminotransferase (AST/SGOT)
Release from heart, liver, skeletal muscle, kidney, pancreas
Alanine aminotransferase (ALT/SGPT)
Release from liver, kidney, heart
Alkaline phosphatase (ALP) Release from liver, bone Amylase Release from pancreas Aldolase Release from skeletal muscle, heart
*The rapidity of enzyme transfer is a function of the weight of the enzyme and the concentration gradient across the cellular membrane. The specific metabolic and excretory rates of the enzymes determine how long levels of enzymes remain elevated.
Cellular Death In response to significant external stimuli, cell injury becomes irreversible and cells are forced to die. Cell death has historically been classified as necrosis and apoptosis. Necrosis is characterized by rapid loss of the plasma membrane structure, swelling of organelles, dysfunction of the mitochondria, and lack of typical features of apoptosis.70 Apoptosis is known as a regulated or programmed cell process characterized by the “dropping off” of cellular fragments called apoptotic bodies. Too little or too much apoptosis is linked to many disorders, including neurodegenerative diseases, ischemic damage, autoimmune disorders, and cancers. Yet, apoptosis can have normal functions, and unlike necrosis it is not always linked with a pathologic process. Until recently, necrosis was only considered passive or accidental cell death occurring after severe and sudden injury. It is the main outcome in several common injuries including ischemia, toxin exposure, certain infections, and trauma. It has now been proposed that under certain conditions, such as activation of death proteases, necrosis may be regulated or programmed in a well-orchestrated way as a back-up for apoptosis (apoptosis may progress to necrosis)71—hence the new term programmed necrosis, or necroptosis. Necroptosis shares traits with both necrosis and apoptosis.72 Although the identification of the signaling mechanisms for necroptosis is incomplete, necroptosis is recognized in both normal physiologic conditions and pathologic conditions, including bone growth plate disorders, cell death in fatty liver disease, acute pancreatitis, reperfusion injury, and certain neurodegenerative disorders, such as Parkinson disease.1 Historically, programmed cell death only referred to apoptosis. Figure 4-29
illustrates the structural changes in cell injury resulting in necrosis or apoptosis. Table 4-11 compares the unique features of necrosis and apoptosis. Other forms of cell loss include autophagy (self-eating) (see p. 105).
FIGURE 4-29 Schematic Illustration of the Morphologic Changes in Cell Injury Culminating in Necrosis or Apoptosis. Myelin figures come from degenerating cellular membranes and are noted within the cytoplasm or extracellularly. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of
disease, ed 9, Philadelphia, 2015, Elsevier.)
TABLE 4-11 Features of Necrosis and Apoptosis
Feature Necrosis Apoptosis Cell size Enlarged (swelling) Reduced (shrinkage) Nucleus Pyknosis → karyorrhexis → karyolysis Fragmentation into nucleosome-size fragments Plasma membrane Disrupted Intact; altered structure, especially orientation of lipids Cellular contents Enzymatic digestion; may leak out of cell Intact; may be released in apoptotic bodies Adjacent inflammation
Frequent No
Physiologic or pathologic role
Invariably pathologic (culmination of irreversible cell injury)
Often physiologic, means of eliminating unwanted cells; may be pathologic after some forms of cell injury, especially DNA damage
From Kumar V et al: Cellular responses to stress and toxic insults: adaptation, injury, and death. In Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.
Necrosis
Cellular death eventually leads to cellular dissolution, or necrosis. Necrosis is the sum of cellular changes after local cell death and the process of cellular self- digestion, known as autodigestion or autolysis (see Figure 4-29). Cells die long before any necrotic changes are noted by light microscopy.71 The structural signs that indicate irreversible injury and progression to necrosis are dense clumping and progressive disruption both of genetic material and of plasma and organelle membranes. Because membrane integrity is lost, necrotic cell contents leak out and may cause the signaling of inflammation in surrounding tissue. In later stages of necrosis, most organelles are disrupted, and karyolysis (nuclear dissolution and lysis of chromatin from the action of hydrolytic enzymes) is under way. In some cells the nucleus shrinks and becomes a small, dense mass of genetic material (pyknosis). The pyknotic nucleus eventually dissolves (by karyolysis) as a result of the action of hydrolytic lysosomal enzymes on DNA. Karyorrhexis means fragmentation of the nucleus into smaller particles or “nuclear dust.” Although necrosis still refers to death induced by nonspecific trauma or injury
(e.g., cell stress or the heat shock response), with the very recent identification of molecular mechanisms regulating the process of necrosis, the study of necrosis has experienced a new twist. Unlike apoptosis, necrosis has been viewed as passive with cell death occurring in a disorganized and unregulated manner. Some molecular regulators governing programmed necrosis have been identified and demonstrated to be interconnected by a large network of signaling pathways.71,73 Emerging evidence shows that programmed necrosis is associated with pathologic diseases and provides innate immune response to viral infection.71,73 Different types of necrosis tend to occur in different organs or tissues and
sometimes can indicate the mechanism or cause of cellular injury. The four major types of necrosis are coagulative, liquefactive, caseous, and fatty. Another type, gangrenous necrosis, is not a distinctive type of cell death but refers instead to larger areas of tissue death. These necroses are summarized as follows:
1. Coagulative necrosis. Occurs primarily in the kidneys, heart, and adrenal glands; commonly results from hypoxia caused by severe ischemia or hypoxia caused by chemical injury, especially ingestion of mercuric chloride. Coagulation is a result of protein denaturation, which causes the protein albumin to change from a gelatinous, transparent state to a firm, opaque state (Figure 4-30, A). The area of coagulative necrosis is called an infarct.
FIGURE 4-30 Types of Necrosis. A, Coagulative necrosis. A wedge-shaped kidney infarct (yellow). B, Liquefactive necrosis of the brain. The area of infarction is softened as a result of liquefaction necrosis. C, Caseous necrosis. Tuberculosis of the lung, with a large area of caseous necrosis containing yellow-white and cheesy debris. D, Fat necrosis of pancreas.
Interlobular adipocytes are necrotic; acute inflammatory cells surround these. (A and C from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier. B from Damjanov I: Pathology for the health professions, ed 4, St Louis, 2012, Saunders. D from Damjanov I, Linder J, editors: Anderson's pathology, ed 10, St Louis, 1996, Mosby.)
2. Liquefactive necrosis. Commonly results from ischemic injury to neurons and glial cells in the brain (Figure 4-30, B). Dead brain tissue is readily affected by liquefactive necrosis because brain cells are rich in digestive hydrolytic enzymes and lipids and the brain contains little connective tissue. Cells are digested by their own hydrolases, so the tissue becomes soft, liquefies, and segregates from healthy tissue, forming cysts. This can be caused by bacterial infection, especially Staphylococci, Streptococci, and Escherichia coli.
3. Caseous necrosis. Usually results from tuberculous pulmonary infection, especially by Mycobacterium tuberculosis (Figure 4-30, C). It is a combination of coagulative and liquefactive necroses. The dead cells disintegrate, but the debris is not completely digested by the hydrolases. Tissues resemble clumped cheese in that they are soft and granular. A granulomatous inflammatory wall encloses areas of
caseous necrosis.
4. Fatty necrosis. Fat necrosis is cellular dissolution caused by powerful enzymes, called lipases, that occur in the breast, pancreas, and other abdominal structures (Figure 4-30, D). Lipases break down triglycerides, releasing free fatty acids that then combine with calcium, magnesium, and sodium ions, creating soaps (saponification). The necrotic tissue appears opaque and chalk-white.
5. Gangrenous necrosis. Refers to death of tissue but is not a specific pattern of cell death and results from severe hypoxic injury, commonly occurring because of arteriosclerosis, or blockage, of major arteries, particularly those in the lower leg (Figure 4-31). With hypoxia and subsequent bacterial invasion, the tissues can undergo necrosis. Dry gangrene is usually the result of coagulative necrosis. The skin becomes very dry and shrinks, resulting in wrinkles, and its color changes to dark brown or black. Wet gangrene develops when neutrophils invade the site, causing liquefactive necrosis. This usually occurs in internal organs, causing the site to become cold, swollen, and black. A foul odor is present, and if systemic symptoms become severe, death can ensue.
FIGURE 4-31 Gangrene, a Complication of Necrosis. In certain circumstances, necrotic tissue will be invaded by putrefactive organisms that are both saccharolytic and proteolytic. Foul-
smelling gases are produced, and the tissue becomes green or black as a result of breakdown of hemoglobin. Obstruction of the blood supply to the bowel almost inevitably is followed by
gangrene.
6. Gas gangrene. Refers to a special type of gangrene caused by infection of injured tissue by one of many species of Clostridium. These anaerobic bacteria produce hydrolytic enzymes and toxins that destroy connective tissue and cellular membranes and cause bubbles of gas to form in muscle cells. This can be fatal if enzymes lyse the membranes of red blood cells, destroying their oxygen-carrying capacity. Death is caused by shock.
Apoptosis Apoptosis (“dropping off”) is an important distinct type of cell death that differs from necrosis in several ways (see Figure 4-29 and Table 4-11). Apoptosis is an active process of cellular self-destruction called programmed cell death and is implicated in both normal and pathologic tissue changes. Cells need to die; otherwise, endless proliferation would lead to gigantic bodies. The average adult may create 10 billion new cells every day—and destroy the same number.74 Death by apoptosis causes loss of cells in many pathologic states including the following: • Severe cell injury. When cell injury exceeds repair mechanisms, the cell triggers apoptosis. DNA damage can result either directly or indirectly from production of free radicals.
• Accumulation of misfolded proteins. This may result from genetic mutations or free radicals. Excessive accumulation of misfolded proteins in the ER leads to a condition known as endoplasmic reticulum stress (ER stress) (see Chapter 1). ER stress results in apoptotic cell death. This mechanism has been linked to several degenerative diseases of the CNS and other organs (Figure 4-32).
FIGURE 4-32 The Unfolded Protein Response, Endoplasmic Stress, and Apoptosis. A, In normal or healthy cells the newly made proteins are folded with help from chaperones and then
incorporated into the cell or secreted. B, Various stressors can cause ER stress whereby the cell is challenged to cope with the increased load of misfolded proteins. The accumulation of the protein load initiates the unfolded protein response in the ER; if restoration of the protein fails, the cell dies by apoptosis. An example of a disease caused by misfolding of proteins is
Alzheimer disease. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
• Infections (particularly viral). Apoptosis may be the result of the virus directly or indirectly by the host immune response. Cytotoxic T lymphocytes respond to viral infections by inducing apoptosis and, therefore, eliminating the infectious cells. This process can cause tissue damage and it is the same for cell death in tumors and rejection of tissue transplants.
• Obstruction in tissue ducts. In organs with duct obstruction, including the pancreas, kidney, and parotid gland, apoptosis causes pathologic atrophy. Excessive or insufficient apoptosis is known as dysregulated apoptosis. A low
rate of apoptosis can permit the survival of abnormal cells, for example, mutated cells that can increase cancer risk. Defective apoptosis may not eliminate lymphocytes that react against host tissue (self-antigens), leading to autoimmune disorders. Excessive apoptosis is known to occur in several neurodegenerative diseases, from ischemic injury (such as myocardial infarction and stroke), and from death of virus-infected cells (such as seen in many viral infections). Apoptosis depends on a tightly regulated cellular program for its initiation and
execution.74 This death program involves enzymes that divide other proteins— proteases, which are activated by proteolytic activity in response to signals that induce apoptosis. These proteases are called caspases, a family of aspartic acid– specific proteases. The activated suicide caspases cleave and, thereby, activate other members of the family, resulting in an amplifying “suicide” cascade. The activated caspases then cleave other key proteins in the cell, killing the cell quickly and neatly. The two different pathways that converge on caspase activation are called the mitochondrial (intrinsic) pathway and the death receptor (extrinsic) pathway (Figure 4-33). Cells that die by apoptosis release chemical factors that recruit phagocytes that quickly engulf the remains of the dead cell, thus reducing chances of inflammation. With necrosis, cell death is not tidy because cells that die as a result of acute injury swell, burst, and spill their contents all over their neighbors, causing a likely damaging inflammatory response.
FIGURE 4-33 Mechanisms of Apoptosis. The two pathways of apoptosis differ in their induction and regulation, and both culminate in the activation of “executioner” caspases. The induction of
apoptosis by the mitochondrial pathway involves the Bcl-2 family, which causes leakage of mitochondrial proteins. The regulators of the death receptor pathway involve the proteases, called caspases. (Adapted from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
Autophagy The Greek term autophagy means “eating of self.” Autophagy, as a “recycling factory,” is a self-destructive process and a survival mechanism. Basically, autophagy involves the delivery of cytoplasmic contents to the lysosome for degradation. Box 4-3 contains the terms used to describe autophagy.
Box 4-3
The Major Forms of Autophagy
Macroautophagy, the most common term to refer to autophagy, involves the sequestration and transportation of parts (cargo) of the cytosol in an autophagic vacuole (autophagosome).
Microautophagy is the inward invagination of the lysosomal membrane for cargo delivery.
Chaperone-mediated autophagy is the chaperone-dependent proteins that direct cargo across the lysosomal membrane.
When cells are starved or nutrient deprived, the autophagic process institutes cannibalization and recycles the digested contents.48,75 Autophagy can maintain cellular metabolism under starvation conditions and remove damaged organelles under stress conditions, improving the survival of cells. With the central role of autophagy in cell homeostasis, autophagy has been implicated in cancer, heart disease, neurodegeneration diseases, inflammation, and infection.76 Autophagy begins with a membrane, also known as a phagophore (although controversial) (Figure 4-34).75 This cup-shaped, curved phagophore expands and engulfs intracellular cargo—organelles, ribosomes, proteins—forming a double membrane autophagosome. The cargo-laden autophagosome fuses with the lysosome, now called an autophagolysosome, which promotes the degradation of the autophagosome by lysosomal acid proteases. The phagophore membrane is highly curved along the rim of the open cup, suggesting that mechanisms responsible for its formation and growth may depend on membrane curvature-dependent events.77 Lysosomal transporters export amino acids and other byproducts of degradation out of the cytoplasm where they can be reused for the synthesis of macromolecules and for metabolism.78,79 ATP is generated and cellular damage is reduced during autophagy that removes nonfunctional proteins and organelles.75
FIGURE 4-34 Autophagy. Cellular stresses, such as nutrient deprivation, activate autophagy genes that create vacuoles in which cellular organelles are sequestered and then degraded
following fusion of the vesicles with lysosomes. The digested materials are recycled to provide nutrients for the cell.
Investigators are excited about the utilization of autophagy for therapeutic strategies. Autophagy is a critical garbage collecting and recycling process in healthy cells, and this process becomes less efficient and less discriminating as the cell ages. Consequently, harmful agents accumulate in cells, damaging cells and leading to aging: for example, failure to clear protein products in neurons of the CNS can cause dementia; failure to clear ROS-producing mitochondria can lead to nuclear DNA mutations and cancer. Thus these processes may even partially define aging. Therefore normal autophagy may potentially rejuvenate an organism and prevent cancer development as well as other degenerative diseases.80 In addition, autophagy may be the last immune defense against infectious microorganisms that penetrate intracellularly.81
Quick Check 4-4
1. Why is an increase in the concentration of intracellular calcium injurious?
2. Compare and contrast necrosis and apoptosis.
3. Why is apoptosis significant?
4. Define autophagy.
Aging and Altered Cellular and Tissue Biology The terms aging and life span tend to be used synonymously; however, they are not equivalent. Aging is usually defined as a normal physiologic process that is universal and inevitable, whereas life span is the time from birth to death and has been used to study the aging process.82 Aging is associated with a gradual loss of homeostatic mechanisms whose underlying cause is perplexing,83 and is a complex process because of a multiplicity of factors. Investigators are focused on genetic, epigenetic, inflammatory, oxidative stress, and metabolic origins of aging, including the study of genetic signatures in humans with exceptional longevity; the identification and recent discovery of epigenetic mechanisms that modulate gene expression; the role of intrauterine environment and lifelong patterns of health; the effects of personality, behavior, and social support; the influence of insulin/insulin- like growth factor 1 (IGF-1) signaling; and the contributions of cellular dysfunction and senescence to an inflammatory microenvironment that leads to chronic disease, frailty, and decreased life span. To focus more simply, the factors that may be most important for aging include increased damage to the cell, reduced capacity to divide (replicative senescence), reduced ability to repair damaged DNA, and increased likelihood of defective protein balance or homeostasis.1 A major challenge of aging research has been to separate the causes of cell and tissue aging from the vast changes that accompany it.83 Public health issues related to healthy aging require understanding of the nature of aging and the factors that predict healthy aging and delayed transition to increasing vulnerability and frailty. Aging traditionally has not been considered a disease because it is “normal”;
disease is usually considered “abnormal.” Conceptually, this distinction seems clear until the concept of “injury” or “damage” is introduced; disease has been defined by some pathologists as the result of injury. Chronologic aging has been defined as the time-dependent loss of structure and function that proceeds very slowly and in such small increments that it appears to be the result of the accumulation of small, imperceptible injuries—a gradual result of wear and tear. One of the hallmarks of aging is the accumulation of damaged macromolecules. DNA damage can lead to cellular dysfunction both directly and indirectly as a consequence of cellular responses to damage that can lead to altered gene expression.84,85 Age-related changes to macromolecules for long-lived cells, such as neurons and myofibers, lead to gradual loss of structure and function. Replicative aging or senescence is the accumulation of cellular damage in
continuously dividing cells, for example, epithelia of the skin or gastrointestinal
tract. One mechanism of replicative senescence is the progressive shortening of telomeres—the repeated sequences of DNA at the ends of chromosomes. Replicative aging and chronologic aging are particularly important for adult stem cells because they divide throughout life.86 As mutations increase with age, cell fates include apoptosis, malignant transformation, cell cycle arrest, or senescence.87 Despite the fact that aging and death are inevitable, life span, on the other hand,
can be experimentally changed.83 Genetic and environmental interventions have extended the life span of model organisms, such as the nematode worm Caenorhabditis elegans (C. elegans), the fruit fly Drosophilia melanogaster, and mice.88,89 Extending life span, however, is not equivalent to delaying aging!83 For example, treatment of an acute infection can prevent death but the fundamental rate of aging continues. Yet, investigators will study and try to isolate, manipulate, and reset so-called longevity genes to slow the rate of aging. Recent advances in stem cell biology have begun to reveal the molecular
mechanisms behind reprogramming events that occur during fertilization and when the nucleus of a mature somatic cell is transferred to an enucleated oocyte. Called somatic cell nuclear transfer (SCNT), this process gave rise to the first cloned mammal, Dolly the sheep, and lead to the explosion of research in cloning.83 SCNT is important in terms of demonstrating the ability of the oocyte cytoplasm to reprogram the donor nucleus. These reprogramming events have led to the process to create induced pluripotent stem cells (iPSCs).90 The major emphasis of reprogramming research is the reversal of the differentiated program and attainment of a pluripotent state (differentiated cells in all three germ layers of the embryo) and not the reversal of aging.83,91 Nevertheless, each of these processes is discussed in the context of resetting the aging clock. Restoration of youthfulness to aged cells and tissues has created so-called
rejuvenating interventions. Experiments to test whether cells and tissues from an old animal can be restored to a younger self include the approach called heterochronic (i.e., young-to-old or old-to-young) transplantations and heterochronic parabiosis, when the systemic circulations of two animals are joined. The systemic environment may become more youthful with restoration of protein components in the blood and tissues, especially chemokines and cytokines.92 For example, investigators found a protein, GDF-11, may reverse age-associated cardiac hypertrophy when injected into old animals.93 Administration of the drug rapamycin, an mTOR inhibitor, can extend the life
span of mice.94 These and future studies may not just change differentiation programs of cells and tissue, but also possibly alter the aging clock. Observations in C. elegans suggest strongly that the causes of aging may be largely epigenetic.83,95,96
Normal Life Span, Life Expectancy, and Quality- Adjusted Life Year The maximal life span of humans is between 80 and 100 years and does not vary significantly among populations. Life expectancy is the average number of years of life remaining at a given age, however, it does not include quality of life. The quality-adjusted life year (QALY) is a measure of disease burden including quality and not just quantity of live lived. The Centers for Disease Control and Prevention reported in 2009 that the overall life expectancy at birth was 78.5 years. Between 2008 and 2009, life expectancy at birth increased for all groups reviewed. It increased for males, from 75.6 to 76.0 years, and females, 80.6 to 80.9 years; for the white population, 78.5 to 78.8 years; the black population, 74.0 to 74.5 years; the Hispanic population, 81.0 to 81.2 years; the non-Hispanic white population, 78.4 to 78.7 years; and the non-Hispanic black population, 73.7 to 74.0 years.97
Degenerative Extracellular Changes Extracellular factors that affect the aging process include the binding of collagen; the increase in the effects of free radicals on cells; the structural alterations of fascia, tendons, ligaments, bones, and joints; and the development of peripheral vascular disease, particularly arteriosclerosis (see Chapter 24). Aging affects the extracellular matrix with increased cross-linking (e.g., aging
collagen becomes more insoluble, chemically stable but rigid, resulting in decreased cell permeability), decreased synthesis, and increased degradation of collagen. The extracellular matrix determines the tissue's physical properties.98 These changes, together with the disappearance of elastin and changes in proteoglycans and plasma proteins, cause disorders of the ground substance that result in dehydration and wrinkling of the skin (see Chapter 41). Other age-related defects in the extracellular matrix include skeletal muscle alterations (e.g., atrophy, decreased tone, loss of contractility), cataracts, diverticula, hernias, and rupture of intervertebral disks. Free radicals of oxygen that result from oxidative cellular metabolism, oxidative
stress (e.g., respiratory chain, phagocytosis, prostaglandin synthesis), damage tissues during the aging process. The oxygen radicals produced include superoxide radical, hydroxyl radical, and hydrogen peroxide (see p. 81). These oxygen products are extremely reactive and can damage nucleic acids, destroy polysaccharides, oxidize proteins, peroxidize unsaturated fatty acids, and kill and lyse cells. Oxidant effects on target cells can lead to malignant transformation, presumably through DNA damage. That progressive and cumulative damage from
oxygen radicals may lead to harmful alterations in cellular function is consistent with those alterations of aging. This hypothesis is founded on the wear-and-tear theory of aging, which states that damages accumulate with time, decreasing the organism's ability to maintain a steady state. Because these oxygen-reactive species not only can permanently damage cells but also may lead to cell death, there is new support for their role in the aging process. Of much interest is the relationship between aging and the disappearance or
alteration of extracellular substances important for vessel integrity. With aging, lipid, calcium, and plasma proteins are deposited in vessel walls. These depositions cause serious basement membrane thickening and alterations in smooth muscle functioning, resulting in arteriosclerosis (a progressive disease that causes such problems as stroke, myocardial infarction, renal disease, and peripheral vascular disease).
Cellular Aging Cellular changes characteristic of aging include atrophy, decreased function, and loss of cells, possibly caused by apoptosis (Figure 4-35). Loss of cellular function from any of these causes initiates the compensatory mechanisms of hypertrophy and hyperplasia of the remaining cells, which can lead to metaplasia, dysplasia, and neoplasia. All of these changes can alter receptor placement and function, nutrient pathways, secretion of cellular products, and neuroendocrine control mechanisms. In the aged cell, DNA, RNA, cellular proteins, and membranes are most susceptible to injurious stimuli. DNA is particularly vulnerable to such injuries as breaks, deletions, and additions. Lack of DNA repair increases the cell's susceptibility to mutations that may be lethal or may promote the development of neoplasia (see Chapter 10).
FIGURE 4-35 Some Biologic Changes Associated with Aging. Insets show the proportion of remaining functions in the organs of a person in late adulthood compared with those of a 20-
year-old.
Mitochondria are the organelles responsible for the generation of most of the energy used by eukaryotic cells. Mitochondrial DNA (mtDNA) encodes some of the proteins of the electron-transfer chain, the system necessary for the conversion of adenosine diphosphate (ADP) to ATP. Mutations in mtDNA can deprive the cell of ATP, and mutations are correlated with the aging process. The accumulation of mutations could be caused by errors in replication or by unrepaired damage.99,100 The most common age-related mtDNA mutation in humans is a large
rearrangement called the 4977 deletion, or common deletion, and is found in humans older than 40 years. It is a deletion that removes all or part of 7 of the 13 protein- encoding mtDNA genes and 5 of the 22 tRNA genes. Individual cells containing this deletion have a condition known as heteroplasmy. Heteroplasmy levels rise with aging. Cumulative damage of mtDNA is implicated in the progression of such common diseases as diabetes, cancer, heart failure, and neurodegenerative
disorders.
Tissue and Systemic Aging It is probably safe to say that every physiologic process functions less efficiently with increasing age. The most characteristic tissue change with age is a progressive stiffness or rigidity that affects many systems, including the arterial, pulmonary, and musculoskeletal systems. A consequence of blood vessel and organ stiffness is a progressive increase in peripheral resistance to blood flow. The movement of intracellular and extracellular substances also decreases with age, as does the diffusion capacity of the lung. Blood flow through organs also decreases. Changes in the endocrine and immune systems include thymus atrophy. Although
this occurs at puberty, causing a decreased immune response to T-dependent antigens (foreign proteins), increased formation of autoantibodies and immune complexes (antibodies that are bound to antigens) and an overall decrease in the immunologic tolerance for the host's own cells further diminish the effectiveness of the immune system later in life. In women the reproductive system loses ova, and in men spermatogenesis decreases. Responsiveness to hormones decreases in the breast and endometrium. The stomach experiences decreases in the rate of emptying and secretion of
hormones and hydrochloric acid. Muscular atrophy diminishes mobility by decreasing motor tone and contractility. Sarcopenia, loss of muscle mass and strength, can occur into old age. The skin of the aged individual is affected by atrophy and wrinkling of the epidermis and by alterations in the underlying dermis, fat, and muscle. Total body changes include a decrease in height; a reduction in circumference of
the neck, thighs, and arms; widening of the pelvis; and lengthening of the nose and ears. Several of these changes are the result of tissue atrophy and of decreased bone mass caused by osteoporosis and osteoarthritis. Some body composition changes include an increase in body weight, which begins in middle age (men gain until 50 years of age and women until 70 years), and an increase fat mass followed by a decrease in stature, weight, fat-free mass, and body cell mass at older ages. Fat-free mass (FFM) includes all minerals, proteins, and water plus all other constituents except lipids. As the amount of fat increases, the percentage of total body water decreases. Increased body fat and centralized fat distribution (abdominal area) are associated with non–insulin-dependent diabetes and heart disease. Total body potassium concentration also decreases because of decreased cellular mass. An increased sodium/potassium ratio suggests that the decreased cellular mass is accompanied by an increased extracellular compartment.
Although some of these alterations are probably inherent in aging, others represent consequences of the process. Advanced age increases susceptibility to disease, and death occurs after an injury or insult because of diminished cellular, tissue, and organ function.
Frailty Frailty is a common clinical syndrome in older adults, leaving a person vulnerable to falls, functional decline, disability, disease, and death. With an increasing aged population worldwide efforts to promote independence and decrease frailty are challenging and needed. Sarcopenia and cachexia are common as a consequence of aging and many acute and chronic illnesses.101 Investigators are grappling with a common nomenclature to develop consensus for definitions of sarcopenia and cachexia. One proposal has been to define it simply as “muscle wasting disease,” which can be applied in both acute and chronic settings.101 An acceptable vocabulary and classification system is yet to be developed. The determinants of sarcopenia include environmental and genetic factors, which
presently are poorly understood.102 Common themes of mechanisms for sarcopenia include the following: (1) decrease in the number of skeletal muscle fibers, mainly type II fibers; (2) decline in muscle protein synthesis with age; (3) decline in muscle fractions, such as myofibrillar and mitochondrial, with age; (4) reduction in protein turnover adversely affecting muscle function by inducing protein loss and protein accumulation; (5) loss of alpha motor neurons in the spinal column; (6) dysregulation of anabolic hormones; (7) cytokine productions and inflammation; (8) inadequate nutrition; and (9) sedentary history.102,103 For research and clinical purposes, the criteria indicating compromised energetics include low grip strength, slowed walking speed, low physical activity, and unintentional weight loss.104 The syndrome is complex and involves other alterations such as osteopenia, cognitive impairment, anemia, and gender differences.
Somatic Death Somatic death is death of the entire person. Unlike the changes that follow cellular death in a live body, postmortem change is diffuse and does not involve components of the inflammatory response. Within minutes after death, postmortem changes appear, eliminating any difficulty in determining that death has occurred. The most notable manifestations are complete cessation of respiration and circulation. The surface of the skin usually becomes pale and yellowish; however, the lifelike color of the cheeks and lips may persist after death that is caused by carbon monoxide poisoning, drowning, or chloroform poisoning.105 Body temperature falls gradually immediately after death and then more rapidly
(approximately 1.0° to 1.5° F/hour) until, after 24 hours, body temperature equals that of the environment.106 After death caused by certain infective diseases, body temperature may continue to rise for a short time. Postmortem reduction of body temperature is called algor mortis. Blood pressure within the retinal vessels decreases, causing muscle tension to
decrease and the pupils to dilate. The face, nose, and chin become sharp or peaked- looking as blood and fluids drain from these areas.105 Gravity causes blood to settle in the most dependent, or lowest, tissues, which develop a purple discoloration called livor mortis. Incisions made at this time usually fail to cause bleeding. The skin loses its elasticity and transparency. Within 6 hours after death, acidic compounds accumulate within the muscles
because of the breakdown of carbohydrates and the depletion of ATP. This interferes with ATP-dependent detachment of myosin from actin (contractile proteins), and muscle stiffening, or rigor mortis, develops. The smaller muscles are usually affected first, particularly the muscles of the jaw. Within 12 to 14 hours, rigor mortis usually affects the entire body. Signs of putrefaction are generally obvious about 24 to 48 hours after death.
Rigor mortis gradually diminishes, and the body becomes flaccid at 36 to 62 hours. Putrefactive changes vary depending on the temperature of the environment. The most visible is greenish discoloration of the skin, particularly on the abdomen. The discoloration is thought to be related to the diffusion of hemolyzed blood into the tissues and the production of sulfhemoglobin, choleglobin, and other denatured hemoglobin derivatives.106,107 Slippage or loosening of the skin from underlying tissues occurs at the same time. After this, swelling or bloating of the body and liquefactive changes occur, sometimes causing opening of the body cavities. At a microscopic level, putrefactive changes are associated with the release of enzymes and lytic dissolution called postmortem autolysis.
Quick Check 4-5
1. Aging is a complex process, discuss the multitude of mechanisms of aging.
2. What are the body composition changes that occur with aging?
3. Define frailty and possible endocrine-immune system involvement.
Did You Understand? Cellular Adaptation 1. Cellular adaptation is a reversible, structural, or functional response both to normal or physiologic conditions and to adverse or pathologic conditions. Cells can adapt to physiologic demands or stress to maintain a steady state called homeostasis.
2. The most significant adaptive changes include atrophy, hypertrophy, hyperplasia, and metaplasia.
3. Atrophy is a decrease in cellular size caused by aging, disuse, or reduced/absent blood supply, hormonal stimulation, or neural stimulation. The amounts of ER, mitochondria, and microfilaments decrease. The mechanisms of atrophy probably include decreased protein synthesis, increased protein catabolism, or both. A new hypothesis called ribosome biogenesis involves the role of mRNA and protein translation.
4. Hypertrophy is an increase in the size of cells in response to mechanical stimuli and consequently increases the size of the affected organ. The amounts of protein in the plasma membrane, ER, microfilaments, and mitochondria increase. Hypertrophy can be classified as physiologic or pathologic.
5. Hyperplasia is an increase in the number of cells caused by an increased rate of cellular division. Hyperplasia is classified as physiologic (compensatory and hormonal) and pathologic.
6. Metaplasia is the reversible replacement of one mature cell type by another less mature cell type.
7. Dysplasia, or atypical hyperplasia, is an abnormal change in the size, shape, and organization of mature tissue cells. It is considered atypical rather than a true adaptational change.
Cellular Injury 1. Injury to cells and to the extracellular matrix (ECM) leads to injury of tissues and organs and ultimately determining the structural patterns of disease. Cellular injury
occurs if the cell is unable to maintain homeostasis—a normal or adaptive steady state—in the face of injurious stimuli or stress. Injured cells may recover (reversible injury) or die (irreversible injury).
2. Injury is caused by lack of oxygen (hypoxia), free radicals, caustic or toxic chemicals, infectious agents, inflammatory and immune responses, genetic factors, insufficient nutrients, or physical and mechanical trauma from many causes.
3. Four biochemical themes are important to cell injury: (1) ATP depletion, resulting in mitochondrial damage; (2) accumulation of oxygen and oxygen-derived free radicals, causing membrane damage; (3) protein folding defects; and (4) increased intracellular calcium concentration and loss of calcium steady state.
4. The sequence of events leading to cell death is commonly decreased ATP production, failure of active transport mechanisms (the sodium-potassium pump), cellular swelling, detachment of ribosomes from the ER, cessation of protein synthesis, mitochondrial swelling as a result of calcium accumulation, vacuolation, leakage of digestive enzymes from lysosomes, autodigestion of intracellular structures, lysis of the plasma membrane, and death.
5. The initial insult in hypoxic injury is usually ischemia (the cessation of blood flow into vessels that supply the cell with oxygen and nutrients).
6. Free radicals cause cellular injury because they have an unpaired electron that makes the molecule unstable. To stabilize itself, the molecule either donates or accepts an electron from another molecule. Therefore it forms injurious chemical bonds with proteins, lipids, and carbohydrates—key molecules in membranes and nucleic acids.
7. The damaging effects of free radicals, especially activated oxygen species such as , OH•, and H2O2, called oxidative stress, include (1) peroxidation of lipids, (2)
alteration of ion pumps and transport mechanisms, (3) fragmentation of DNA, and (4) damage to mitochondria, releasing calcium into the cytosol.
8. Restoration of oxygen, however, can cause additional injury, called reperfusion injury. The mechanisms discussed for reperfusion-injury include oxidative stress, increased intracellular calcium concentration, inflammation, and complement activation.
9. Humans are exposed to thousands of chemicals that have inadequate toxicologic
data. A systems biology approach is now being used to investigate toxicity pathways that include oxidative stress, heat shock proteins, DNA damage response, hypoxia, ER stress, mental stress, inflammation, and osmotic stress.
10. Unintentional and intentional injuries are an important health problem in the United States. Death as a result of these injuries is more common for men than women and higher among blacks than whites and other racial groups.
11. Injuries by blunt force are the result of the application of mechanical energy to the body, resulting in tearing, shearing, or crushing of tissues. The most common types of blunt-force injuries include motor vehicle accidents and falls.
12. A contusion is bleeding into the skin or underlying tissues as a consequence of a blow. A collection of blood in soft tissues or an enclosed space may be referred to as a hematoma.
13. An abrasion (scrape) results from removal of the superficial layers of the skin caused by friction between the skin and injuring object. Abrasions and contusions may have a patterned appearance that mirrors the shape and features of the injuring object.
14. A laceration is a tear or rip resulting when the tensile strength of the skin or tissue is exceeded.
15. An incised wound is a cut that is longer than it is deep. A stab wound is a penetrating sharp-force injury that is deeper than it is long.
16. Gunshot wounds may be either penetrating (bullet retained in the body) or perforating (bullet exits the body). The most important factors determining the appearance of a gunshot injury are whether it is an entrance or an exit wound and the range of fire.
17. Asphyxial injuries are caused by a failure of cells to receive or utilize oxygen. These injuries can be grouped into four general categories: suffocation, strangulation, chemical, and drowning.
18. Activation of inflammation and immunity, which occurs after cellular injury or infection, involves powerful biochemicals and proteins capable of damaging normal (uninjured and uninfected) cells.
19. Genetic disorders injure cells by altering the nucleus and the plasma membrane's structure, shape, receptors, or transport mechanisms.
20. Deprivation of essential nutrients (proteins, carbohydrates, lipids, vitamins) can cause cellular injury by altering cellular structure and function, particularly of transport mechanisms, chromosomes, the nucleus, and DNA.
21. Injurious physical agents include temperature extremes, changes in atmospheric pressure, ionizing radiation, illumination, mechanical stresses, and noise.
22. Errors in health care are a leading cause of injury or death in the United States. Errors involve medicines, surgery, diagnosis, equipment, and laboratory reports. They can occur anywhere in the healthcare system including hospitals, clinics, outpatient surgery centers, physicians' and nurse practitioners' offices, pharmacies, and the individual's home.
Manifestations of Cellular Injury 1. An important manifestation of cell injury is the resultant metabolic disturbances of intracellular accumulation (infiltration) of abnormal amounts of various substances. Two categories of accumulations are (1) normal cellular substances, such as water, proteins, lipids, and carbohydrate excesses; and (2) abnormal substances, either endogenous (e.g., from abnormal metabolism) or exogenous (e.g., a virus).
2. Most accumulations are attributed to four types of mechanisms, all abnormal: (1) An endogenous substance is produced in excess or at an increased rate; (2) an abnormal substance, often the result of a mutated gene, accumulates; (3) an endogenous substance is not effectively catabolized; and (4) a harmful exogenous substance accumulates because of inhalation, ingestion, or infection.
3. Accumulations harm cells by “crowding” the organelles and by causing excessive (and sometimes harmful) metabolites to be produced during their catabolism. The metabolites are released into the cytoplasm or expelled into the extracellular matrix.
4. Cellular swelling, the accumulation of excessive water in the cell, is caused by the failure of transport mechanisms and is a sign of many types of cellular injury. Oncosis is a type of cellular death resulting from cellular swelling.
5. Accumulations of organic substances—lipids, carbohydrates, glycogen, proteins,
pigments—are caused by disorders in which (1) cellular uptake of the substance exceeds the cell's capacity to catabolize (digest) or use it or (2) cellular anabolism (synthesis) of the substance exceeds the cell's capacity to use or secrete it.
6. Dystrophic calcification (accumulation of calcium salts) is always a sign of pathologic change because it occurs only in injured or dead cells. Metastatic calcification, however, can occur in uninjured cells in individuals with hypercalcemia.
7. Disturbances in urate metabolism can result in hyperuricemia and deposition of sodium urate crystals in tissue—leading to a painful disorder called gout.
8. Systemic manifestations of cellular injury include fever, leukocytosis, increased heart rate, pain, and serum elevations of enzymes in the plasma.
Cellular Death 1. Cellular death has historically been classified as necrosis and apoptosis. Necrosis is characterized by rapid loss of the plasma membrane structure, organelle swelling, mitochondrial dysfunction, and the lack of features of apoptosis. Apoptosis is known as regulated or programmed cell death and is characterized by “dropping off” of cellular fragments, called apoptotic bodies. It is now understood that under certain conditions necrosis is regulated or programmed, hence the new term programmed necrosis, or necroptosis.
2. There are four major types of necrosis: coagulative, liquefactive, caseous, and fatty. Different types of necrosis occur in different tissues.
3. Structural signs that indicate irreversible injury and progression to necrosis are the dense clumping and disruption of genetic material and the disruption of the plasma and organelle membranes.
4. Apoptosis, a distinct type of sublethal injury, is a process of selective cellular self-destruction that occurs in both normal and pathologic tissue changes.
5. Death by apoptosis causes loss of cells in many pathologic states including (1) severe cell injury, (2) accumulation of misfolded proteins, (3) infections, and (4) obstruction in tissue ducts.
6. Excessive accumulation of misfolded proteins in the ER leads to a condition
known as endoplasmic reticulum stress. ER stress results in apoptotic cell death and this mechanism has been linked to several degenerative diseases of the CNS and other organs.
7. Excessive or insufficient apoptosis is known as dysregulated apoptosis.
8. Autophagy means “eating of self,” and as a recycling factory it is a self- destructive process and a survival mechanism. When cells are starved or nutrient deprived, the autophagic process institutes cannibalization and recycles the digested contents. Autophagy can maintain cellular metabolism under starvation conditions and remove damaged organelles under stress conditions, improving the survival of cells. Autophagy declines and becomes less efficient as the cell ages, thus contributing to the aging process.
9. Gangrenous necrosis, or gangrene, is tissue necrosis caused by hypoxia and the subsequent bacterial invasion.
Aging and Altered Cellular and Tissue Biology 1. It is difficult to determine the physiologic (normal) from the pathologic changes of aging. Investigators are focused on genetic, epigenetic, inflammatory, oxidative stress, and metabolic origins of aging.
2. Important factors in aging include increased damage to the cell, reduced capacity to divide, reduced ability to repair damaged DNA, and increased likelihood of defective protein balance or homeostasis.
3. Frailty is a common clinical syndrome in older adults, leaving a person vulnerable to falls, functional decline, disability, disease, and death. Sarcopenia and cachexia are common as a consequence of aging.
Somatic Death 1. Somatic death is death of the entire organism. Postmortem change is diffuse and does not involve the inflammatory response.
2. Manifestations of somatic death include cessation of respiration and circulation, gradual lowering of body temperature, dilation of the pupils, loss of elasticity and transparency in the skin, stiffening of the muscles (rigor mortis), and discoloration
of the skin (livor mortis). Signs of putrefaction are obvious about 24 to 48 hours after death.
Key Terms Adaptation, 73
Aging, 107
Algor mortis, 109
Anoxia, 80
Anthropogenic, 93
Apoptosis, 104
Asphyxial injury, 94
Atrophy, 74
Autolysis, 102
Autophagic vacuole, 74
Autophagy, 105
Bilirubin, 100
Carbon monoxide (CO), 90
Carboxyhemoglobin, 90
Caseous necrosis, 103
Caspase, 105
Cellular accumulations (infiltrations), 96
Cellular swelling, 97
Chemical asphyxiant, 96
Choking asphyxiation, 94
Coagulative necrosis, 102
Compensatory hyperplasia, 76
Cyanide, 96
Cytochrome, 100
Disuse atrophy, 74
Drowning, 96
Dry-lung drowning, 96
Dysplasia (atypical hyperplasia), 77
Dystrophic calcification, 100
Electrophile, 84
Endoplasmic reticulum stress (ER stress), 104
Ethanol, 90
Fat-free mass (FFM), 109
Fatty change (steatosis), 98
Fatty necrosis, 103
Fetal alcohol syndrome, 92
Frailty, 109
Free radical, 81
Gangrenous necrosis, 104
Gas gangrene, 104
Hanging strangulation, 95
Hemoprotein, 100
Hemosiderin, 100
Hemosiderosis, 100
Hormonal hyperplasia, 76
Hydrogen sulfide, 96
Hyperplasia, 76
Hypertrophy, 75
Hypoxia, 78
Hypoxia-inducible factor (HIF), 79
Infarct, 103
Irreversible injury, 78
Ischemia, 79
Ischemia-reperfusion injury, 81
Karyolysis, 102
Karyorrhexis, 102
Lead, 87
Life expectancy, 108
Life span, 107
Ligature strangulation, 96
Lipid peroxidation, 82
Lipofuscin, 75
Liquefactive necrosis, 103
Livor mortis, 109
Manual strangulation, 96
Maximal life span, 108
Melanin, 99
Mesenchymal (tissue from embryonic mesoderm) cell, 77
Metaplasia, 77
Metastatic calcification, 100
Mitochondrial DNA (mtDNA), 109
Necrosis, 102
Nucleophile, 84
Oncosis (vacuolar degeneration), 97
Oxidative stress, 81
Pathologic atrophy, 74
Pathologic hyperplasia, 76
Physiologic atrophy, 74
Postmortem autolysis, 110
Postmortem change, 109
Programmed necrosis (necroptosis), 101
Protein adduct, 85
Proteasome, 74
Psammoma body, 100
Pyknosis, 102
Reperfusion injury, 81
Reversible injury, 78
Rigor mortis, 110
Sarcopenia, 109
Somatic death, 109
Strangulation, 95
Suffocation, 94
Toxicophore, 84
Ubiquitin, 74
Ubiquitin-proteasome pathway, 74
Urate, 101
Vacuolation, 81
Xenobiotic, 84
References 1. Kumar V, et al. Pathology. Elsevier: St Louis; 2014. 2. Chaillou T, et al. Ribosome biogenesis: emerging evidence for a central role in the regulation of skeletal muscle mass. J Cell Physiol. 2014 [Mar 7; Epub ahead of print].
3. Lam MPY, et al. Protein kinetic signatures of the remodeling heart following isoproterenol stimulation. J Clin Invest. 2014;124(4):1734–1744.
4. Cacciapuoti F. Role of ubiquitin-proteosome system (UPS) in left ventricular hypertrophy (LVH). Am J Cardiovasc Dis. 2014;4(1):1–5.
5. Hill JA, Olson EN. Cardiac plasticity. N Engl J Med. 2008;358(13):1370– 1380.
6. Leri A, et al. Role of cardiac stem cells in cardiac pathophysiology: a paradigm shift in human myocardial biology. Circ Res. 2011;109:941–961.
7. Hariharan N, et al. Autophagy plays an essential role in mediating regression of hypertrophy during unloading of the heart. PLos One. 2013;8(1):e51632.
8. Zafeiridis A, et al. Regression of cellular hypertrophy after left ventricular assist device support. Circulation. 1998;98:656d–662d.
9. Depre C, et al. Unloaded heart in vivo replicates fetal gene expression of cardiac hypertrophy. Nat Med. 1998;4:1269–1275.
10. Friddle CJ, et al. Expression profiling reveals distinct sets of genes altered during induction and regression of cardiac hypertrophy. Proc Natl Acad Sci U S A. 2000;97:6745–6750.
11. Jiang Y, et al. Dietary copper supplementation reverses hypertrophic cardiomyopathy induced by chronic pressure overload in mice. J Exp Med. 2007;204:657–666.
12. Zhou Y, et al. Copper reverses cardiomyocyte hypertrophy through vascular endothelial growth factor-mediated reduction in the cell size. J Mol Cell Cardiol. 2008;45:106–117.
13. Zhou Y, et al. Copper-induced regression of cardiomyocyte hypertrophy is associated with enhanced vascular endothelial growth factor receptor-1 signalling pathway. Cardiovasc Res. 2009;84:54–63.
14. Alberts B, et al. Essential cell biology. ed 4. Garland Press: New York; 2014. 15. Hamanaka RB, Chandel NS. Mitochondrial reactive oxygen species regulate
hypoxic signaling. Curr Opin Cell Biol. 2009;21(6):894–899. 16. Eltzschig HK, Carmeliet P. Hypoxia and inflammation. N Engl J Med.
2011;364(7):656–665. 17. Choi HJ, et al. ECM-dependent HIF induction directs trophoblast stem cell
fate via LIMK1-mediated cytoskeletal rearrangement. PLoS One. 2013;8(2):e56949.
18. Choksi S, et al. A HIF-1, ATIA, protects cells from apoptosis by modulating the mitochondrial thioredoxin, TRX2. Mol Cell. 2011;42(5):597–609.
19. Wang ZJ, et al. The effect of intravenous vitamin C infusion on periprocedural myocardial injury for patients undergoing elective percutaneous coronary intervention. Can J Cardiol. 2014;30(1):96–101.
20. Rodrigo R, et al. The effectiveness of antioxidant vitamins C and E in reducing myocardial infarct size in patients subjected to percutaneous coronary angioplasty (PREVEC Trial): study protocol for a pilot randomized double-blind controlled trial. Trials. 2014;15:192.
21. Oka T, et al. Mitochondrial DNA that escapes from autophagy causes inflammation and heart failure. Nature. 2012;485(7397):251–255.
22. Tice RR, et al. Improving the human hazard characterization of chemicals: a TOX21 update. Environ Health Perspect. 2013;121:756–765.
23. Seeff LB. Herbal hepatotoxicity. Clin Liver Dis. 2007;11:577–596. 24. Carithers RL Jr, McClain CJ. Alcoholic liver disease. Sleisenger MH, et al.
Sleisenger and Fordtran's gastrointestinal and liver disease: pathophysiology, diagnosis, management. ed 9. Saunders/Elsevier: Philadelphia; 2010:1383–1400.
25. Gu X, Manautou JE. Molecular mechanisms underlying chemical liver injury. Exp Rev Mol Med. 2013;14:e4.
26. Jones DP, Delong MJ. Detoxification and protective functions of nutrients. Stipanuk M. Biochemical and physiological aspects of nutrition. Saunders: Philadelphia; 2000.
27. Hanzlik RP, et al. The reactive metabolite target protein database (TPDB)—a web-accessible resource. BMC Bioinformatics. 2007;8:95.
28. Liebler DC, Guengerich FP. Elucidating mechanisms of drug-induced toxicity. Nature. 2005;4:410–420.
29. Gunnell D, et al. Use of paracetamol (acetaminophen) for suicide and nonfatal poisoning: worldwide patterns of use and misuse. Suicide Life Threat Behav. 2000;30:313–326.
30. Grespan R, et al. Hepatoprotective effect of pretreatment with thymus vulgaris essential oil in experimental model of acetaminophen-induced injury. Evid Based Complement Alternat Med. 2014;2014:954136.
31. Occupational Safety and Health Administration. Lead. U.S. Department of Labor: Washington, DC; 2012.
32. Murata K, et al. Lead toxicity: does the critical level of lead resulting in adverse effects differ between adults and children? J Occup Health.
2009;51(1):1–12. 33. Gibson JL. A plea for painted railings and painted walls of rooms as the
source of lead poisoning amongst Queensland children, 1904. Public Health Rep. 2005;120:301.
34. Gilbert SG, Weiss B. A rationale for lowering the blood lead action level from 10 to 2 microg/dL. Neurotoxicol. 2006;27:693.
35. Jacobs DE, et al. The prevalence of lead-based paint hazards in U.S. housing. Environ Health Perspect. 2002;110:599.
36. Neal AP, Guilarte TR. Mechanisms of lead and manganese neurotoxicity. Toxicol Res. 2013;2:99–114.
37. Nichani WI, et al. Blood lead levels in children after phase-out of leaded gasoline in Bombay, India. Sci Total Environ. 2006;363:95.
38. Pirkle L, et al. Exposure of the U.S. population to lead, 1991-1994. Environ Health Perspect. 1998;106:745.
39. Stromberg U, et al. Yearly measurements of blood lead in Swedish children since 1978: the declining trend continues in the petrol-lead-free period 1995-2007. Environ Res. 2008;107:332.
40. Luo XS, et al. Distribution, availability, and sources of trace metals in different particle size fractions of urban soils in Hong Kong: implications for assessing the risk to human health. Environ Pollut. 2011;159:1317.
41. United States Environmental Protection Agency. Final Revisions to the National Quality Air Standards for Lead. [Lead fact sheet] Author: Washington, DC; 2008 [Available at] www.epa.gov/air/lead/pdfs/20081015pbfactsheet.pdf.
42. Centers for Disease Control and Prevention (CDC). Blood lead levels in children aged 1-5 years—United States, 1999-2010. CDC: Atlanta, GA; 2013.
43. Morris RG, et al. Selective impairment of learning and blockade of long- term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5. Nature. 1986;319(6056):774–776.
44. Konur S, Ghosh A. Calcium signaling and the control of dendritic development. Neuron. 2005;46:401–405.
45. Waites CL, Garner CC. Presynaptic function in health and disease. Trends Neurosci. 2011;34:326–337.
46. Jomova K, Valko M. Advances in metal-induced oxidative stress and human disease. Toxicology. 2011;283:65–87.
47. Abadin H, et al. Toxicological profile for lead. Agency for Toxic Substances and Disease Registry (US): Atlanta, GA; 2007.
48. Kumar V, et al. Robbins and Cotran pathologic basis of disease. ed 9.
Saunders/Elsevier: St Louis; 2015. 49. Romanoff R, et al. Acute ethanol exposure inhibits renal folate transport, but
repeated exposure upregulates folate transport proteins in rats and human cells. J Nutr. 2007;137:1260–1265.
50. Tsai ST, et al. The interplay between alcohol consumption, oral hygiene, ALDH2, and ADH1B in the risk of head and neck cancer. Int J Cancer. 2014 [Apr10; Epub ahead of print].
51. Hines LM, et al. Alcoholism: the dissection for endophenotypes. Dialogues Clin Neurosci. 2005;7(2):153–163.
52. Rostron B. Epidemiology alcohol consumption and mortality risks in the USA. Alcohol Alcohol. 2012;47(3):334–339.
53. Connor J. The life and times of the j-shaped curve. Alcohol Alcohol. 2006;41:583–584.
54. Krenz M, Korthius RJ. Moderate ethanol ingestion and cardiovascular protection: from epidemiologic associations to cellular mechanisms. J Mol Cell Cardiol. 2013;52(1):93–104.
55. American Heart Association. Alcohol and heart disease. Author: Dallas, TX; 2014.
56. Costanzo S, et al. Cardiovascular and overall mortality risk in relation to alcohol consumption in patients with cardiovascular disease. Circulation. 2010;121:1951–1959.
57. Spörndly-Nees S, et al. Leisure-time physical activity predicts complaints of snoring in women: a prospective cohort study over 10 years. Sleep Med. 2014;15:415–421.
58. Nassir F, Ibdah JA. Role of mitochondria in alcoholic liver disease. World J Gastroenterol. 2014;20(9):2136–2142.
59. Leiber CS. Metabolism of alcohol. Clin Liver Dis. 2005;9(1):1–35. 60. Curtis BJ, et al. Epigenetic targets for reversing immune defects caused by
alcohol exposure. Alcohol Res. 2013;35(1):97–113. 61. Sadrian B, et al. Long-lasting circuit dysfunction following developmental
ethanol exposure. Brain Sci. 2013;3(2):704–727. 62. Govorko D, et al. Male germline transmits fetal alcohol adverse effect on
hypothalamic proopiomelanocortin gene across generations. Biol Psychiatry. 2012;72:378–388.
63. Burd L, et al. Prenatal alcohol exposure, blood alcohol concentrations and alcohol elimination rates for the mother, fetus and newborn. J Perinatol. 2012;32(9):652–659.
64. UNEP. Global mercury assessment 2013: sources, emissions, releases and environmental transport. UNEP Chemicals Branch: Geneva, Switzerland;
2013. 65. Mortazavi SM, et al. High-field MRI and mercury release from dental
amalgam fillings. Int J Occup Med. 2014;5(2):101–105. 66. Geier DA, et al. A dose-dependent relationship between mercury exposure
from dental amalgams and urinary mercury levels: a further assessment of the Casa Pia Children's Dental Amalgam Trial. Hum Exp Toxicol. 2012;31(1):11–17.
67. Homme KG, et al. New science challenges old notion that mercury dental amalgam is safe. Biometals. 2014;27(1):19–24.
68. Schecter R, Grether JK. Continuing increases in autism reported to California's developmental services system: mercury in retrograde. Arch Gen Psychiatry. 2008;65(1):19–24.
69. Centers for Disease Control and Prevention: CDC 24/7. Saving lives, protecting people. All injuries. Health, United States. 2013.
70. Hitomi J, et al. Identification of a molecular signaling network that regulates a cellular necrotic cell death pathway by a genome wide siRNA screen. Cell. 2008;135(7):1311–1323.
71. Moquin D, Chan F. The molecular regulation of programmed necrotic cell injury. Trends Biochem Sci. 2010;35(8):434–441.
72. Smith CC, Yellon DM. Necroptosis, necrostatins and tissue injury. J Cell Mol Med. 2011;15(9):1797–1806.
73. Wang Z, et al. The mitochondrial phosphatase PGAM5 functions at the convergence point of multiple necrotic death pathways. Cell. 2012;148:228– 243.
74. Raloff J. Coming to terms with death: accurate descriptions of a cell's demise may offer clues to diseases and treatments. Sci News. 2001;159:378– 380.
75. Glick D, et al. Autophagy: cellular and molecular mechanisms. J Pathol. 2010;221(1):3–12.
76. Ge L, et al. The protein-vesicle network of autophagy. Curr Opin Cell Biol. 2014;29:18–24.
77. Dancourt J, Melia TJ. Lipidation of the autophagy proteins LC3 and GABARAP is a membrane-curvature dependent process. Autophagy. 2014;10(8) [June 12; Epub ahead of print].
78. Mizushima N. Autophagy: process and function. Genes Dev. 2007;21(22):2861–2873 [review].
79. Shen HM, Mizushima N. At the end of the autophagic road: an emerging understanding of lysosomal functions in autophagy. Trends Biochem Sci. 2014;39(2):61–71.
80. Butow RA, Avadhani NG. Mitochondrial signaling: the retrograde response. Mol Cell. 2004;14(1):1–15.
81. Levine B, et al. Autophagy in immunity and inflammation. Nature. 2011;469:323–335.
82. Tissenbaum HA. Genetics, life span, health span, and the aging process in Caenorhabditis elegans. J Gerontol A Biol Sci Med Sci. 2012;67A(5):503– 510.
83. Rando TA, Chang HY. Aging rejuvenation, and epigenetic reprogramming: resetting the aging clock. Cell. 2012;148:46–57.
84. Campisi J, Vijg J. Does damage to DNA and other macromolecules play a role in aging? If so, how? J Gerontol A Biol Sci Med Sci. 2009;64:175–178.
85. Seviour EG, Lin SY. The DNA damage response: balancing the scale between cancer and ageing. Aging (Albany, NY). 2010;2:900–907.
86. Charville GW, Rando TA. Stem cell ageing and non-random chromosome segregation. Philos Trans R Soc Lond B Biol Sci. 2011;366:85–93.
87. Kuilman T, et al. The essence of senescence. Genes Dev. 2010;24:2463–2479. 88. Fontana L, et al. Extending healthy life span—from yeast to humans. Science.
2010;328:321–326. 89. Kenyon CJ. The genetics of ageing. Nature. 2010;464:504–512. 90. Takahashi K, Yamanaka S. Induction of pluripotent stem cells from mouse
embryonic and adult fibroblast cultures by defined factors. Cell. 2006;126:663–676.
91. Hanna JH, et al. Pluripotency and cellular reprogramming: facts, hypotheses, unresolved issues. Cell. 2010;143:508–525.
92. Villeda SA, et al. Age-related changes in the systemic milieu regulate adult neurogenesis. Nature. 2011;477:90–94.
93. Loffredo FS, et al. Growth differentiation factor 11 is a circulating factor that reverses age-related cardiac hypertrophy. Cell. 2013;153:828–839.
94. Harrison DE, et al. Rapamycin fed late in life extends lifespan in genetically heterogeneous mice. Nature. 2009;460:392–395.
95. Armstrong L, et al. The epigenetic contribution to stem cell ageing—can we rejuvenate our older cells? Stem Cells. 2014 [Apr 16; Epub ahead of print].
96. Greer EL, et al. Transgenerational epigenetic inheritance of longevity in Caenorhabditis elegans. Nature. 2011;479:365–371.
97. Centers for Disease Control and Prevention (CDC). National Vital Statist Rep. 2009;62(7) [United States Life Tables, Washington, DC, 2014, U.S. Department of Health and Human Services].
98. Fausto A, et al. Liver regeneration. Hepatology. 2006;43:S45–S53. 99. Itsara LS, et al. Oxidative stress is not a major contributor to somatic
mitochondrial DNA mutations. PLoS Genet. 2014;10(2):e1003974. 100. Sevini F, et al. mtDNA mutations in human aging and longevity:
controversies and new perspectives opened by high-throughput technologies. Exp Gerontol. 2014 [Apr 5; Epub ahead of print; Available at] http://dx.doi.org/10.1016/j.exger.2014.03.022.
101. Anker SD, et al. Muscle wasting disease: a proposal for a new disease classification. J Cachexia Sarcopenia Muscle. 2014;5(1):1–3.
102. Walrand S, et al. Physiopathological mechanism of sarcopenia. Clin Geriatr Med. 2011;27:365–385.
103. Ali S, Garcia JM. Sarcopenia, cachexia and aging: diagnosis, mechanism and therapeutic options—a mini-review. Gerontology. 2014 [Apr 8; Epub ahead of print].
104. Walston JD. Clin Geriatr Med. 2011;27(1). 105. Shennan T. Postmortems and morbid anatomy. ed 3. William Wood:
Baltimore; 1935. 106. Riley MW. Foreword: the gender paradox. Ory MG, Warner HR. Gender,
health, and longevity: multidisciplinary perspectives. Springer: New York; 1990.
107. Katsumata Y, et al. Green pigments in epidermal blisters of decomposed cadavers. Forensic Sci Int. 1985;28(3-4):167–174.
5
Fluids and Electrolytes, Acids and Bases Sue E. Huether
CHAPTER OUTLINE
Distribution of Body Fluids and Electrolytes, 114
Water Movement Between Plasma and Interstitial Fluid, 115 Water Movement Between ICF and ECF, 115
Alterations in Water Movement, 115
Edema, 115 Sodium, Chloride, and Water Balance, 116 Alterations in Sodium, Chloride, and Water Balance, 119
Isotonic Alterations, 119 Hypertonic Alterations, 119 Hypotonic Alterations, 121
Alterations in Potassium and Other Electrolytes, 122
Potassium, 122 Other Electrolytes—Calcium, Phosphate, and Magnesium, 125
Acid-Base Balance, 125
Hydrogen Ion and pH, 125 Buffer Systems, 125 Acid-Base Imbalances, 127
PEDIATRIC CONSIDERATIONS: Distribution of Body Fluids, 131 GERIATRIC CONSIDERATIONS: Distribution of Body Fluids, 131
The cells of the body live in a fluid environment with electrolyte and acid-base concentrations maintained within a narrow range. Changes in electrolyte concentration affect the electrical activity of nerve and muscle cells and cause shifts of fluid from one compartment to another. Alterations in acid-base balance disrupt cellular functions. Fluid fluctuations also affect blood volume and cellular function. Disturbances in these functions are common and can be life-threatening. Understanding how alterations occur and how the body compensates or corrects the disturbance is important for comprehending many pathophysiologic conditions.
Distribution of Body Fluids and Electrolytes The sum of fluids within all body compartments constitutes total body water (TBW)—about 60% of body weight in adults (Table 5-1). The volume of TBW is usually expressed as a percentage of body weight in kilograms. One liter of water weighs 2.2 lb (1 kg). The rest of the body weight is composed of fat and fat-free solids, particularly bone.
TABLE 5-1 Total Body Water (%) in Relation to Body Weight*
Body Build Adult Male Adult Female Child (1-10 yr) Infant (1 mo to 1 yr) Newborn (Up to 1 mo) Normal 60 50 65 70 70-80 Lean 70 60 50-60 80 Obese 50 42 50 60
*NOTE: Total body water is a percentage of body weight.
Body fluids are distributed among functional compartments, or spaces, and provide a transport medium for cellular and tissue function. Intracellular fluid (ICF) comprises all the fluid within cells, about two thirds of TBW. Extracellular fluid (ECF) is all the fluid outside the cells (about one third of TBW) and includes the interstitial fluid (the space between cells and outside the blood vessels) and the intravascular fluid (blood plasma) (Table 5-2). The total volume of body water for a 70-kg person is about 42 liters. Other ECF compartments include lymph and transcellular fluids, such as synovial, intestinal, and cerebrospinal fluid; sweat; urine; and pleural, peritoneal, pericardial, and intraocular fluids.
TABLE 5-2 Distribution of Body Water (70-kg Man)
Fluid Compartment % of Body Weight Volume (L) Intracellular fluid (ICF) 40 28 Extracellular fluid (ECF) 20 14 Interstitial 15 11 Intravascular 5 3 Total body water (TBW) 60 42
Electrolytes and other solutes are distributed throughout the intracellular and extracellular fluid (Table 5-3). Note that the extracellular fluid contains a large amount of sodium and chloride and a small amount of potassium, whereas the opposite is true of the intracellular fluid. The concentrations of phosphates and magnesium are greater in the intracellular fluid and the concentration of calcium is greater in the extracellular fluid. These differences are important for the
maintenance of electroneutrality between the extracellular and intracellular compartments, the transmission of electrical impulses, and the movement of water among body compartments (see Chapter 1).
TABLE 5-3 Representative Distribution of Electrolytes in Body Compartments
Electrolytes ECF (mEq/L) ICF (mEq/L) Cations Sodium 142 12 Potassium 4.2 150 Calcium 5 0 Magnesium 2 24 TOTAL 153.2 186 Anions Bicarbonate 24 12 Chloride 103 4 Phosphate 2 100 Proteins 16 65 Other anions 8 6 TOTAL 153 187
ECF, Extracellular fluid; ICF, intracellular fluid.
Although the amount of fluid within the various compartments is relatively constant, solutes (e.g., salts) and water are exchanged between compartments to maintain their unique compositions. The percentage of TBW varies with the amount of body fat and age. Because fat is water repelling (hydrophobic), very little water is contained in adipose (fat) cells. Individuals with more body fat have proportionately less TBW and tend to be more susceptible to dehydration. The distribution and the amount of TBW change with age (see the Pediatric
Considerations and Geriatric Considerations boxes), and although daily fluid intake may fluctuate widely, the body regulates water volume within a relatively narrow range. Water obtained by drinking, water ingested in food, and water derived from oxidative metabolism are the primary sources of body water. Normally, the largest amounts of water are lost through renal excretion, with lesser amounts lost through the stool and vaporization from the skin and lungs (insensible water loss) (Table 5- 4).
TABLE 5-4 Normal Water Gains and Losses (70-kg Man)
Daily Intake (mL) Daily Output (mL) Drinking 1400-1800 Urine 1400-1800 Water in food 700-1000 Stool 100 Water of oxidation 300-400 Skin 300-500
Lungs 600-800 TOTAL 2400-3200 TOTAL 2400-3200
Water Movement Between Plasma and Interstitial Fluid The distribution of water and the movement of nutrients and waste products between the capillary and interstitial spaces occur as a result of changes in hydrostatic pressure (pushes water) and osmotic/oncotic pressure (pulls water) at the arterial and venous ends of the capillary (see Figure 1-24). Water, sodium, and glucose readily move across the capillary membrane. The plasma proteins normally do not cross the capillary membrane and maintain effective osmolality by generating plasma oncotic pressure (particularly albumin). As plasma flows from the arterial to the venous end of the capillary, four forces
determine if fluid moves out of the capillary and into the interstitial space (filtration) or if fluid moves back into the capillary from the interstitial space (reabsorption). These forces acting together are described as net filtration or Starling forces:
1. Capillary hydrostatic pressure (blood pressure) facilitates the outward movement of water from the capillary to the interstitial space.
2. Capillary (plasma) oncotic pressure osmotically attracts water from the interstitial space back into the capillary.
3. Interstitial hydrostatic pressure facilitates the inward movement of water from the interstitial space into the capillary.
4. Interstitial oncotic pressure osmotically attracts water from the capillary into the interstitial space. The forces moving fluid back and forth across the capillary wall are summarized below:
At the arterial end of the capillary, hydrostatic pressure exceeds capillary oncotic pressure and fluid moves into the interstitial space (filtration). At the venous end of the capillary, capillary oncotic pressure exceeds capillary hydrostatic pressure and fluids are attracted back into the circulation (reabsorption). Interstitial hydrostatic pressure promotes the movement of about 10% of the interstitial fluid along with small amounts of protein into the lymphatics, which then returns to the circulation. Because albumin does not normally cross the capillary membrane, interstitial oncotic pressure is normally minimal. Figure 5-1 illustrates net filtration.
FIGURE 5-1 Net Filtration—Fluid Movement between Plasma and Interstitial Space. The movement of fluid between the vascular, interstitial spaces and the lymphatics is the result of net filtration of fluid across the semipermeable capillary membrane. Capillary hydrostatic
pressure is the primary force for fluid movement out of the arteriolar end of the capillary and into the interstitial space. At the venous end, capillary oncotic pressure (from plasma proteins) attracts water back into the vascular space. Interstitial hydrostatic pressure promotes the movement of fluid and proteins into the lymphatics. Osmotic pressure accounts for the movement of fluid between the interstitial space and the intracellular space. Normally,
intracellular and extracellular fluid osmotic pressures are equal (280 to 294 mOsm) and water is equally distributed between the interstitial and intracellular compartments.
Water Movement Between ICF and ECF Water moves between ICF and ECF compartments primarily as a function of osmotic forces (see Chapter 1 for definitions). Water moves freely by diffusion
through the lipid bilayer cell membrane and through aquaporins, a family of water channel proteins that provide permeability to water.1 Sodium is responsible for the ECF osmotic balance, and potassium maintains the ICF osmotic balance. The osmotic force of ICF proteins and other nondiffusible substances is balanced by the active transport of ions out of the cell. Water crosses cell membranes freely, so the osmolality of TBW is normally at equilibrium. Normally the ICF is not subject to rapid changes in osmolality, but when ECF osmolality changes, water moves from one compartment to another until osmotic equilibrium is reestablished (see Figure 5-7, p. 120).
Alterations in Water Movement Edema Edema is excessive accumulation of fluid within the interstitial spaces. The forces favoring fluid movement from the capillaries or lymphatic channels into the tissues are increased capillary hydrostatic pressure, decreased plasma oncotic pressure, increased capillary membrane permeability, and lymphatic channel obstruction2 (Figure 5-2).
FIGURE 5-2 Mechanisms of Edema Formation.
Pathophysiology Capillary hydrostatic pressure increases as a result of venous obstruction or salt and water retention. Venous obstruction causes hydrostatic pressure to increase behind the obstruction, pushing fluid from the capillaries into the interstitial spaces. Thrombophlebitis (inflammation of veins), hepatic obstruction, tight clothing around the extremities, and prolonged standing are common causes of venous obstruction. Congestive heart failure, renal failure, and cirrhosis of the liver are associated with excessive salt and water retention, which cause plasma volume
overload, increased capillary hydrostatic pressure, and edema. Since plasma albumin acts like a magnet to attract water, the loss or diminished
production (e.g., from liver disease or protein malnutrition) contributes to decreased plasma oncotic pressure. Plasma proteins are lost in glomerular diseases of the kidney, serous drainage from open wounds, hemorrhage, burns, and cirrhosis of the liver. The decreased oncotic attraction of fluid within the capillary causes filtered capillary fluid to remain in the interstitial space, resulting in edema. Capillaries become more permeable with inflammation and immune responses,
especially with trauma such as burns or crushing injuries, neoplastic disease, and allergic reactions. Proteins escape from the vascular space and produce edema through decreased capillary oncotic pressure and interstitial fluid protein accumulation. The lymphatic system normally absorbs interstitial fluid and a small amount of
proteins. When lymphatic channels are blocked or surgically removed, proteins and fluid accumulate in the interstitial space, causing lymphedema.3 For example, lymphedema of the arm or leg occurs after surgical removal of axillary or femoral lymph nodes, respectively, for treatment of carcinoma. Inflammation or tumors may cause lymphatic obstruction, leading to edema of the involved tissues.
Clinical manifestations Edema may be localized or generalized. Localized edema is usually limited to a site of trauma, as in a sprained finger. Another kind of localized edema occurs within particular organ systems and includes cerebral, pulmonary, and laryngeal edema; pleural effusion (fluid accumulation in the pleural space); pericardial effusion (fluid accumulation within the membrane around the heart); and ascites (accumulation of fluid in the peritoneal space). Edema of specific organs, such as the brain, lung, or larynx, can be life-threatening. Generalized edema is manifested by a more uniform distribution of fluid in interstitial spaces. Dependent edema, in which fluid accumulates in gravity-dependent areas of the body, might signal more generalized edema. Dependent edema appears in the feet and legs when standing and in the sacral area and buttocks when supine (lying on back). It can be identified by pressing on tissues overlying bony prominences. A pit left in the skin indicates edema (hence the term pitting edema) (Figure 5-3).
FIGURE 5-3 Pitting Edema. (From Bloom A, Ireland J: Color atlas of diabetes, ed 2, St Louis, 1992, Mosby.)
Edema usually is associated with weight gain, swelling and puffiness, tight-fitting clothes and shoes, limited movement of affected joints, and symptoms associated with the underlying pathologic condition. Fluid accumulations increase the distance required for nutrients and waste products to move between capillaries and tissues. Blood flow may be impaired also. Therefore wounds heal more slowly, and with prolonged edema the risks of infection and pressure sores over bony prominences increase. As edematous fluid accumulates, it is trapped in a “third space” (i.e., the interstitial space, pleural space, pericardial space) and is unavailable for metabolic processes or perfusion. Dehydration can develop as a result of this sequestering. Such sequestration occurs with severe burns, where large amounts of vascular fluid are lost to the interstitial spaces, reducing plasma volume and causing shock (see Chapter 24).
Evaluation and treatment Specific conditions causing edema require diagnosis. Edema may be treated symptomatically until the underlying disorder is corrected. Supportive measures include elevating edematous limbs, using compression stockings, avoiding prolonged standing, restricting salt intake, and taking diuretics. Administration of
IV albumin can be required in severe cases.
Quick Check 5-1
1. How does an increase in capillary hydrostatic pressure cause edema?
2. How does a decrease in capillary oncotic pressure cause edema?
Sodium, Chloride, and Water Balance The kidneys and hormones have a central role in maintaining sodium and water balance. Because water follows the osmotic gradients established by changes in salt concentration, sodium concentration and water balance are intimately related. Sodium concentration is regulated by renal effects of aldosterone (see Figure 18- 18). Water balance is regulated primarily by antidiuretic hormone (ADH; also known as vasopressin). Sodium (Na+) accounts for 90% of the ECF cations (positively charged ions) (see
Table 5-3). Along with its constituent anions (negatively charged ions) chloride and bicarbonate, sodium regulates extracellular osmotic forces and therefore regulates water balance. Sodium is important in other functions, including maintenance of neuromuscular irritability for conduction of nerve impulses (in conjunction with potassium and calcium; see Figure 1-29), regulation of acid-base balance (using sodium bicarbonate and sodium phosphate), participation in cellular chemical reactions, and transport of substances across the cellular membrane. The kidney, in conjunction with neural and hormonal mediators, maintains
normal serum sodium concentration within a narrow range (135 to 145 mEq/L) primarily through renal tubular reabsorption. Hormonal regulation of sodium (and potassium) balance is mediated by aldosterone, a mineralocorticoid synthesized and secreted from the adrenal cortex as a component of the renin-angiotensin- aldosterone system. Aldosterone secretion is influenced by circulating blood volume, by blood pressure, and by plasma concentrations of sodium and potassium. When circulating blood volume or blood pressure is reduced, or sodium levels are depressed or potassium levels are increased, renin, an enzyme secreted by the juxtaglomerular cells of the kidney, is released. Renin stimulates the formation of angiotensin I, an inactive polypeptide. Angiotensin-converting enzyme (ACE) in pulmonary vessels converts angiotensin I to angiotensin II, which stimulates the secretion of aldosterone and antidiuretic hormone (see below) and also causes vasoconstriction. The aldosterone promotes renal sodium and water reabsorption and excretion of potassium, increasing blood volume (Figure 5-4; also see Figure 29-9). Vasoconstriction elevates the systemic blood pressure and restores renal perfusion (blood flow). This restoration inhibits the further release of renin.
FIGURE 5-4 The Renin-Angiotensin-Aldosterone System. ADH, Antidiuretic hormone; BP, blood pressure; ECF, extracellular fluid; Na, sodium. (Modified from Herlihy B, Maebius N: The human body in health and disease, ed 4, Philadelphia, 2011, Saunders. Borrowed from Lewis et al: Medical-surgical nursing: and management of clinical problems, ed 9, St
Louis, 2014, Mosby.)
Natriuretic peptides are hormones primarily produced by the myocardium. Atrial natriuretic hormone (ANH) is produced by the atria. B-type natriuretic peptide (BNP) is produced by the ventricles. Urodilatin (an ANP analog) is synthesized within the kidney. Natriuretic peptides are released when there is an increase in transmural atrial pressure (increased volume), which may occur with congestive heart failure or when there is an increase in mean arterial pressure4 (Figure 5-5). They are natural antagonists to the renin-angiotensin-aldosterone system. Natriuretic peptides cause vasodilation and increase sodium and water excretion, decreasing blood pressure. Natriuretic peptides are sometimes called a “third factor” in sodium regulation. (Increased glomerular filtration rate is thus the first factor and aldosterone the second factor.)
FIGURE 5-5 The Natriuretic Peptide System. ANH, Atrial natriuretic hormone; BNP, brain natriuretic peptide; GFR, glomerular filtration rate; Na+, sodium ion.
Chloride (Cl−) is the major anion in the ECF and provides electroneutrality, particularly in relation to sodium. Chloride transport is generally passive and follows the active transport of sodium so that increases or decreases in chloride concentration are proportional to changes in sodium concentration. Chloride
concentration tends to vary inversely with changes in the concentration of bicarbonate ( ), the other major anion. Water balance is regulated by the secretion of ADH (also known as vasopressin).
ADH is secreted when plasma osmolality increases or circulating blood volume decreases and blood pressure drops (Figure 5-6). Increased plasma osmolality occurs with water deficit or sodium excess in relation to total body water. The increased osmolality stimulates hypothalamic osmoreceptors. In addition to causing thirst, these osmoreceptors signal the posterior pituitary gland to release ADH. Thirst stimulates water drinking and ADH increases water reabsorption into the plasma from the distal tubules and collecting ducts of the kidney (see Chapter 29). The reabsorbed water decreases plasma osmolality, returning it toward normal, and urine concentration increases.
FIGURE 5-6 The Antidiuretic Hormone (ADH) System.
With fluid loss (dehydration) from vomiting, diarrhea, or excessive sweating, a decrease in blood volume and blood pressure often occurs. Volume-sensitive receptors and baroreceptors (nerve endings that are sensitive to changes in volume and pressure) also stimulate the release of ADH from the pituitary gland and stimulate thirst. The volume receptors are located in the right and left atria and thoracic vessels; baroreceptors are found in the aorta, pulmonary arteries, and carotid sinus. ADH secretion also occurs when atrial pressure drops, as occurs with decreased blood volume and with the release of angiotensin II (see Figure 29-9). The reabsorption of water mediated by ADH then promotes the restoration of plasma volume and blood pressure (see Figure 5-6).
Quick Check 5-2
1. What forces promote net filtration?
2. How do hormones regulate salt and water balance?
3. What are aquaporins?
Alterations in Sodium, Chloride, and Water Balance Alterations in sodium and water balance are closely related. Sodium imbalances occur with gains or losses of body water. Water imbalances develop with gains or losses of salt. In general, these alterations can be classified as changes in tonicity, the change in the concentration of solutes in relation to water: isotonic, hypertonic, or hypotonic (Table 5-5 and Figure 5-7; also see Figure 1-25). Changes in tonicity also alter the volume of water in the intracellular and extracellular compartments, resulting in isovolemia, hypervolemia, or hypovolemia.
TABLE 5-5 Water and Solute Imbalances
Tonicity Mechanism Isotonic (isoosmolar) imbalance Serum osmolality = 280- 294 mOsm/kg
Gain or loss of ECF resulting in concentration equivalent to 0.9% sodium chloride solution (normal saline); no shrinking or swelling of cells
Hypertonic (hyperosmolar) imbalance Serum osmolality >294 mOsm/kg
Imbalances that result in ECF concentration >0.9% salt solution (i.e., water loss or solute gain); cells shrink in hypertonic fluid
Hypotonic (hypoosmolar) imbalance Serum osmolality <280 mOsm/kg
Imbalance that results in ECF <0.9% salt solution (i.e., water gain or solute loss); cells swell in hypotonic fluid
Formula for calculating serum osmolarity
(2 × [Na] + [Glu])/18 + BUN/2.8
BUN, Blood serum urea nitrogen level (mg/dl); ECF, extracellular fluid; [Glu], serum glucose concentration (mg/dl); [Na], serum sodium concentration (mEq/dl).
FIGURE 5-7 Effects of Alterations in Extracellular Sodium Concentration in RBC, Body Cell, and Neuron. A, Hypotonic alteration: Decrease in ECF sodium (Na+) concentration (hyponatremia)
results in ICF osmotic attraction of water with swelling and potential bursting of cells. B, Isotonic alteration: Normal concentration of sodium in the ECF and no change in shifts of fluid in or out of cells. C, Hypertonic alteration: An increase in ECF sodium concentration (hypernatremia) results
in osmotic attraction of water out of cells with cell shrinkage. RBC, Red blood cell.
Isotonic Alterations Isotonic alterations are the most common and occur when TBW changes are accompanied by proportional changes in the concentrations of electrolytes (see Figure 5-7). Isotonic fluid loss causes dehydration and hypovolemia. For example, if an individual loses pure plasma or ECF, fluid volume is depleted but the concentration and type of electrolytes and the osmolality remain in the normal range (280 to 294 milliosmoles [mOsm]). Causes include hemorrhage, severe
wound drainage, excessive diaphoresis (sweating), and inadequate fluid intake. There is loss of extracellular fluid volume with weight loss, dryness of skin and mucous membranes, decreased urine output, and symptoms of hypovolemia. Indicators of hypovolemia include a rapid heart rate, flattened neck veins, and normal or decreased blood pressure. In severe states, hypovolemic shock can occur (see Chapter 24). Isotonic fluids containing electrolytes and glucose are given orally, intravenously (i.e., 0.9% saline solution or 5% dextrose in 0.225% saline solution), or, in some cases, subcutaneously (hypodermoclysis). Isotonic fluid excess causes hypervolemia. Common causes include excessive
administration of intravenous fluids, hypersecretion of aldosterone, or the effects of drugs such as cortisone (which causes renal reabsorption of sodium and water). As plasma volume expands, hypervolemia develops with weight gain. The diluting effect of excess plasma volume leads to decreased hematocrit and decreased plasma protein concentration. The neck veins may distend, and the blood pressure increases. Increased capillary hydrostatic pressure leads to edema formation. Ultimately, pulmonary edema and heart failure may develop. Diuretics are commonly used for treatment.
Hypertonic Alterations Hypertonic fluid alterations develop when the osmolality of the ECF is elevated above normal (greater than 294 mOsm). The most common causes are increased concentration of ECF sodium (hypernatremia) or deficit of ECF water, or both. In both instances, ECF hypertonicity attracts water from the intracellular space, causing ICF dehydration (see Figure 5-7).
Hypernatremia
Pathophysiology Hypernatremia occurs when serum sodium levels exceed 145 mEq/L. Increased levels of serum sodium cause hypertonicity. Hypernatremia can be isovolemic, hypovolemic, or hypervolemic depending on the accompanying ECF water volume. Isovolemic hypernatremia is the most common and occurs when there is a loss of
free water with a near normal body sodium concentration. Causes include inadequate water intake; excessive sweating (sweat is hypotonic), fever, or respiratory tract infections, which increase the respiratory rate and enhance water loss from the lungs; burns; vomiting; diarrhea; and central or nephrogenic diabetes insipidus (lack of ADH or inadequate renal response to ADH). Infants with severe diarrhea are vulnerable and have increased risk because they cannot communicate
thirst. Insufficient water intake occurs particularly in individuals who are comatose, confused, or immobilized or are receiving gastric feedings. Dehydration refers to water deficit but also is commonly used to indicate both sodium and water loss (isotonic or isoosmolar dehydration).5 Hypovolemic hypernatremia occurs where there is loss of sodium accompanied
by a relatively greater loss of body water. Causes include use of loop diuretics, osmotic diuresis (i.e., from hyperglycemia related to uncontrolled diabetes mellitus or use of mannitol), or failure of the kidneys to concentrate urine. Hypervolemic hypernatremia is rare and occurs when there is increased total
body water and a greater increase in total body sodium level, resulting in hypervolemia. Causes include infusion of hypertonic saline solutions (e.g., as sodium replacement for treatment of salt depletion, which can occur with renal impairment, heart failure, or gastrointestinal [GI] losses); oversecretion of adrenocorticotropic hormone (ACTH) or aldosterone (e.g., Cushing syndrome, adrenal hyperplasia); and near salt water drowning.6 High amounts of dietary sodium rarely cause hypernatremia in a healthy individual because the sodium is eliminated by the kidneys. Because chloride follows sodium, hyperchloremia (elevation of serum chloride
concentration greater than 105 mEq/L) often accompanies hypernatremia, as well as plasma bicarbonate deficits (such as in metabolic acidosis)7 (see p. 127). There are no specific symptoms or treatment for chloride excess.
Clinical manifestations When there is excessive sodium intake or decreased sodium loss in relation to water, water is osmotically redistributed to the hypertonic extracellular space, resulting in hypervolemia, and intracellular dehydration ensues. Clinical manifestations include thirst, weight gain, bounding pulse, and increased blood pressure. Central nervous system signs are the most serious and are related to alterations in membrane potentials and shrinking of brain cells (sodium cannot cross brain capillaries because of their tight endothelial junctions). Signs include muscle twitching and hyperreflexia (hyperactive reflexes), confusion, coma, convulsions, and cerebral hemorrhage from stretching of veins. Hypernatremia with marked water deficit is manifested by signs and symptoms of intracellular and extracellular dehydration with volume depletion (Box 5-1).
Box 5-1 Signs and Symptoms of Dehydration
Increased serum sodium concentration
Thirst
Headache
Weight loss
Oliguria and concentrated urine
Hard stools
Decreased skin turgor
Dry mucous membranes
Decreased sweating and tears
Elevated temperature
Soft eyeballs
Sunken fontanels in infants
Prolonged capillary refill time
Tachycardia
Weak pulses
Low blood pressure
Postural hypotension
Hypovolemic shock
Confusion
Coma
Evaluation and treatment Serum sodium levels are greater than 147 mEq/L and urine specific gravity will be
greater than 1.030. The history and physical examination provide information about underlying disorders and events. The treatment of hypernatremia and water deficit is to give oral fluids or isotonic salt-free fluid (5% dextrose in water) until the serum sodium level returns to normal. Fluid replacement must be given slowly to prevent cerebral edema. Serum sodium levels need to be monitored. Hypervolemia or hypovolemia requires treatment of the underlying clinical condition.
Hypotonic Alterations Hypotonic fluid imbalances occur when the osmolality of the ECF is less than 280 mOsm (see Figure 5-7). The most common causes are sodium deficit or water excess. Either leads to intracellular overhydration (cellular edema) and cell swelling. When there is a sodium deficit, the osmotic pressure of the ECF decreases and water moves into the cell where the osmotic pressure is greater. The plasma volume then decreases, leading to symptoms of hypovolemia. With water excess, increases in both the ICF and ECF volume occur, causing symptoms of hypervolemia and water intoxication with cerebral and pulmonary edema.
Hyponatremia
Pathophysiology Hyponatremia develops when the serum sodium concentration falls below 135 mEq/L. Hyponatremia occurs when there is loss of sodium, inadequate intake of sodium, or dilution of sodium by water excess.8 Sodium depletion usually causes hypoosmolality with movement of water into cells with rupture of cell membranes. Isovolemic hyponatremia occurs when there is loss of sodium without a
significant loss of water (pure sodium deficit). Causes can include syndrome of inappropriate antidiuretic hormone9 (SIADH [see Chapter 19], which enhances water retention), hypothyroidism, pneumonia, and glucocorticoid deficiency. Inadequate intake of dietary sodium is rare but possible in individuals on low-sodium diets, particularly with use of diuretics. Hypervolemic hyponatremia occurs when total body sodium level increases. The
increased sodium leads to an increase in total body water and dilution of sodium in the extracellular space. Causes include congestive heart failure, cirrhosis of the liver, and nephrotic syndrome. Edema is present. Hypovolemic hyponatremia occurs with a loss of total body water, but there is a
greater loss of body sodium. The extracelluar volume is decreased. Causes include prolonged vomiting, severe diarrhea, inadequate secretion of aldosterone (e.g., adrenal insufficiency), and renal losses from diuretics.
Dilutional hyponatremia (water intoxication) occurs when there is intake of large amounts of free water or replacement of fluid loss with intravenous 5% dextrose in water, which dilutes sodium. The glucose is metabolized to carbon dioxide and water, leaving a hypotonic solution with a diluting effect. Excessive sweating stimulates thirst and intake of large amounts of free water (as can occur in endurance athletes), which dilutes sodium. Some individuals with psychogenic disorders develop water intoxication from compulsive water drinking. Other causes can include tap water enemas, near fresh water drowning, and use of selective serotonin reuptake inhibitors (SSRIs). When the body is functioning normally, it is almost impossible to produce an excess of TBW because water balance is regulated by the kidneys. Hypochloremia, a low level of serum chloride (less than 97 mEq/L), usually
occurs with hyponatremia or an elevated bicarbonate concentration, as in metabolic alkalosis (see p. 127). Sodium deficit related to restricted intake, use of diuretics, vomiting, or nasogastric suction is accompanied by chloride deficiency. Cystic fibrosis is characterized by hypochloremia (see Chapter 28). Treatment of the underlying cause is required.
Clinical manifestations The serum sodium concentration will be less than 135 mEq/L. Sodium depletion usually causes hypoosmolality with movement of water into cells. The hematocrit is reduced from the dilutional effect of water excess in dilutional hyponatremia. The high amount of intracellular solutes compared to the low amount of extracellular solutes as a result of the hyponatremia causes an intracellular osmotic shift of water, resulting in cell swelling. The most life-threatening consequence is cerebral edema and increased intracranial pressure. Neurologic changes include lethargy, confusion, apprehension, seizures, and coma. A decrease in sodium concentration changes the cell's ability to depolarize and repolarize normally, altering the action potential in neurons and muscle (see Chapter 1). Muscle twitching, depressed reflexes, and weakness are common. Nausea and vomiting are more common with less severe hyponatremia (i.e., decreases between 120 and 130 mEq/L). Hypovolemic hyponatremia has signs of hypotension, tachycardia, and decreased urine output. Hypervolemic hyponatremia is accompanied by weight gain, edema, ascites, and jugular vein distention. Hyponatremia is a major cause of morbidity and mortality in intensive care units and in the elderly (see Health Alert: Hyponatremia and the Elderly).
Health Alert
Hyponatremia and the Elderly
Hyponatremia is the most common of the electrolyte disorders and prevalence is highest among elderly hospitalized individuals. Isovolemic hyponatremia caused by SIADH is thought to be the most common cause and can occur with central nervous system injury, pulmonary disease, malignancies, nausea, pain, and aging changes. Other contributing factors include use of thiazide diuretics, proton pump inhibitors, age-related decrease in thirst with dehydration, and diminished urine concentrating ability. Hyponatremia contributes to cognitive deficits, gait disturbances, falls, fractures, long-term hospitalization, the need for long-term care, and death. The elderly need to be assessed for risk, implementation of preventive strategies, and early intervention. SIADH, Syndrome of inappropriate antidiuretic hormone.
From Ayus JC et al: Nephrol Dial Transplant 27(10):3725-3731, 2012; Berl T: Clin J Am Soc Nephrol 8(3):469-475; Cowen LE et al: Endocrinol Metab Clin North Am 42(2):349-370, 2013; Cumming K et al: PLoS One 9(2):e88272, 2014; Mannesse CK et al: Ageing Res Rev 12(1):165-173, 2013; Schrier RW et al: Nat Rev Nephrol 9(1):37-50, 2013 (Erratum in: Nat Rev Nephrol 9[3]:124, 2013).
Evaluation and treatment The cause of hyponatremia must be determined and treatment planned accordingly. Small amounts of intravenous hypertonic sodium chloride (i.e., 3% sodium chloride) can be given when neurologic manifestations are severe but must be given slowly to prevent osmotic demyelination syndrome in the brain.9 Restriction of water intake is required in most cases of dilutional hyponatremia because body sodium levels may be normal or increased even though serum sodium levels are low. Arginine vasopressin (ADH) receptor antagonists (vaptans) are a class of drugs used for the treatment of hypervolemic and euvolemic hyponatremia.10 Serum sodium concentration must be monitored.8
Quick Check 5-3
1. What causes isotonic imbalance?
2. What are some causes of hypernatremia?
3. What is the most severe complication of hyponatremia?
Alterations in Potassium and Other Electrolytes Potassium Potassium (K+) is the major intracellular electrolyte and is essential for normal cellular functions. Total body potassium content is about 4000 mEq, with most of it (98%) located in the cells. The ICF concentration of potassium is 150 to 160 mEq/L; the ECF potassium concentration is 3.5 to 5.0 mEq/L. The difference in concentration is maintained by a sodium-potassium adenosinetriphosphatase active transport system (Na+-K+ ATPase pump) (see Figure 1-26). As the predominant ICF ion, potassium exerts a major influence on the regulation
of ICF osmolality and fluid balance as well as on intracellular electrical neutrality in relation to hydrogen (H+) and sodium. Potassium is required for glycogen and glucose deposition in liver and skeletal muscle cells. It also maintains the resting membrane potential, as reflected in the transmission and conduction of nerve impulses (see Figure 1-29), the maintenance of normal cardiac rhythms, and the contraction of skeletal muscle and smooth muscle. Dietary potassium moves rapidly into cells after ingestion. However, the
distribution of potassium between intracellular and extracellular fluids is influenced by several factors. Insulin, aldosterone, epinephrine, and alkalosis facilitate the shift of potassium into cells. Insulin deficiency, aldosterone deficiency, acidosis, cell lysis, and strenuous exercise facilitate the shift of potassium out of cells. Glucagon blocks entry of potassium into cells, and glucocorticoids promote potassium excretion. Potassium also will move out of cells along with water when there is increased ECF osmolarity. Although potassium is found in most body fluids, the kidney is the most efficient
regulator of potassium balance. Potassium is freely filtered by the renal glomerulus, and 90% is reabsorbed by the proximal tubule and loop of Henle. In the distal tubules, principal cells secrete potassium and intercalated cells reabsorb potassium. These cells determine the amount of potassium excreted from the body. The gut may also sense the amount of K+ ingested and stimulate renal K+ excretion independent of aldosterone.11 The potassium concentration in the distal tubular cells is determined primarily by
the plasma concentration in the peritubular capillaries. When plasma potassium concentration increases from increased dietary intake or shifts of potassium from the ICF to the ECF occur, potassium is secreted into the urine by the distal tubules. Decreased levels of plasma potassium result in decreased distal tubular secretion,
although approximately 5 to 15 mEq per day will continue to be lost. Changes in the rate of filtrate (urine) flow through the distal tubule also influence the concentration gradient for potassium secretion. When the urine flow rate is high, as with the use of diuretics, potassium concentration in the distal tubular urine is lower, leading to the secretion of potassium into the urine. Changes in pH and thus in hydrogen ion concentration also affect potassium
balance. During acute acidosis, hydrogen ions accumulate in the ICF and potassium shifts out of the cell to the ECF to maintain a balance of cations across the cell membrane. This occurs in part because of a decrease in sodium-potassium ATPase pump activity. Decreased ICF potassium results in decreased secretion of potassium by the distal tubular cells, contributing to hyperkalemia. In acute alkalosis, intracellular fluid levels of hydrogen diminish and potassium shifts into the cell; in addition, the distal tubular cells increase their secretion of potassium, further contributing to hypokalemia.12 Besides conserving sodium, aldosterone also regulates potassium concentration.
Elevated plasma potassium concentration causes the release of renin by renal juxtaglomerular cells and the adrenal secretion of aldosterone through the renin- angiotensin-aldosterone system. Aldosterone then stimulates the release of potassium into the urine by the distal renal tubules. Aldosterone also increases the secretion of potassium from sweat glands. Insulin helps regulate plasma potassium levels by stimulating the sodium-
potassium ATPase pump, thus promoting the movement of potassium into liver and muscle cells, particularly after eating. Insulin can also be used to treat hyperkalemia. Dangerously low levels of plasma potassium can result when insulin is given while potassium levels are depressed. Potassium balance is especially significant in the treatment of conditions requiring insulin administration, such as insulin-dependent diabetes mellitus. Potassium adaptation is the ability of the body to adapt to increased levels of
potassium intake over time. A sudden increase in potassium may be fatal, but if the intake of potassium is slowly increased by amounts of more than 120 mEq per day, the kidney can increase the urinary excretion of potassium and maintain potassium balance.
Hypokalemia
Pathophysiology Potassium deficiency, or hypokalemia, develops when the serum potassium concentration falls to less than 3.5 mEq/L. Because cellular and total body stores of potassium are difficult to measure, changes in potassium balance are described,
although not always accurately, by the plasma concentration. Generally, lowered serum potassium level indicates loss of total body potassium. With potassium loss from the ECF, the concentration gradient change favors movement of potassium from the cell to the ECF. The ICF/ECF concentration ratio is maintained, but the amount of total body potassium is depleted. Factors contributing to the development of hypokalemia include reduced intake
of potassium, increased entry of potassium into cells, and increased losses of body potassium. Dietary deficiency of potassium is more common in elderly individuals with both low protein intake and inadequate intake of fruits and vegetables and in individuals with alcoholism or anorexia nervosa (see Health Alert: Potassium Intake: Hypertension and Stroke). Reduced potassium intake generally becomes a problem when combined with other causes of potassium depletion.
Health Alert Potassium Intake: Hypertension and Stroke
Enriched dietary intake of potassium is associated with lower risk of hypertension and stroke. The American diet often exceeds recommendations for sodium intake and a deficiency in potassium intake. There is increased risk of high blood pressure, cardiovascular disease, and mortality when the plasma ratio of sodium concentration to potassium concentration is high. Potassium attenuates the effects of high dietary salt with reduction in blood pressure, stroke rates, and cardiovascular disease risk. The exact mechanism of how potassium affects blood pressure is unknown but is thought to be related to renal handling of sodium, endothelial cell function, decreased vascular resistance, and reduced oxidative stress. A large prospective study of older women showed they were found to have lower risk of ischemic but not hemorrhagic stroke associated with higher intakes of potassium, especially in women without hypertension. Lower risk of mortality was found in all women with higher intakes of potassium. Increased dietary intake of potassium is recommended for most individuals without impaired renal handling of potassium.
Data from Aaron KJ, Sanders PW: Mayo Clin Proc 88(9):987-995, 2013; Arjun S et al: Stroke September 4, 2014 [Epub ahead of print]; Castro H, Raij L: Semin Nephrol 33(3):277-289, 2013; Whelton PK, He J: Curr Opin Lipidol 25(1):75-79, 2014.
ECF hypokalemia can develop without losses of total body potassium. For example, potassium shifts from the ECF to the ICF in exchange for hydrogen to maintain plasma acid-base balance during respiratory or metabolic alkalosis.
Insulin promotes cellular uptake of potassium and insulin administration may cause an ECF potassium deficit. Potassium shifts from the ICF to the ECF in conditions such as diabetic
ketoacidosis, in which the increased hydrogen ion concentration in the ECF causes H+ to shift into the cell in exchange for potassium. A normal level of potassium is maintained in the plasma, but potassium continues to be lost in the urine, causing a deficit in the amount of total body potassium. Severe, even fatal, hypokalemia may occur if insulin is administered without also providing potassium supplements. Thus total body potassium depletion becomes evident when insulin treatment and rehydration therapy are initiated. Potassium replacement is instituted cautiously to prevent hyperkalemia. Losses of potassium from body stores are usually caused by gastrointestinal and
renal disorders. Diarrhea, intestinal drainage tubes or fistulae, and laxative abuse also result in hypokalemia. Normally, only 5 to 10 mEq of potassium and 100 to 150 ml of water are excreted in the stool each day. With diarrhea, fluid and electrolyte losses can be voluminous, with several liters of fluid and 100 to 200 mEq of potassium lost per day. Vomiting or continuous nasogastric suctioning often is associated with potassium depletion, partly because of the potassium lost from the gastric fluid but principally because of renal compensation for volume depletion and the metabolic alkalosis (elevated bicarbonate levels) that occurs from sodium, chloride, and hydrogen ion losses. The loss of fluid and sodium stimulates the secretion of aldosterone, which in turn causes renal losses of potassium. Renal potassium losses occur with increased secretion of potassium by the distal
tubule. Use of potassium-wasting diuretics, excessive aldosterone secretion, increased distal tubular flow rate, and low plasma magnesium concentration all may contribute to urinary losses of potassium. The elevated flow of bicarbonate at the distal tubule during alkalosis also contributes to renal excretion of potassium because the increased tubular lumen electronegativity attracts potassium. Many diuretics inhibit the reabsorption of sodium chloride, causing the diuretic effect. The distal tubular flow rate then increases, promoting potassium excretion. If sodium loss is severe, the compensating aldosterone secretion may further deplete potassium stores. Primary hyperaldosteronism with excessive secretion of aldosterone from an adrenal adenoma (tumor) also causes potassium wasting. Many kidney diseases reduce the ability to conserve sodium. The disordered sodium reabsorption produces a diuretic effect, and the increased distal tubule flow rate favors the secretion of potassium. Magnesium deficits increase renal potassium secretion and promote hypokalemia. Certain antibiotics (i.e., carbenicillin disodium and amphotericin B) are known to cause hypokalemia by increasing the rate of potassium excretion. Rare hereditary defects in renal potassium transport (e.g.,
Bartter and Gitelman syndromes) also can cause hypokalemia.
Clinical manifestations Mild losses of potassium are usually asymptomatic. Severe loss of potassium results in neuromuscular and cardiac manifestations. Neuromuscular excitability decreases, causing skeletal muscle weakness, smooth muscle atony, cardiac dysrhythmias, glucose intolerance, and impaired urinary concentrating ability.13 Symptoms occur in relation to the rate of potassium depletion. Because the body
can accommodate slow losses of potassium, the decrease in ECF concentration may allow potassium to shift from the intracellular space, restoring the potassium concentration gradient toward normal, with less severe neuromuscular changes. With acute and severe losses of potassium, changes in neuromuscular excitability are more profound. Skeletal muscle weakness occurs initially in the larger muscles of the legs and arms and ultimately affects the diaphragm and depresses ventilation. Paralysis and respiratory arrest can occur with severe losses. Loss of smooth muscle tone is manifested by constipation, intestinal distention, anorexia, nausea, vomiting, and paralytic ileus (paralysis of the intestinal muscles). The cardiac effects of hypokalemia are related also to changes in membrane
excitability. As ECF potassium concentration decreases, the resting membrane potential becomes more negative (i.e., from −90 millivolts to −100 millivolts [hypopolarization]). Because potassium contributes to the repolarization phase of the action potential, hypokalemia delays ventricular repolarization. Various dysrhythmias may occur, including sinus bradycardia, atrioventricular block, and paroxysmal atrial tachycardia. The characteristic changes in the electrocardiogram (ECG) reflect delayed repolarization. For instance, the amplitude of the T wave decreases, the amplitude of the U wave increases, and the ST segment is depressed (Figure 5-8). In severe states of hypokalemia, P waves peak, the QT interval is prolonged, and T wave inversions may be seen. Hypokalemia enhances the therapeutic effect of digitalis and increases the risk of digitalis toxicity.
FIGURE 5-8 Electrocardiogram Changes with Potassium Imbalance.
A wide range of metabolic dysfunctions may result from potassium deficiency (Table 5-6). Carbohydrate metabolism is affected because hypokalemia depresses insulin secretion and alters hepatic and skeletal muscle glycogen synthesis. Renal function is impaired, with a decreased ability to concentrate urine. Polyuria (increased urine) and polydipsia (increased thirst) are associated with decreased
responsiveness to ADH. Long-term potassium deficits lasting more than 1 month may damage renal tissue, with interstitial fibrosis and tubular atrophy.
TABLE 5-6 Clinical Manifestations of Potassium Level Alterations
Organ System Hypokalemia Hyperkalemia Cardiovascular Dysrhythmias
ECG changes (flattened T waves, U waves, ST depression, peaked P wave, prolonged QT interval)
Cardiac arrest Weak, irregular pulse rate Postural hypotension
Dysrhythmias ECG changes (peaked T waves, prolonged PR interval, absent P wave with widened QRS complex)
Bradycardia Heart block Cardiac arrest
Nervous Lethargy Fatigue Confusion Paresthesias
Anxiety Tingling Numbness
Gastrointestinal Nausea and vomiting Decreased motility Distention Decreased bowel sounds Ileus
Nausea and vomiting Diarrhea Colicky pain
Kidney Water loss Thirst Inability to concentrate urine Increased tubular production of ammonia and ammonium Kidney damage
Oliguria Kidney damage
Skeletal and smooth muscle
Weakness Flaccid paralysis Respiratory arrest Constipation Bladder dysfunction
Early: hyperactive muscles Late: weakness and flaccid paralysis
Evaluation and treatment The diagnosis of hypokalemia is significantly related to the medical history and the identification of disorders associated with potassium loss or shifts of extracellular potassium to the intracellular space. Treatment involves an estimation of total body potassium losses and correction of acid-base imbalances. Further losses of potassium should be prevented and the individual should be encouraged to eat foods rich in potassium. The maximal rate of oral replacement is 40 to 80 mEq/day if renal function is normal. A maximal safe rate of intravenous replacement is 20 mEq/hr. Because potassium is irritating to blood vessels, a maximal concentration of 40 mEq/L should be used. Serum potassium values are monitored until normokalemia is achieved.
Hyperkalemia
Pathophysiology Elevation of ECF potassium concentration greater than 5.5 mEq/L constitutes
hyperkalemia.14 Because of efficient renal excretion, increases in total body potassium level are relatively rare. Acute increases in serum potassium level are handled quickly through increased cellular uptake and renal excretion of body potassium excesses. Hyperkalemia may be caused by increased intake, a shift of potassium from cells
to the ECF, decreased renal excretion, or drugs that decrease renal potassium excretion (i.e., ACE inhibitors, angiotensin receptor blockers, and aldosterone antagonists). If renal function is normal, slow, long-term increases in potassium intake are usually well tolerated through potassium adaptation, although short-term potassium loading can exceed renal excretion rates. Dietary excesses of potassium are uncommon but accidental ingestion of potassium salt substitutes can cause toxicity. Use of stored whole blood and intravenous boluses of potassium penicillin G or replacement potassium can precipitate hyperkalemia, particularly with impaired renal function. Potassium moves from the ICF to the ECF with cell trauma or a change in cell membrane permeability, acidosis, insulin deficiency, or cell hypoxia. Burns, massive crushing injuries, and extensive surgeries can cause release of potassium to the ECF as a result of cell trauma. If renal function is sustained, potassium is excreted. As cell repair begins, hypokalemia develops without an adequate replacement of potassium. In acidosis, ECF hydrogen ions shift into cells in exchange for ICF potassium and
sodium; hyperkalemia and acidosis therefore often occur simultaneously. Because insulin promotes cellular entry of potassium, insulin deficits, which occur with such conditions as diabetic ketoacidosis, are accompanied by hyperkalemia. Hypoxia can lead to hyperkalemia by diminishing the efficiency of cell membrane active transport, resulting in the escape of potassium to the ECF. Digitalis overdose (toxicity) may cause hyperkalemia by inhibiting the Na+-K+ ATPase pump, and thus allowing potassium to remain outside the cell, Decreased renal excretion of potassium commonly is associated with
hyperkalemia. Renal failure that results in oliguria (urine output of 30 ml/hr or less) is accompanied by elevations of serum potassium level. The severity of hyperkalemia is related to the amount of potassium intake, the degree of acidosis, and the rate of renal cell damage. Decreases in the secretion or renal effects of aldosterone also can cause decreases in the urinary excretion of potassium. For example, Addison disease (a disease of adrenal cortical insufficiency) results in decreased production and secretion of aldosterone (and other steroids) and thus contributes to hyperkalemia.
Clinical manifestations Symptoms vary with the severity of hyperkalemia. During mild attacks, increased
neuromuscular irritability may be manifested as restlessness, intestinal cramping, and diarrhea. Severe hyperkalemia decreases the resting membrane potential (i.e., from −90 millivolts to −70 millivolts [hyperpolarization]) and causes muscle weakness, loss of muscle tone, and paralysis. In mild states of hyperkalemia, there is more rapid repolarization, reflected in the ECG as narrow and taller T waves with a shortened QT interval. Severe hyperkalemia causes delayed cardiac conduction and prevents repolarization of heart muscle. Severe hyperkalemia depresses the ST segment, prolongs the PR interval, and widens the QRS complex because of decreased conduction velocity from inactivated sodium channels (see Figure 5-8). Bradydysrhythmias and delayed conduction are common in hyperkalemia; severe hyperkalemia can cause ventricular fibrillation or cardiac arrest.15 As with hypokalemia, changes in the ratio of intracellular to extracellular
potassium concentration contribute to the symptoms of hyperkalemia (see Table 5- 6). The neuromuscular effects of hyperkalemia are related to the increase in rate of repolarization and the presence of other contributing factors, such as acidosis and calcium balance. Long-term increases in ECF potassium concentration result in shifts of potassium into the cell, because the tendency is to maintain a normal ratio of ICF to ECF potassium concentrations. Acute elevations of extracellular potassium concentration affect neuromuscular irritability because this ratio is disrupted. Increases in extracellular fluid calcium concentration can override the neuromuscular effects of hyperkalemia because calcium is also a cation and affects the threshold potential (see Chapter 1).
Evaluation and treatment Hyperkalemia should be investigated when there is a history of renal disease, massive trauma, insulin deficiency, Addison disease, use of potassium salt substitutes, or metabolic acidosis. The acuity of the onset of symptoms may be related to the underlying cause. Management of hyperkalemia includes treating the contributing causes and
correcting the potassium excess. When serum potassium levels are dangerously high, calcium gluconate can be administered to restore normal neuromuscular irritability and to stabilize the resting cardiac membrane potential by making the threshold potential less negative. Administration of glucose (which readily stimulates insulin secretion) or administration of both glucose and insulin for diabetic individuals facilitates cellular entry of potassium. Sodium bicarbonate corrects metabolic acidosis and lowers serum potassium concentration. Oral or rectal administration of cation exchange resins, which exchange sodium for potassium in the intestine, can be effective. Dialysis effectively removes potassium when renal failure has occurred.
Quick Check 5-4
1. What role does potassium play in the body? What metabolic dysfunctions occur in potassium deficiency? In potassium excess?
2. Explain how a person can have normal total body potassium levels but still exhibit hypokalemia.
3. What is the most prominent ECG change associated with hyperkalemia? With hypokalemia?
Other Electrolytes—Calcium, Phosphate, and Magnesium The specifics of balance for the other body electrolytes—calcium (Ca++), phosphate (PO4
3−), and magnesium (Mg++)—are summarized in Table 5-7. Parathyroid hormone and vitamin D are important for the regulation of these minerals16 (see Chapter 18).
TABLE 5-7 Alterations in Calcium, Phosphate, and Magnesium
Parameter Calcium Phosphate Magnesium Normal values
Serum: 8.8-10.5 mg/dl (total), 4.5-5.6 mg/dl (ionized); 99% in bone as hydroxyapatite; remainder in plasma and body cells with 50% bound to plasma proteins; 40% free or ionized; ionized form most important physiologically
Serum: 2.5-5.0 mg/dl, but may be as high as 6.0-7.0 mg/dl in infants and young children; mainly in bone with some in ICF and ECF; exists as phospholipids, phosphate esters, and inorganic phosphate (ionized form)
Serum: 1.8-3.0 mEq/L; 40-60% stored in bone, 33% bound to plasma proteins; primary intracellular divalent cation
Function Needed for fundamental metabolic processes; major cation for structure of bone and teeth; enzymatic cofactor for blood clotting; required for hormone secretion and function of cell receptors; directly related to plasma membrane stability and permeability, transmission of nerve impulses, and contraction of muscles; parathyroid hormone, vitamin D3, and calcitonin act together to control calcium absorption and excretion (see Chapter 18)
Intracellular and extracellular anion buffer in regulation of acid-base balance; provides energy for muscle contraction (as ATP); parathyroid hormone, vitamin D3, and calcitonin act together to control phosphate absorption and excretion (see Chapter 18)
Cofactor in intracellular enzymatic reactions and causes neuromuscular excitability; often interacts with calcium and potassium in reactions at cellular level and has important role in smooth muscle contraction and relaxation; magnesium is absorbed in the intestine and eliminated by the kidney
Excess Hypercalcemia (serum concentrations >10- 12 mg/dl)
Hyperphosphatemia (serum concentrations >4.7 mg/dl)
Hypermagnesemia (serum concentrations >3.0 mEq/L)
Causes Hyperparathyroidism; bone metastases with calcium resorption from breast, prostate, renal, and cervical cancer; sarcoidosis; excess vitamin D; many tumors that produce PTH
Acute or chronic renal failure with significant loss of glomerular filtration; treatment of metastatic tumors with chemotherapy that releases large amounts of phosphate into serum; long-term use of laxatives or enemas containing phosphates; hypoparathyroidism
Usually renal insufficiency or failure; also excessive intake of magnesium-containing antacids, adrenal insufficiency
Effects Many nonspecific; fatigue, weakness, lethargy, anorexia, nausea, constipation; impaired renal function, kidney stones; dysrhythmias, bradycardia, cardiac arrest; bone pain, osteoporosis
Symptoms primarily related to low serum calcium levels (caused by high phosphate levels) similar to results of hypocalcemia; when prolonged, calcification of soft tissues in lungs, kidneys, joints
Skeletal smooth muscle contraction; excess nerve function; loss of deep tendon reflexes; nausea and vomiting; muscle weakness; hypotension; bradycardia; respiratory distress
Deficit Hypocalcemia (serum calcium concentration <8.5 mg/dl)
Hypophosphatemia (serum phosphate concentration <2.0 mg/dl)
Hypomagnesemia (serum magnesium concentration <1.5 mEq/L)
Causes Related to inadequate intestinal absorption, deposition of ionized calcium into bone or soft tissue, blood administration, or decreases in PTH and vitamin D; nutritional deficiencies occur with inadequate sources of dairy products or green leafy vegetables
Most commonly by intestinal malabsorption related to vitamin D deficiency, use of magnesium- and aluminum-containing antacids, long-term alcohol abuse, and malabsorption syndromes; respiratory alkalosis; increased renal excretion of phosphate associated with hyperparathyroidism
Malnutrition, malabsorption syndromes, alcoholism, urinary losses (renal tubular dysfunction, loop diuretics)
Effects Increased neuromuscular excitability; tingling, muscle spasm (particularly in hands, feet, and facial muscles), intestinal cramping, hyperactive bowel sounds; severe cases show convulsions and tetany; prolonged QT interval, cardiac arrest
Conditions related to reduced capacity for oxygen transport by red blood cells and disturbed energy metabolism; leukocyte and platelet dysfunction; deranged nerve and muscle function; in severe cases, irritability, confusion, numbness, coma, convulsions; possibly respiratory failure (because of muscle weakness), cardiomyopathies, bone resorption (leading to rickets or osteomalacia)
Behavioral changes, irritability, increased reflexes, muscle cramps, ataxia, nystagmus, tetany, convulsions, tachycardia, hypotension
ATP, Adenosine triphosphate; PTH, parathyroid hormone.
Acid-Base Balance Acid-base balance must be regulated within a narrow range for the body to function normally. Slight changes in amounts of hydrogen and changes in pH can significantly alter biologic processes in cells and tissues.17 Hydrogen ion is needed to maintain membrane integrity and the speed of metabolic enzyme reactions. Most pathologic conditions disturb acid-base balance, producing circumstances possibly more harmful than the disease process itself.
Hydrogen Ion and pH The concentration of hydrogen ions in body fluids is very small—approximately 0.0000001 mg/L. This number, which may be expressed as 10−7 mg/L, is indicated as pH 7.0. The symbol pH represents the acidity or alkalinity of a solution. As the pH changes 1 unit (e.g., from pH 7.0 to pH 6.0), the [H+] ([H+] = hydrogen ion concentration) changes tenfold. The greater the [H+], the more acidic the solution and the lower the pH. The lower the [H+], the more alkaline or basic the solution and the higher the pH. In biologic fluids, a pH of less than 7.4 is defined as acidic and a pH greater than 7.4 is defined as alkaline or basic (Table 5-8).
TABLE 5-8 pH of Body Fluids
Body Fluid pH Factors Affecting pH Gastric juices 1.0-3.0 Hydrochloric acid production Urine 5.0-6.0 H+ ion excretion from waste products Arterial blood 7.35-7.45 pH is slightly higher because there is less carbonic acid (H2CO3) Venous blood 7.37 pH is slightly lower because there is more carbonic acid Cerebrospinal fluid 7.32 Decreased bicarbonate and higher carbon dioxide content decrease pH Pancreatic fluid 7.8-8.0 Contains bicarbonate produced by exocrine cells Bile 7.0-8.0 Contains bicarbonate Small intestine fluid 6.5-7.5 Contains alkaline fluid from pancreas, liver, and gallbladder
Body acids are formed as end products of protein, carbohydrate, and fat metabolism and acids can release hydrogen ion. Acids must be balanced by the amount of basic substances in the body to maintain normal pH. The lungs, kidneys, and bones are the major organs involved in regulating acid-base balance. The systems work together to regulate short- and long-term changes in acid-base status. Body acids exist in two forms: volatile (can be eliminated as CO2 gas) and
nonvolatile (can be eliminated by the kidney). The volatile acid is carbonic acid (H2CO3), a weak acid (i.e., it does not release its hydrogen easily). In the presence of the enzyme carbonic anhydrase, it readily dissociates into carbon dioxide (CO2) and
water (H2O). The carbon dioxide is then eliminated by pulmonary ventilation. Nonvolitile acids are sulfuric, phosphoric, and other organic acids. They are
strong acids (readily release their hydrogens). Nonvolatile acids are secreted into the urine by the renal tubules in amounts of about 60 to 100 mEq of hydrogen per day or about 1 mEq per kilogram of body weight.
Buffer Systems Buffering occurs in response to changes in acid-base status. Buffers can absorb excessive hydrogen ion (H+) (acid) or hydroxyl ion (OH−) (base) and prevent a significant change in pH. The buffer systems are located in both the ICF and the ECF compartments, and they function at different rates (Table 5-9). The most important plasma buffer systems are carbonic acid–bicarbonate and the protein hemoglobin (Figure 5-9). Phosphate and protein are the most important intracellular buffers and provide a first line of defense. Ammonia and phosphate can attach hydrogen ions and are important renal buffers.
TABLE 5-9 Buffer Systems
Buffer Pairs Buffer System Chemical Reaction Rate Bicarbonate Instantaneously
Hb−/HHb Hemoglobin HHb ⇌ H+ + Hb− Instantaneously Phosphate Instantaneously
Pr−/HPr Plasma proteins HPr ⇌ H+ + Pr− Instantaneously Organs Physiologic Mechanism Rate Lung ventilation Regulates retention or elimination of CO2 and therefore H2CO3 concentration Minutes to hours Ionic shifts Exchange of intracellular potassium and sodium for hydrogen 2-4 hours Kidney tubules Bicarbonate reabsorption and regeneration, ammonia formation, phosphate buffering Hours to days Bone Exchanges of calcium and phosphate and release of carbonate Hours to days
CO2, Carbon dioxide; Hb −, hemoglobin; , bicarbonate; H2CO3, carbonic acid; HHb, hydrogenated
hemoglobin; , dibasic phosphate; , monobasic phosphate; HPr, hydrogenated protein; Pr−, protein.
FIGURE 5-9 Integration of pH Control Mechanisms (example for acidosis). CO2 is produced in tissue cells and diffuses to plasma, where it is transported as dissolved CO2, or it combines
with water to form carbonic acid (H2CO3), or it combines with protein from which hydrogen has
been released. Most of the CO2 diffuses into the red blood cells and combines with water to
form H2CO3. The H2CO3 dissociates to form hydrogen ion (H +) and bicarbonate ( ).
Hydrogen combines with hemoglobin that has released its oxygen to form HHb, which buffers the hydrogen and makes venous blood slightly more acidic than arterial blood. The increase in H+ coupled with elevated CO2 levels results in HHbCO3 and an increase in the respiratory rate
and secretion of H+ by the kidneys.
Carbonic Acid–Bicarbonate Buffering The carbonic acid–bicarbonate buffer pair operates in both the lung and the kidney and is a major extracellular buffer. The lungs are a second line of defense and can relatively quickly (within seconds to minutes) decrease the amount of carbonic acid by blowing off carbon dioxide and leaving water. The kidneys are a third line of defense (hours to days) and can reabsorb bicarbonate (a type of base) or regenerate new bicarbonate from carbon dioxide and water. The relationship between bicarbonate ( ) and carbonic acid (H2CO3) is usually expressed as a ratio. Normal bicarbonate level is about 24 mEq/L, and normal carbonic acid level is about 1.2 mEq/L (when the arterial CO2 partial pressure [PaCO2] is 40 mm Hg), producing a 20 : 1 (24/1.2) ratio and the normal pH of 7.4 (Figure 5-10). These two systems are very effective together because the lungs can adjust acid concentration rapidly by ventilation and bicarbonate is easily reabsorbed or regenerated by the kidney tubules, although more slowly.
FIGURE 5-10 Ratio of Carbonic Acid and Bicarbonate Concentration in Maintaining pH Within
Normal Limits. An increase in H2CO3 or decrease in concentration causes acidosis. A
decrease in H2CO3 or increase in concentration causes alkalosis. H2CO3, Carbonic acid;
, bicarbonate. (From Monahan FD: Medical-surgical nursing: health and illness perspectives, ed 8, St Louis, 2007, Mosby.)
Renal and respiratory adjustments to primary changes in pH are known as compensation. The respiratory system compensates for changes in pH by increasing or decreasing the concentration of carbon dioxide (carbonic acid) by changing ventilation. The renal system compensates by producing more acidic or more alkaline urine. The values for PaCO2 and bicarbonate will vary from normal levels in an attempt to maintain a ratio of 20 : 1. Correction occurs when the values for both components of the buffer pair (carbonic acid and bicarbonate) return to normal levels.
Protein Buffering Both intracellular and extracellular proteins have negative charges and can serve as buffers for hydrogen, but because most proteins are inside cells, they are primarily an intracellular buffer system. Hemoglobin (Hb) is an excellent intracellular blood buffer because it can bind with hydrogen ion (H+) (forming HHb) and carbon dioxide (forming HHbCO2). Hemoglobin bound to hydrogen ion becomes a weak acid. Hemoglobin not saturated with oxygen (venous blood) is a better buffer than
hemoglobin saturated with oxygen (arterial blood). The pH control mechanism is illustrated in Figure 5-9.
Renal Buffering The distal tubule of the kidney regulates acid-base balance by secreting hydrogen into the urine and reabsorbing bicarbonate into the plasma. Dibasic phosphate ( ) and ammonia (NH3) are two important renal buffers because they can attach hydrogen ions and be secreted into the urine. The renal buffering of hydrogen ions requires the use of carbon dioxide (CO2) and water (H2O) to form carbonic acid (H2CO3). The enzyme carbonic anhydrase catalyzes the reaction. The hydrogen in the carbonic acid is then secreted from the tubular cell and buffered in the lumen by phosphate and ammonia (i.e., forms and ). The remaining bicarbonate is reabsorbed. The end effect is the addition of new bicarbonate to the plasma, which contributes to the alkalinity of the plasma because the hydrogen ion is excreted from the body (Figure 5-11).
FIGURE 5-11 Renal Excretion of Acid. 1, Conservation of filtered bicarbonate. Filtered bicarbonate combines with secreted hydrogen ion in the presence of carbon anhydrase (CA) to form carbonic acid (H2CO3), which then dissociates to water (H2O) and carbon dioxide (CO2); both diffuse into the epithelial cell. The CO2 and H2O combine to form H2CO3 in the presence of
CA, and the resulting bicarbonate ion ( ) is reabsorbed into the capillary. 2, Formation of
titratable acid. Hydrogen ion is secreted and combines with dibasic phosphate ( ) to form
monobasic phosphate ( ). The secreted hydrogen ion is formed from the dissociation of
H2CO3, and the remaining is reabsorbed into the capillary. 3, Formation of ammonium. Ammonia (NH3) is produced from glutamine in the epithelial cell and diffuses to the tubular
lumen, where it combines with H+ to form ammonium ion ( ). Once has been formed, it cannot return to the epithelial cell (diffusional trapping), and the bicarbonate remaining in the
epithelial cell is reabsorbed into the capillary.
Acid-Base Imbalances Pathophysiologic changes in the concentration of hydrogen ion in the blood lead to acid-base imbalances.18,19 In acidemia the pH of arterial blood is less than 7.4. A systemic increase in hydrogen ion concentration or a loss of base is termed acidosis. In alkalemia the pH of arterial blood is greater than 7.4. A systemic decrease in hydrogen ion concentration or an excess of base is termed alkalosis. These changes may be caused by metabolic or respiratory processes. Figure 5-10 summarizes the relationship among pH, the partial pressure of carbon dioxide (respiratory regulation), and the concentration of bicarbonate (renal regulation) during alkalosis and acidosis. Acid-base imbalances are assessed using measurement of arterial blood gases, which includes the reporting of pH, PaCO2, and
. The medical history and clinical symptoms are important in determining the cause of the disorder. Figure 5-12 summarizes the relationships among pH, PCO2, and bicarbonate during different acid-base alterations.
FIGURE 5-12 Primary and Compensatory Acid-Base Changes. A systematic approach can be used to interpret the cause of an acid-base imbalance. 1, Is the pH low or high? 2, If the pH is
low (acidemia), is the cause respiratory (high PaCO2) or metabolic (low )? 3, If the pH is
high (alkalemia), is the cause respiratory (low PaCO2) or metabolic (high )? 4, Is there
compensation for the primary acid-base disorder? (a) will be ≥24 mEq/L if there is renal compensation for a primary respiratory acidosis; (b) PaCO2 will be <40 mm Hg if there is
respiratory compensation of a primary metabolic acidosis; (c) will be <24 mEq/L if there is renal compensation for primary respiratory alkalosis; (d) PaCO2 will be >40 mm Hg if there is
respiratory compensation for primary metabolic alkalosis. NOTE: Examine the pH first to
determine if there is acidemia or alkalemia. Then examine the changes in and PaCO2. 1,
will be elevated when there is primary metabolic alkalosis or renal compensation for
primary respiratory acidosis. 2, will be decreased when there is primary metabolic acidosis or renal compensation for primary respiratory alkalosis. 3, PaCO2 will be elevated
when there is primary respiratory acidosis or respiratory compensation for primary metabolic alkalosis. 4, PaCO2 will be decreased when there is primary respiratory alkalosis or respiratory
compensation for metabolic acidosis. H2CO3, Carbonic acid; , bicarbonate; PaCO2, arterial partial pressure of carbon dioxide.
Metabolic Acidosis In metabolic acidosis the concentrations of non–carbonic acids increase or bicarbonate is lost from extracellular fluid or cannot be regenerated by the kidney
(Table 5-10). This can occur either quickly, as in lactic acidosis caused by poor perfusion or hypoxemia, or slowly over an extended time, as in renal failure, diabetic ketoacidosis, or starvation (anion gap acidosis).20 There is a decrease in the 20 : 1 ratio of to H2CO3.
TABLE 5-10 Causes of Metabolic Acidosis
Increased Non–Carbonic Acids (Elevated Anion Gap*) Bicarbonate Loss or Hyperchloremic Acidosis (Normal Anion Gap)
Increased H+ load Diarrhea Ketoacidosis (e.g., diabetes mellitus, starvation) Lactic acidosis (e.g., shock, hypoxemia) Ingestion (e.g., ammonium chloride, ethylene glycol, methanol, salicylates, paraldehyde)
Ureterosigmoidoscopy (chloride absorbed in excess of sodium in small intestine) Renal failure (loss of bicarbonate) Proximal renal tubular acidosis (loss of more renal sodium in relation to chloride)Decreased renal H+ excretion
Uremia Distal renal tubule acidosis
*Anion gap refers to anions not usually measured in laboratory reports (e.g., sulfate, phosphate, and
lactate). The anions usually measured are chloride (Cl−) and bicarbonate ( ). When the sum of the concentrations of measured anions (e.g., chloride and bicarbonate) is subtracted from the sum of the concentrations of measured cations (e.g., sodium and potassium), there is a “gap” of approximately 10 to 12 mEq/L; this is the normal anion gap. An elevated anion gap provides clues to the cause of the acidosis (i.e., to the addition of endogenously or exogenously generated acids). In a normal anion gap acidosis, chloride is retained to replace lost bicarbonate.
The buffering systems normally compensate for excess acid and maintain arterial pH within normal range. When acidosis is severe, buffers become depleted and cannot compensate, and the ratio of the concentrations of bicarbonate to carbonic acid decreases to less than 20 : 1 (see Figure 5-10). An increase in the plasma concentration of chloride out of proportion of sodium causes hyperchloremic acidosis (nonainion gap acidosis). The specific type of acidosis can be determined by examining the serum anion gap (see Table 5-10). Metabolic acidosis is manifested by changes in the function of the neurologic,
respiratory, gastrointestinal, and cardiovascular systems. Early symptoms include headache and lethargy, which progress to confusion and coma in severe acidosis. The respiratory system's efforts to compensate for the increase in metabolic acids result in what are termed Kussmaul respirations (a form of hyperventilation), which are deep and rapid. This represents the body's attempt to increase pH by expelling carbon dioxide, which decreases carbonic acid concentration. Other symptoms include anorexia, nausea, vomiting, diarrhea, and abdominal discomfort. Death can result in the most severe and prolonged cases preceded by dysrhythmias and hypotension. The underlying condition must be diagnosed to establish effective treatment.
Metabolic Alkalosis When excessive loss of metabolic acids occurs, bicarbonate concentration increases, causing metabolic alkalosis21 (see Figure 5-12). When acid loss is caused by vomiting, renal compensation is not very effective because loss of chloride (an anion) in hydrochloric acid (HCl) stimulates renal retention of bicarbonate (an anion). The result is known as hypochloremic metabolic alkalosis.21 Hyperaldosteronism also can lead to alkalosis as a result of sodium bicarbonate retention and loss of hydrogen and potassium. Diuretics may produce a mild alkalosis because they promote greater excretion of sodium, potassium, and chloride than of bicarbonate. Some common signs and symptoms of metabolic alkalosis are weakness, muscle
cramps, hyperactive reflexes, tetany, confusion, convulsions, and atrial tachycardia. Respirations may be shallow and slow ventilation as the lungs attempt to compensate by increasing carbon dioxide retention. The manifestations vary with the cause and severity of the alkalosis. The symptoms of hyperactive reflexes and tetany occur because alkalosis increases binding of Ca++ to plasma proteins, thus decreasing ionized calcium concentration. The decreased ionized calcium concentration causes excitable cells to become hypopolarized, initiating an action potential more easily and causing muscle contraction. Treatments are related to the underlying cause of the condition. With
hypochloremic alkalosis or contraction alkalosis with volume depletion, a sodium chloride solution is required for correction because chloride must be replaced before bicarbonate can be excreted by the kidney.
Respiratory Acidosis Respiratory acidosis occurs when there is alveolar hypoventilation, resulting in an excess of carbon dioxide in the blood (hypercapnia). The arterial carbon dioxide tension (or pressure) (PaCO2) is >45 mm Hg and the pH is less than 7.35 (see Figure 5-12). A decrease in alveolar ventilation in relation to the metabolic production of carbon dioxide produces respiratory acidosis by an increase in the concentration of carbonic acid. Respiratory acidosis can be acute or chronic.22 Common causes include depression of the respiratory center (e.g., from drugs or head injury), paralysis of the respiratory muscles, disorders of the chest wall (e.g., kyphoscoliosis or broken ribs), and disorders of the lung parenchyma (e.g., pneumonia, pulmonary edema, emphysema, asthma, bronchitis). Renal compensation occurs by elimination of hydrogen ion and retention of bicarbonate. The signs and symptoms seen often include headache, blurred vision,
breathlessness, restlessness, and apprehension followed by lethargy, disorientation,
muscle twitching, tremors, convulsions, and coma. Respiratory rate is rapid at first and gradually becomes depressed as the respiratory center adapts to increasing levels of carbon dioxide. The skin may be warm and flushed because the elevated carbon dioxide concentration causes vasodilation. The restoration of adequate alveolar ventilation is necessary to remove the excess CO2 (↓H2CO3).
Respiratory Alkalosis Respiratory alkalosis occurs when there is alveolar hyperventilation (deep, rapid respirations). Excessive reduction in plasma carbon dioxide levels (hypocapnia) decrease carbonic acid concentration.23,24 The PaCO2 is <35 mm Hg and the pH is greater than normal (see Figure 5-12). Respiratory alkalosis can be chronic or acute. Hypoxemia (caused by pulmonary disease, congestive heart failure, or high altitudes), hypermetabolic states (e.g., fever, anemia, thyrotoxicosis), early salicylate intoxication, hysteria, cirrhosis, and gram-negative sepsis stimulate hyperventilation. Improper use of mechanical ventilators also can cause iatrogenic (treatment-related) respiratory alkalosis, and secondary alkalosis may develop as a result of hyperventilation stimulated by metabolic or respiratory acidosis. The kidneys compensate by decreasing hydrogen excretion and bicarbonate reabsorption. The central and peripheral nervous systems are stimulated by respiratory
alkalosis, causing dizziness, confusion, tingling of extremities (paresthesias), convulsions, and coma. Cerebral vasoconstriction reduces cerebral blood flow. Carpopedal spasm (spasm of muscles in the fingers and toes), tetany, and other symptoms of hypocalcemia (see Table 5-7, p. 126) are similar to those of metabolic alkalosis. The underlying disturbance must be treated, particularly hypoxemia.
Quick Check 5-5
1. What is the difference between compensation and correction of acid-base disturbances?
2. What two chemicals are altered in metabolic acid-base disturbances?
3. How do alterations in carbon dioxide concentration influence acid-base status?
Pediatric Considerations
Distribution of Body Fluids
Newborn Infants At birth TBW represents about 75% to 80% of body weight and decreases to about 67% during the first year of life. Physiologic loss of body water amounting to 5% of body weight occurs as an infant adjusts to a new environment. Infants are particularly susceptible to significant changes in TBW because of a high metabolic rate and greater body surface area, as compared to adults. Consequently, they have a greater fluid intake and output in relation to their body size. Renal mechanisms of fluid and electrolyte conservation may not be mature enough to counter abnormal losses related to vomiting or diarrhea, thereby allowing dehydration to occur. Symptoms of dehydration include increased thirst, decreased urine output, decreased body weight, decreased skin elasticity, sunken fontanels, absent tears, dry mucous membranes, increased heart rate, and irritability.
Children and Adolescents TBW slowly decreases to 60% to 65% of body weight. At adolescence the percentage of TBW approaches adult levels and differences according to gender appear. Males have a greater percentage of body water because of increased muscle mass, and females have more body fat because of the influence of estrogen and thus less water.
Geriatric Considerations Distribution of Body Fluids
The further decline in the percentage of TBW in the elderly is in part the result of a decreased free fat mass and decreased muscle mass, as well as a reduced ability to regulate sodium and water balance. Kidneys are less efficient in producing either a concentrated or a diluted urine, and sodium-conserving responses are sluggish. Thirst perception also may decline and loss of cognitive function can influence access to beverages. Healthy older adults can adequately maintain their hydration status. When disease is present, a decrease in TBW, dehydration, and hypernatremia can become life-threatening.
Did You Understand? Distribution of Body Fluids 1. Body fluids are distributed among functional compartments and are classified as intracellular fluid (ICF) and extracellular fluid (ECF).
2. The sum of all fluids is the total body water (TBW), which varies with age and amount of body fat.
3. Water moves between the ICF and ECF compartments principally by osmosis.
4. Water moves between the plasma and interstitial fluid by osmosis (pulling of water) and hydrostatic pressure (pushing of water), which occur across the capillary membrane.
5. Movement across the capillary wall is called net filtration and is described according to Starling law (the balance between hydrostatic and osmotic forces).
Alterations in Water Movement 1. Edema is a problem of fluid distribution that results in accumulation of fluid within the interstitial spaces.
2. The pathophysiologic process that leads to edema is related to an increase in forces favoring fluid filtration from the capillaries or lymphatic channels into the tissues.
3. Edema is caused by arterial dilation, venous or lymphatic obstruction, increased vascular volume, loss of plasma proteins, or increased capillary permeability.
4. Edema may be localized or generalized and usually is associated with weight gain, swelling and puffiness, tighter-fitting clothes and shoes, and limited movement of the affected area.
Sodium, Chloride, and Water Balance 1. There is an intimate relationship between the balance of sodium and water levels; chloride levels are generally proportional to changes in sodium levels.
2. Water balance is regulated by the sensation of thirst and by antidiuretic hormone (ADH), which is secreted in response to an increase in plasma osmolality or a decrease in circulating blood volume.
3. Sodium balance is regulated by aldosterone, which increases reabsorption of sodium from the urine into the blood by the distal tubule of the kidney.
4. Renin and angiotensin are enzymes that promote secretion of aldosterone and thus regulate sodium and water balance.
5. Natriuretic hormones are involved in decreasing tubular reabsorption and promoting urinary excretion of sodium.
Alterations in Sodium, Water, and Chloride Balance 1. Alterations in sodium and water balance may be classified as isotonic, hypertonic, or hypotonic.
2. Isotonic alterations occur when changes in TBW are accompanied by proportional changes in electrolytes.
3. Hypertonic alterations develop when the osmolality of the ECF is elevated above normal, usually because of an increased concentration of ECF sodium or a deficit of ECF water.
4. Hypernatremia (sodium levels more than 145 mEq/L) may be caused by an acute increase in sodium level or a loss of water.
5. Hypernatremia can be isovolemic, hypovolemic, or hypervolemic depending on accompanying changes in the level of body water.
6. Hypernatremia with marked water deficit is manifested by hypovolemia and dehydration.
7. Hyperchloremia is caused by an excess of sodium or a deficit of bicarbonate.
8. Hypotonic alterations occur when the osmolality of the ECF is less than normal.
9. Hyponatremia (serum sodium concentration less than 135 mEq/L) usually causes movement of water into cells.
10. Hyponatremia may be caused by sodium loss, inadequate sodium intake, or dilution of the body's sodium level with excess water.
11. Hyponatremia can be isovolemic, hypervolemic or hypovolemic, or dilutional depending on accompanying changes in the amount of body water.
12. Hypochloremia usually is the result of hyponatremia or elevated bicarbonate concentrations.
Alterations in Potassium and Other Electrolytes 1. Potassium is the predominant ICF ion; it regulates ICF osmolality, maintains the resting membrane potential, and is required for deposition of glycogen in liver and skeletal muscle cells.
2. Potassium balance is regulated by the kidney, by aldosterone and insulin secretion, and by changes in pH.
3. Potassium adaptation allows the body to accommodate slowly to increased levels of potassium intake.
4. Hypokalemia (serum potassium concentration less than 3.5 mEq/L) indicates loss of total body potassium, although ECF hypokalemia can develop without losses of total body potassium, and plasma potassium levels may be normal or elevated when total body potassium is depleted.
5. Hypokalemia may be caused by reduced potassium intake, a shift of potassium from the ECF to the ICF, increased aldosterone secretion, increased renal excretion, and alkalosis.
6. Hyperkalemia (potassium levels that are greater than 5.5 mEq/L) may be caused by increased potassium intake, a shift of potassium from the ICF to the ECF, or decreased renal excretion.
7. Calcium is an ion necessary for bone and teeth formation, blood coagulation, hormone secretion and cell receptor function, and membrane stability.
8. Phosphate acts as a buffer in acid-base regulation and provides energy for muscle contraction.
9. Calcium and phosphate concentrations are rigidly controlled by parathyroid hormone (PTH), vitamin D, and calcitonin.
10. Hypocalcemia (serum calcium concentration less than 8.5 mg/dl) is related to inadequate intestinal absorption, deposition of calcium into bone or soft tissue, blood administration, or decreased PTH and vitamin D levels.
11. Hypercalcemia (serum calcium concentration greater than 12 mg/dl) can be caused by a number of diseases, including hyperparathyroidism, bone metastases, sarcoidosis, and excess vitamin D.
12. Hypophosphatemia is usually caused by intestinal malabsorption and increased renal excretion of phosphate.
13. Hyperphosphatemia develops with acute or chronic renal failure when there is significant loss of glomerular filtration.
14. Magnesium is a major intracellular cation and is regulated principally by PTH.
15. Magnesium functions in enzymatic reactions and often interacts with calcium at the cellular level.
16. Hypomagnesemia (serum magnesium concentrations less than 1.5 mEq/L) may be caused by malabsorption syndromes.
17. Hypermagnesemia (serum magnesium concentrations greater than 2.5 mEq/L) is rare and usually is caused by renal failure.
Acid-Base Balance 1. Hydrogen ions, which maintain membrane integrity and the speed of enzymatic reactions, must be concentrated within a narrow range if the body is to function normally.
2. Hydrogen ion concentration, [H+], is expressed as pH, which represents the negative logarithm (i.e., 10−7) of hydrogen ions in solution (i.e., 0.0000001 mg/L).
3. Different body fluids have different pH values; values less than 7.4 are more acidic and values greater than 7.4 are more basic.
4. The renal and respiratory systems, together with the body's buffer systems, are the principal regulators of acid-base balance.
5. Buffers are substances that can absorb excessive acid or base without a significant change in pH.
6. Buffers exist as acid-base pairs; the principal plasma buffers are carbonic acid (H2CO3), bicarbonate ( ), protein (hemoglobin), and phosphate.
7. The lungs and kidneys act to compensate for primary changes in pH by increasing or decreasing ventilation and by producing more acidic or more alkaline urine.
8. Correction is a process different from compensation; correction occurs when the values for both components of the buffer pair return to normal as the primary disorder is treated or resolves.
9. Acid-base imbalances are caused by changes in the concentration of hydrogen ion in the blood; an increase causes acidosis, and a decrease causes alkalosis.
10. An abnormal increase or decrease in bicarbonate concentration causes metabolic alkalosis or metabolic acidosis; changes in the rate of alveolar ventilation and removal of carbon dioxide produce respiratory acidosis or respiratory alkalosis.
11. Metabolic acidosis is caused by an increase in the levels of non–carbonic acids or by the loss of bicarbonate from the extracellular fluid.
12. Metabolic alkalosis occurs with an increase in bicarbonate concentration, which is usually caused by loss of metabolic acids from conditions such as vomiting or gastrointestinal suctioning or by excessive bicarbonate intake, hyperaldosteronism, and diuretic therapy.
13. Respiratory acidosis occurs with decreased alveolar ventilation, which in turn causes hypercapnia (an increase in carbon dioxide concentration) and increased carbonic acid concentration.
14. Respiratory alkalosis occurs with alveolar hyperventilation and excessive reduction of carbon dioxide level, or hypocapnia with decreases in carbonic acid concentration.
Key Terms Acidemia, 127
Acidosis, 127
Aldosterone, 117
Alkalemia, 127
Alkalosis, 127
Angiotensin I, 117
Angiotensin II, 117
Anion gap, 127
Aquaporin, 115
Antidiuretic hormone (ADH), 117
Baroreceptor, 119
Buffer, 125
Buffering, 125
Capillary hydrostatic pressure (blood pressure), 115
Capillary (plasma) oncotic pressure, 115
Carbonic acid–bicarbonate buffer, 125
Chloride (Cl−), 117
Compensation, 125
Correction, 125
Dehydration, 119
Dilutional hyponatremia (water intoxication), 121
Edema, 115
Extracellular fluid (ECF), 114
Hypercapnia, 128
Hyperchloremia, 120
Hyperkalemia, 124
Hypernatremia, 119
Hypertonic fluid alterations, 119
Hypervolemic hypernatremia, 119
Hypervolemic hyponatremia, 121
Hypocapnia, 130
Hypochloremia, 121
Hypochloremic metabolic alkalosis, 127
Hypokalemia, 122
Hyponatremia, 121
Hypotonic fluid imbalance, 121
Hypovolemic hypernatremia, 119
Hypovolemic hyponatremia, 121
Interstitial fluid, 114
Interstitial hydrostatic pressure, 115
Interstitial oncotic pressure, 115
Intracellular fluid (ICF), 114
Intravascular fluid, 114
Isotonic alteration, 119
Isotonic fluid excess, 119
Isotonic fluid loss, 119
Isovolemic hypernatremia, 119
Isovolemic hyponatremia, 121
Lymphedema, 116
Metabolic acidosis, 127
Metabolic alkalosis, 127
Natriuretic peptide, 117
Net filtration, 115
Nonvolatile, 125
Osmoreceptor, 118
Potassium (K+), 122
Potassium adaptation, 122
Renin, 117
Renin-angiotensin-aldosterone system, 117
Respiratory acidosis, 128
Respiratory alkalosis, 130
Sodium (Na+), 117
Starling forces, 115
Total body water (TBW), 114
Volatile, 125
Volume-sensitive receptor, 119
Water balance, 118
References 1. Day RE, et al. Human aquaporins: regulators of transcellular water flow. Biochim Biophys Acta. 2014;1840(5):1492–1506.
2. Trayes KP, et al. Edema: diagnosis and management. Am Fam Physician. 2013;88(2):102–110.
3. Ridner SH. Pathophysiology of lymphedema. Semin Oncol Nurs. 2013;29(1):4–11.
4. Motiwala SR, Januzzi JL Jr. The role of natriuretic peptides as biomarkers for guiding the management of chronic heart failure. Clin Pharmacol Ther. 2013;93(1):57–67.
5. Cheuvront SN, et al. Physiologic basis for understanding quantitative dehydration assessment. Am J Clin Nutr. 2013;97(3):455–462.
6. Sam R, Feizi I. Understanding hypernatremia. Am J Nephrol. 2012;36(1):97– 104.
7. Berend K, van Hulsteijn LH, Gans RO. Chloride: the queen of electrolytes? Eur J Intern Med. 2012;23(3):203–211.
8. Sterns RH. Disorders of plasma sodium—causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
9. Gross P. Clinical management of SIADH. Ther Adv Endocrinol Metab. 2012;3(2):61–73.
10. Lehrich RW, et al. Role of vaptans in the management of hyponatremia. Am J Kidney Dis. 2013;62(2):364–376.
11. Youn JH. Gut sensing of potassium intake and its role in potassium homeostasis. Semin Nephrol. 2013;33(3):248–256.
12. Lee Hamm L, et al. Acid-base and potassium homeostasis. Semin Nephrol. 2013;33(3):257–264.
13. Pepin J, Shields C. Advances in diagnosis and management of hypokalemic and hyperkalemic emergencies. Emerg Med Pract. 2012;14(2):1–17.
14. Lim S. Approach to hyperkalemia. Acta Med Indones. 2007;39(2):99–103. 15. Maxwell AP, et al. Management of hyperkalaemia. J R Coll Physicians
Edinb. 2013;43(3):246–251. 16. Moe SM. Disorders involving calcium, phosphorus and magnesium. Prim
Care. 2008;35(2):215–237. 17. Adeva-Andany MM, et al. The importance of the ionic product for water to
understand the physiology of the acid-base balance in humans. Biomed Res Int. 2014;2014:695281.
18. Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434–1445 [Erratum
in: N Engl J Med 371(20):1948]. 19. Carmody JB, Norwood VF. A clinical approach to paediatric acid-base
disorders. Postgrad Med J. 2012;88(1037):143–151. 20. Kraut JA, Madias NE. Differential diagnosis of nongap metabolic acidosis:
value of a systematic approach. Clin J Am Soc Nephrol. 2012;7(4):671–679. 21. Soifer JT, Kim HT. Approach to metabolic alkalosis. Emerg Med Clin North
Am. 2014;32(2):453–463. 22. Bruno CM, Valenti M. Acid-base disorders in patients with chronic
obstructive pulmonary disease: a pathophysiological review. J Biomed Biotechnol. 2012;2012:915150.
23. Palmer BF. Evaluation and treatment of respiratory alkalosis. Am J Kidney Dis. 2012;60(5):834–838.
24. Madias NE. Renal acidification responses to respiratory acid base disorders. J Nephrol. 2010;16(Suppl 16):S85–S91.
UNIT 2 Mechanisms of Self-Defense
OUTLINE 6 Innate Immunity: Inflammation and Wound Healing 7 Adaptive Immunity 8 Infection and Defects in Mechanisms of Defense 9 Stress and Disease
6
Innate Immunity
Inflammation and Wound Healing Neal S. Rote
CHAPTER OUTLINE
Human Defense Mechanisms, 134 Innate Immunity, 134
First Line of Defense: Physical and Biochemical Barriers and the Human Microbiome, 135 Second Line of Defense: Inflammation, 137 Plasma Protein Systems and Inflammation, 138 Cellular Components of Inflammation, 141
Acute and Chronic Inflammation, 147
Local Manifestations of Acute Inflammation, 149 Systemic Manifestations of Acute Inflammation, 149 Chronic Inflammation, 149
Wound Healing, 151
Phase I: Inflammation, 152 Phase II: Proliferation and New Tissue Formation, 152 Phase III: Remodeling and Maturation, 153
Dysfunctional Wound Healing, 153 PEDIATRIC CONSIDERATIONS: Age-Related Factors Affecting Innate Immunity in the Newborn Child, 154 GERIATRIC CONSIDERATIONS: Age-Related Factors Affecting Innate Immunity in the Elderly, 154
The human body is continually exposed to a large variety of conditions that result in damage, such as sunlight, pollutants, agents that can cause physical trauma, and infectious agents (viruses, bacteria, fungi, parasites). Damage can also arise from within, such as cancers. The damage may be at the level of a single cell, which can be easily repaired, or may be at the level of multiple cells or tissues or organs, which can result in disease and potentially the death of the individual. To protect us from these conditions, the body has developed a highly sophisticated, multilevel system of interactive defense mechanisms.
Human Defense Mechanisms The human body has developed several means of protecting itself from injury and infection. Innate immunity, also known as natural or native immunity, includes natural barriers (physical, mechanical, and biochemical) and inflammation. Innate barriers form the first line of defense at the body's surfaces and are in place at birth to prevent damage by substances in the environment and thwart infection by pathogenic microorganisms. Surface barriers may also harbor a group of microorganisms known as the “normal flora” that can protect us from pathogens. If the surface barriers are breached, the second line of defense, the inflammatory response, is activated to protect the body from further injury, prevent infection of the injured tissue, and promote healing. The inflammatory response is a rapid activation of biochemical and cellular mechanisms that are relatively nonspecific, with similar responses being initiated against a wide variety of causes of tissue damage. The third line of defense, adaptive immunity (also known as acquired or specific immunity), is induced in a relatively slower and more specific process and targets particular invading microorganisms for the purpose of eradicating them. Adaptive immunity also involves “memory,” which results in a more rapid response during future exposure to the same microorganism. Comparisons among defense mechanisms are described in Table 6-1. The information presented in this chapter introduces the components and processes of innate immunity and sets the stage for Chapter 7, which presents an overview of adaptive immunity, and Chapter 8, which discusses processes of infection and alterations in immune defenses.
TABLE 6-1 Overview of Human Defenses
Characteristics Barriers Innate Immunity Adaptive (Acquired) Immunity
Level of defense First line of defense against infection and tissue injury
Second line of defense; occurs as response to tissue injury or infection (inflammatory response)
Third line of defense; initiated when innate immune system signals cells of adaptive immunity
Timing of defense
Constant Immediate response Delay between primary exposure to antigen and maximal response; immediate against secondary exposure to antigen
Specificity Broadly specific Broadly specific Response is very specific toward “antigen”
Cells Epithelial cells Microbiome
Mast cells, granulocytes (neutrophils, eosinophils, basophils), monocytes/ macrophages, natural killer (NK) cells, platelets, endothelial cells
T lymphocytes, B lymphocytes, macrophages, dendritic cells
Memory No memory involved No memory involved Specific immunologic memory by T and B lymphocytes
Active molecules Defensins, cathelicidins, collectins, lactoferrin, bacterial toxins
Complement, clotting factors, kinins, cytokines Antibodies, complement, cytokines
Protection Protection includes anatomic barriers (i.e., skin and mucous membranes), cells and secretory molecules (e.g., lysozymes, low pH of stomach and urine), and ciliary activity
Protection includes vascular responses, cellular components (e.g., mast cells, neutrophils, macrophages), secretory molecules or cytokines, and activation of plasma protein systems
Protection includes activated T and B lymphocytes, cytokines, and antibodies
Innate Immunity Innate immunity includes natural barriers (physical, mechanical, and biochemical) that form the first line of defense at the body's surfaces and are in place at birth. Surface barriers also may harbor a group of frequently benign microorganisms known as the “normal microbiome” that can protect us from pathogenic microorganisms. Innate immunity in the newborn and changes associated with aging are reviewed in the Pediatric and Geriatric Considerations boxes.
First Line of Defense: Physical and Biochemical Barriers and the Human Microbiome Physical Barriers The physical barriers that cover the external parts of the human body offer considerable protection from damage and infection. These barriers are composed of tightly associated epithelial cells of the skin and of the linings of the gastrointestinal, genitourinary, and respiratory tracts (Figure 6-1). When pathogens attempt to penetrate this physical barrier, they may be removed by mechanical means—sloughed off with dead skin cells as they are routinely replaced, expelled by coughing or sneezing, vomited from the stomach, or flushed from the urinary tract by urine. Epithelial cells of the upper respiratory tract also produce mucus and have hair-like cilia that trap and move pathogens upward to be expelled by coughing or sneezing. Additionally, the low temperature (such as on the skin) and the low pH (such as of the skin and stomach) generally inhibit microorganisms, most of which routinely require temperatures near 37° C (98.6° F) and pH near neutral for efficient growth.
FIGURE 6-1 The Closed Barrier. The digestive, respiratory, and genitourinary tracts and skin form closed barriers between the internal organs and the environment. (From Grimes DE: Infectious
diseases, St. Louis, 1991, Mosby.)
Epithelial Cell–Derived Chemicals Epithelial cells secrete an array of substances that protect against infection,
including mucus, perspiration (or sweat), saliva, tears, and earwax. These can trap potential invaders and contain substances that will kill microorganisms. Perspiration, tears, and saliva contain an enzyme (lysozyme) that attacks the cell walls of gram-positive bacteria. Sebaceous glands in the skin also secrete fatty acids and lactic acid that kill bacteria and fungi. These glandular secretions create an acidic (pH 3 to 5) and inhospitable environment for most bacteria. Epithelial cell secretions also contain small-molecular-weight antimicrobial
peptides that kill or inhibit the growth of disease-causing bacteria, fungi, and viruses.1 These are generally positively charged polypeptides of approximately 15 to 95 amino acids. More than a thousand antimicrobial peptides have been found, but the best studied are cathelicidins and defensins. Several cathelicidins have been discovered in other species, but only one is
currently known to function in humans. Bacteria have cholesterol-free cell membranes into which cathelicidin can insert and disrupt the membrane, killing the bacteria. Cathelicidin is produced by epithelial cells of the skin, gut, urinary tract, and respiratory tract, and is stored in neutrophils, mast cells, and monocytes and can be released during inflammation. In contrast, many different human defensins have been identified. Defensin
molecules can be further subdivided into α (at least six identified in humans) and β types (at least six identified, but perhaps up to 40 different molecules). The α- defensins often require activation by proteolytic enzymes, whereas the β-defensins are synthesized in active forms. Given the similarity in their chemical charges, defensins may kill bacteria in the same way as cathelicidin. The α-defensins are particularly rich in the granules of neutrophils and may contribute to the killing of bacteria by those cells. They are also found in Paneth cells lining the small intestine, where they protect against a variety of disease-causing microorganisms. The β- defensins are found in epithelial cells lining the respiratory, urinary, and intestinal tracts, as well as in the skin. In addition to antibacterial properties, β-defensins may also help protect epithelial surfaces from infection with adenovirus (one of the causes of the common cold) and human immunodeficiency virus (HIV). Both classes of antimicrobial peptides also can activate cells of the next levels of defense: innate and acquired immunity. The lung also produces and secretes a family of glycoproteins, collectins, which
includes surfactant proteins A through D and mannose-binding lectin. Collectins react with carbohydrates on the surface of a wide array of pathogenic microorganisms and help cells of the innate immune system (macrophages) to recognize and kill the microorganism. Mannose-binding lectin (MBL) recognizes a sugar commonly found on the surface of microbes and is a powerful activator of a plasma protein system (complement) resulting in damage to bacteria or increased
recognition by macrophages.
The Normal Microbiome The body's surfaces are colonized with an array of microorganisms, the normal microbiome previously known as normal flora. Each surface (the skin and the mucous membranes of the eyes, upper and lower gastrointestinal tracts, upper respiratory tract, urethra, and vagina) is colonized by a combination of bacteria and fungi that is unique to the particular location and individual2 (Table 6-2). The microorganisms in the microbiome do not normally cause disease, and although their relationship with humans has been referred to as commensal (to the benefit of one organism without affecting the other), the relationship may be more mutualistic (to the benefit of both organisms). Using the colon for an example, at birth the lower gut is relatively sterile but colonization with bacteria begins quickly, with the number, diversity, and concentration increasing progressively during the first year of life.
TABLE 6-2 The Human Microbiome
Location Microorganisms Skin Predominantly gram-positive cocci and rods; Staphylococcus epidermidis, corynebacteria, mycobacteria, and streptococci are primary
inhabitants; Staphylococcus aureus in some people; also yeasts (Candida, Pityrosporum) in some areas of skin Numerous transient microorganisms may become temporary residents In moist areas, gram-negative bacteria Around sebaceous glands, Propionibacterium and Brevibacterium Mite Demodex folliculorum lives in hair follicles and sebaceous glands around face
Nose Predominantly gram-positive cocci and rods, especially S. epidermidis Some people are nasal carriers of pathogenic bacteria, including S. aureus, β-hemolytic streptococci, and Corynebacterium diphtheria
Mouth Complex of bacteria that includes several species of streptococci, Actinomyces, lactobacilli, and Haemophilus Anaerobic bacteria and spirochetes colonize gingival crevices
Pharynx Similar to flora in mouth plus staphylococci, Neisseria, and diphtheroids Some asymptomatic persons also harbor pathogens: pneumococcus, Haemophilus influenzae, Neisseria meningitidis, and C. diphtheria
Distal small intestine
Enterobacteria, streptococci, lactobacilli, anaerobic bacteria, and C. albicans
Colon Bacteroides, lactobacilli, clostridia, Salmonella, Shigella, Klebsiella, Proteus, Pseudomonas, enterococci, and other streptococci, bacilli, and Escherichia coli
Distal urethra
Typical bacteria found on skin, especially S. epidermidis and diphtheroids; also lactobacilli and nonpathogenic streptococci
Vagina Birth to 1 month: similar to adult 1 month to puberty: S. epidermidis, diphtheroids, E. coli, and streptococci Puberty to menopause: Lactobacillus acidophilus, diphtheroids, staphylococci, streptococci, and variety of anaerobes Postmenopause: similar to prepubescence
Adapted from Bennett JE et al, editors: Mandell, Douglas, and Bennett's principles and practice of infectious diseases, ed 8, Philadelphia, 2015, Saunders.
The normal microbiome benefits us in many ways; bacteria in the gastrointestinal (GI) tract produce (1) enzymes that facilitate the digestion and utilization of many molecules in the human diet, such as fatty acids and large polysaccharides; (2)
usable metabolites (e.g., vitamin K, B vitamins); and (3) antibacterial factors that prevent colonization by pathogenic microorganisms (see Chapter 8) For instance, members of the normal microbiome in the colon produce chemicals (ammonia, phenols, indoles, and other toxic materials) and proteins (bacteriocins) that are toxic to more pathogenic microorganisms. They also compete with pathogens for nutrients and block attachment to the epithelium, which is an obligatory first step in the infectious process by most pathogens. Additionally, the normal microbiome of the gut helps train the adaptive immune system by inducing growth of gut-associated lymphoid tissue (where most cells of the adaptive immune system reside) and the development of both local and systemic adaptive immunity. Bidirectional communication between the brain and GI tract (brain-gut axis) is influenced by GI bacteria with importance for cognitive function, behavior, pain modulation, and stress responses.3 Prolonged treatment with broad-spectrum antibiotics can alter the normal
microbiome, decreasing its protective activity, and lead to an overgrowth of pathogenic microorganisms. In the intestine, overgrowth of the yeast Candida albicans or the bacteria Clostridium difficile (a cause of pseudomembranous colitis, an infection of the colon) may occur. The bacterium Lactobacillus is a major constituent of the normal gastrointestinal and vaginal microbiome in healthy women.4 This microorganism produces a variety of chemicals (e.g., hydrogen peroxide, lactic acid, bacteriocins) that help prevent infections of the vagina and urinary tract by other bacteria and yeast. Prolonged antibiotic treatment can diminish colonization with Lactobacillus and increase the risk for urologic or vaginal infections, such as vaginosis. The mutualistic relationship with the microbiome is maintained through the
physical integrity of the skin and mucosal epithelium and other mechanisms that protect the microbiome from the immune and inflammatory systems. Some members of the normal bacterial microbiome are opportunistic; opportunistic microorganisms can cause disease if the individual's defenses are compromised. These microorganisms are normally controlled by the innate and adaptive immune systems and contribute to our defenses. For example, Pseudomonas aeruginosa is a member of the normal microbiome of the skin and produces a toxin that protects against infections with staphylococcal and other bacteria. However, severe burns compromise the integrity of the skin and may lead to life-threatening systemic infections with Pseudomonas.
Quick Check 6-1
1. How do physical and mechanical barriers contribute to defense mechanisms?
2. What are antimicrobial peptides?
3. What two types of defensins contribute to the biochemical barrier?
4. What is the normal bacterial flora? What is its role in defense?
5. What are opportunistic microorganisms?
Second Line of Defense: Inflammation Whereas the physical and chemical barriers of the innate immune system are relatively static, inflammation is programmed to respond to cellular or tissue damage, whether the damaged tissue is septic or sterile. The response is a rapid initiation of an interactive system of humoral (soluble in the blood) and cellular systems designed to limit the extent of tissue damage, destroy contaminating infectious microorganisms, initiate the adaptive immune response, and begin the healing process. The inflammatory response (1) occurs in tissues with a blood supply
(vascularized); (2) is activated rapidly (within seconds) after damage occurs; (3) depends on the activity of both cellular and chemical components; and (4) is nonspecific, meaning that it takes place in approximately the same way regardless of the type of stimulus or whether exposure to the same stimulus has occurred in the past. Inflammation will be activated by virtually any injury to vascularized tissues,
including infection or tissue necrosis (e.g., ischemia, trauma, physical or chemical injury, foreign bodies, immune reactions). The classic or cardinal signs of acute inflammation were described in the first century by a Roman named Celsus and included rubor (redness), calor (heat), tumor (swelling), and dolor (pain). A fifth sign, functio laesa (loss of function), was added later. Microscopic inflammatory changes occur within seconds in the microcirculation (arterioles, capillaries, and venules) near the site of an injury and include the following processes (Figure 6-2):
1. Vasodilation (increased size of the blood vessels), which causes slower blood velocity and increases blood flow to the injured site
2. Increased vascular permeability (the blood vessels become porous from contraction of endothelial cells) and leakage of fluid out of the vessel (exudation), causing swelling (edema) at the site of injury; as plasma moves outward, blood in
the microcirculation becomes more viscous and flows more slowly, and the increased blood flow and increasing concentration of red cells at the site of inflammation cause locally increased redness (erythema) and warmth
3. White blood cell adherence to the inner walls of vessels and their migration through enlarged junctions between the endothelial cells lining the vessels into the surrounding tissue
FIGURE 6-2 The Major Local Changes in the Process of Inflammation. Compared with the normal circulation, inflammation is characterized by (1) dilation of the blood vessels and
increased blood flow, leading to erythema and warmth; (2) increased vascular permeability with leakage of plasma from the vessels, leading to edema; and (3) movement of leukocytes from
the vessels into the site of injury. (From Kumar V et al: Robbins and Cotran pathological basis of disease, ed 8, Philadelphia, 2009, Saunders.)
Each of the characteristic changes associated with inflammation is the direct result of the activation and interactions of a host of chemicals and cellular components found in the blood and tissues. The vascular changes deliver leukocytes (particularly neutrophils), plasma proteins, and other biochemical mediators to the site of injury, where they act in concert. Some of these chemical mediators activate pain fibers. The tissue injury, pain, and swelling contribute to loss of function. Figure 6-3 summarizes the process of inflammation. The lymphatic vessels drain the extravascular fluid to the lymph nodes and may, themselves, become secondarily inflamed; lymphangitis of the lymph vessels and lymphadenitis of the nodes, which become hyperplastic, enlarged, and frequently painful.
FIGURE 6-3 Acute Inflammatory Response. Inflammation is usually initiated by cellular injury and may be complicated by infection. Mast cell degranulation, the activation of three plasma systems, and the release of subcellular components from the damaged cells occur as a consequence. These systems are interdependent, so that induction of one (e.g., mast cell
degranulation) can result in the induction of the other two. The result is the development of the characteristic microscopic and clinical hallmarks of inflammation. The figure numbers refer to
additional figures in which more detailed information may be found on that portion of the response.
There are several benefits of inflammation, including the following:
1. Prevents infection and further damage by invading microorganisms. The inflammatory exudate dilutes toxins produced by bacteria and released from dying cells. The activation of plasma protein systems (e.g., complement and clotting systems) helps contain and destroy bacteria. The influx of phagocytes (e.g., neutrophils, macrophages) destroys cellular debris and microorganisms.
2. Limits and controls the inflammatory process. The influx of plasma protein systems (e.g., clotting system), plasma enzymes, and cells (e.g., eosinophils) prevents the inflammatory response from spreading to areas of healthy tissue.
3. Interacts with components of the adaptive immune system to elicit a more specific response to contaminating pathogen(s) through the influx of macrophages and lymphocytes that destroy pathogens.
4. Prepares the area of injury for healing and repair through removal of bacterial products, dead cells, and other products of inflammation (e.g., by way of channels through the epithelium or drainage by lymphatic vessels).
Fluid and debris that accumulate at an inflamed site are drained by lymphatic vessels. This process also facilitates the development of acquired immunity because microbial antigens in lymphatic fluid pass through the lymph nodes, where they encounter lymphocytes.
Quick Check 6-2
1. Why are innate immunity and inflammation described as “nonspecific”?
2. How are the five classic superficial symptoms of inflammation related to the process of inflammation?
3. Describe the basic steps in acute inflammation.
4. What are the benefits of inflammation?
Plasma Protein Systems and Inflammation Three key plasma protein systems are essential to an effective inflammatory response (Figure 6-4). These are the complement system, the clotting system, and the kinin system. Although each system has a unique role in inflammation, they have
many similarities. Each system consists of multiple proteins found in the blood, usually in inactive forms; several are enzymes that circulate as proenzymes. Each system contains a few proteins that can be activated early in inflammation. Activation of the first components results in sequential activation of other components of the system, leading to a biologic function that helps protect the individual. This sequential activation is referred to as a cascade. Thus, we occasionally refer to the complement cascade, the clotting cascade, or the kinin cascade. In some cases, activation of a particular protein in the system may require that it be enzymatically cut into two pieces of different size. Usually the larger fragment continues the cascade by activating the next component, and the smaller fragment frequently has potent proinflammatory activities.
FIGURE 6-4 Plasma Protein Systems in Inflammation: Complement, Clotting, and Kinin Systems. Each plasma protein system consists of a family of proteins that are activated in
sequence to create potent biologic effects. The complement system can be activated by three mechanisms, each of which results in proteolytic activation of C3. The fragments of C3
activation, C3a and C3b, are major components of inflammation. C3a is a potent anaphylatoxin, which induces degranulation of mast cells. C3b can bind to the surface or cells, such as
bacteria, and either serve as an opsonin for phagocytosis or proteolytically activate the next component of the complement cascade, C5. The smaller fragment of C5 activation is C5a, a powerful anaphylatoxin, and is also chemotactic for neutrophils, attracting them to the site of inflammation. The larger fragment, C5b, activates the components of the membrane attack complex (C5-C9), which damage the bacterial membrane and kill the bacteria. The clotting system can be activated by the tissue factor (extrinsic) pathway and the contact activation (intrinsic) pathway. All routes of clotting initiation lead to activation of factor X and thrombin.
Thrombin is an enzyme that proteolytically activates fibrinogen to form fibrin and small fibrinopeptides (FPs). Fibrin polymerizes to form a clot, and the FPs are highly active as
chemotactic factors and cause increased vascular permeability. The XIIa produced by the clotting system can also be activated by kallikrein of the kinin system (red arrow). Prekallikrein
is enzymatically converted to kininogen, which activates bradykinin. Bradykinin functions similar to histamine and increases vascular permeability. Bradykinin can also stimulate nerve
endings to cause pain. FP, Fibrinopeptide; TF, tissue factor.
Complement System The complement system consists of a large number of proteins (sometimes called complement factors) that together constitute about 10% of the total circulating serum protein. Activation of the complement system produces several factors that can destroy pathogens directly or can activate or increase the activity of many other components of the inflammatory and adaptive immune response. Factors produced
during activation of the complement system are among the body's most potent defenders, particularly against bacterial infection. The most important function of the complement cascade is activation of C3 and
C5, which results in a variety of molecules that are (1) opsonins, (2) chemotactic factors, or (3) anaphylatoxins.5 Opsonins coat the surface of bacteria and increase their susceptibility to being phagocytized (eaten) and killed by inflammatory cells, such as neutrophils and macrophages. Chemotactic factors diffuse from a site of inflammation and attract phagocytic cells to that site. Anaphylatoxins induce rapid degranulation of mast cells (i.e., release of histamine that induces vasodilation and increased capillary permeability), a major cellular component of inflammation. The most potent complement products are C3b (opsonin), C3a (anaphylatoxin), and C5a (anaphylatoxin, chemotactic factor). Activation of terminal complement components C5b through C9 (membrane attack complex, or MAC) results in a complex that creates pores in the outer membranes of cells or bacteria. The pores disrupt the cell's membrane and permit water to enter, causing the death of the cell. Three major pathways control the activation of complement (see Figure 6-4). The
classical pathway is primarily activated by antibodies, which are proteins of the acquired immune system. Antibodies must first bind to their targets, called antigens, which can be proteins or carbohydrates from bacteria or other infectious agents. Antibodies activate the first component of complement, C1, which leads to activation of other complement components, leading to activation of C3 and C5. Thus, antibodies of the acquired immune response can use the complement system to kill bacteria and activate inflammation. The alternative pathway is activated by several substances found on the surface
of infectious organisms (e.g., lipopolysaccharides [endotoxin] on the bacterial surface or yeast cell wall carbohydrates [zymosan]). This pathway uses unique proteins (factor B, factor D, and properdin) to form a complex that activates C3. C3 activation leads to C5 activation and convergence with the classical pathway. Thus, the complement system can be directly activated by certain infectious organisms without antibody being present. The lectin pathway is similar to the classical pathway but is independent of
antibody. It is activated by several plasma proteins, particularly mannose-binding lectin (MBL). MBL binds to bacterial polysaccharides containing the carbohydrate mannose and activates complement through two proteins that are similar to C1— MASP-1 (mannose-binding lectin-associated serine protease) and MASP-2.6 Thus, infectious agents that do not activate the alternative pathway may be susceptible to complement through the lectin pathway. In summary, the complement cascade can be activated by at least three different
means, and its products have four functions: (1) opsonization (C3b), (2)
anaphylatoxic activity resulting in mast cell degranulation (C3a, C5a), (3) leukocyte chemotaxis (C5a), and (4) cell lysis (C5b-C9; membrane attack complex [MAC]).
Clotting System The clotting (coagulation) system is a group of plasma proteins that, when activated sequentially, form a blood clot. A blood clot is a meshwork of protein (fibrin) strands that contains platelets (the primary cellular initiator of clotting) and traps other cells, such as erythrocytes, phagocytes, and microorganisms. Clots (1) plug damaged vessels and stop bleeding, (2) trap microorganisms and prevent their spread to adjacent tissues, and (3) provide a framework for future repair and healing. Specific details and illustrations of the clotting system are presented in Chapter 20 (also see Figure 20-18) and only the relationship between clotting and inflammation is presented here. The clotting system can be activated by many substances that are released during
tissue injury and infection, including collagen, proteinases, kallikrein, and plasmin, as well as by bacterial products such as endotoxins. Like the complement cascade, the coagulation cascade can be activated through different pathways that converge and result in the formation of a clot (see Figure 6-4). The tissue factor (extrinsic) pathway is activated by tissue factor (TF) (also called tissue thromboplastin) that is released by damaged endothelial cells in blood vessels and reacts with activated factor VII (VIIa). The contact activation (intrinsic) pathway is activated when the vessel wall is damaged and Hageman factor (factor XII) in plasma contacts negatively-charged subendothelial substances. The pathways converge at factor X. Activation of factor X begins a common pathway leading to activation of fibrin that polymerizes to form a fibrin clot. As with the complement system, activation of the clotting system produces
protein fragments known as fibrinopeptides (FPs) A and B that enhance the inflammatory response. Fibrinopeptides are released from fibrinogen when fibrin is produced. Both fibrinopeptides (especially fibrinopeptide B) are chemotactic for neutrophils and increase vascular permeability by enhancing the effects of bradykinin (formed from the kinin system) on endothelial cells.
Kinin System The third plasma protein system, the kinin system (see Figure 6-4), interacts closely with the coagulation system. Both the clotting and kinin systems can be initiated through activation of Hageman factor (factor XII) to factor XIIa. Another name for factor XIIa is prekallikrein activator because it enzymatically activates the first component of the kinin system, prekallikrein. The final product of the kinin system
is a small-molecular-weight molecule, bradykinin, which is produced from a larger precursor molecule, kininogen. Bradykinin causes dilation of blood vessels, acts with prostaglandins to induce pain, causes smooth muscle cell contraction, and increases vascular permeability.
Control and Interaction of Plasma Protein Systems The three plasma protein systems are highly interactive so that activation of one results in production of a large number of very potent, biologically active substances that further activate the other systems. Very tight regulation of these processes is essential for the following two reasons.
1. The inflammatory process is critical for an individual's survival; thus efficient activation must be guaranteed regardless of the cause of tissue injury. Interaction among the plasma systems may result in activation of the entire inflammatory response regardless of which system is activated initially.
2. The biochemical mediators generated during these processes are potent and potentially detrimental to the individual, and their actions must be strictly confined to injured or infected tissues.
Therefore, multiple mechanisms are available to either activate or inactivate (regulate) these plasma protein systems. For instance, the plasma that enters the tissues during inflammation (edema) contains enzymes that destroy mediators of inflammation. Carboxypeptidase inactivates the anaphylatoxic activities of C3a and C5a, and kininases degrade kinins. Histaminase degrades histamine and kallikrein and down-regulates the inflammatory response. The formation of clots also activates a fibrinolytic system that is designed to
limit the size of the clot and remove the clot after bleeding has ceased. Thrombin of the clotting system activates plasminogen in the blood to form the enzyme plasmin. The primary activity of plasmin is to degrade fibrin polymers in clots. However, plasmin can also activate the complement cascade through components C1, C3, and C5 and the kinin cascade by activating factor XII and producing prekallikrein activator. Another example of a common regulator is C1 esterase inhibitor (C1 inh). C1
inh inhibits complement activation through C1 (classical pathway), MASP-2 (lectin pathway), and C3b (alternative pathway). It is also a major inhibitor of the clotting and kinin pathway components (e.g., kallikrein, factor XIIa). A genetic defect in C1 inh (C1 inh deficiency) results in hereditary angioedema, which is a self-limiting edema of cutaneous and mucosal layers resulting from stress, illness, or relative
minor or unapparent trauma. The disease is characterized by hyperactivation of all three plasma protein systems, although excessive production of bradykinin appears to be the principal cause of increased vascular permeability. Many cells are protected from inadvertent complement system damage by factors
linked to the external surface of the plasma membrane. Two examples are decay accelerating factor (DAF) and CD59; DAF prevents activation of C3 and CD59 inhibits the membrane attack complex.
Quick Check 6-3
1. What are the three most important products of the complement system?
2. How is the coagulation cascade activated? How is it related to the plasma kinin cascade?
3. What factors control the plasma protein systems of inflammation?
Cellular Components of Inflammation Inflammation is a process in vascular tissue; thus the cellular components can be found in the blood or in tissue surrounding the blood vessels. The blood vessels are lined with endothelial cells, which under normal conditions actively maintain blood flow. During inflammation the vascular endothelium becomes a principal coordinator of blood clotting and the passage of cells and fluid into the tissue. The tissues close to the vessels contain mast cells, which are probably the most important activators of inflammation, and dendritic cells, which connect the innate and acquired immune responses. The blood contains a complex mixture of cells (Figure 6-5 and see Chapter 20). Blood cells are divided into erythrocytes (red blood cells), platelets, and leukocytes (white blood cells). Erythrocytes carry oxygen to the tissues and platelets are small cell fragments involved in blood clotting. Leukocytes are subdivided into granulocytes (containing many enzyme- filled cytoplasmic granules), monocytes, and lymphocytes. Granulocytes are the most common leukocytes and are classified by the type of stains needed to visualize enzyme-containing granules in their cytoplasm (basophils, eosinophils, and neutrophils). Monocytes are precursors of macrophages that are found in the tissue. Various forms of lymphocytes participate in the innate immune response (e.g., natural killer [NK] cells) and the acquired immune response (B and T cells).
FIGURE 6-5 Cellular Components of the Blood. Cells in the blood can be classified as red blood cells (erythrocytes), cellular fragments (platelets), or white blood cells (leukocytes). Leukocytes
consist of lymphocytes, monocytes, and granulocytes (neutrophils, eosinophils, basophils). (Erythrocyte plate from Goldman L, Schafer AI, editors: Goldman's Cecil medicine, ed 24, Philadelphia, 2012, Saunders; rest of plates from McPherson RA, editor: Henry's clinical diagnosis and management by laboratory methods, ed 22, Philadelphia, 2012, Saunders.)
Cells of both innate and acquired immune systems respond to molecules produced at a site of cellular damage and are recruited to that site to augment the protective response. These molecules originate from destroyed or damaged cells, contaminating microbes, activation of the plasma protein systems, or secretions by other cells of the innate or acquired immune systems. Each cell has a set of cell surface receptors that specifically bind these molecules, resulting in activation of intracellular signaling pathways and activation of the cell itself. Activation may result in the cell gaining a function critical to the inflammatory response or the induction of the release of additional cellular products that increase inflammation, or both. Most of these inflammatory cells and protein systems, along with the substances they produce, act at the site of tissue injury to confine the extent of damage; kill microorganisms; remove the cellular debris; and activate healing,
tissue regeneration (a process known as resolution), or repair.
Cellular Receptors As will be discussed in Chapter 7, B and T lymphocytes of the adaptive immune system have evolved surface receptors (i.e., the T-cell receptor, or TCR, and the B- cell receptor, or BCR) that bind a large spectrum of antigens. Cells involved in innate resistance have evolved a different set of receptors that recognize a much more limited array of specific molecules (ligands). These are referred to as pattern recognition receptors (PRRs). PRRs recognize two types of molecular patterns: molecules that are expressed by infectious agents, either found on their surface or released as soluble molecules (pathogen-associated molecular patterns, or PAMPs); or products of cellular damage (damage-associated molecular patterns, or DAMPs). Thus cells of the innate immune system can respond to both sterile (through DAMPs) and septic (through PAMPs and DAMPs) tissue damage. It is estimated that at least 100 different PRRs are expressed that recognize more than 1000 different molecules. PRRs are generally expressed on cells in tissues near the body's surface (i.e., skin,
respiratory tract, gastrointestinal tract, genitourinary tract) where they monitor the environment for products of cellular damage and potentially infectious microorganisms. Classes of cellular PRRs primarily differ in the specificity of ligands they bind. PRRs can be found as cell surface receptors that bind extracellular ligands, in endosomes in contact with ingested microbes and other materials, in the cytosol where they bind intracellular materials resulting from cellular damage, or secreted into the extracellular environment. An example of a secreted PRR is mannose-binding lectin of the lectin pathway of complement activation (see p. 139). Toll-like receptors (TLRs) primarily recognize a large variety of PAMPs
located on the microorganism's cell wall or surface (e.g., bacterial lipopolysaccharide [LPS], peptidoglycans, lipoproteins, yeast zymosan, viral coat proteins), other surface structures (e.g., bacterial flagellin), or microbial nucleic acid (e.g., bacterial DNA, viral double-stranded RNA).7 Ten different TLRs have been described in humans (Table 6-3). They are expressed on the surface of many cells that have direct and early contact with potential pathogenic microorganisms, including mucosal epithelial cells, mast cells, neutrophils, macrophages, dendritic cells, and some subpopulations of lymphocytes. TLRs are linked to pathways that produce two groups of transcription factors: NF-κB, which controls synthesis and release of cytokines; and interferon regulatory factors (IRFs), which control the production of anti-viral type I interferons.8
TABLE 6-3 Cellular Source and Microbial Target for Each Toll-like Receptor (TLR)
Receptor Cellular Expression Pattern PAMP Recognition TLR1 Cell surface (ubiquitous): neutrophils, monocytes/macrophages,
dendritic cells, T cells, B cells, NK cells Fungal, bacterial, viral; forms heterodimer with TLR2 (see TLR2 recognition)
TLR2 Cell surface: neutrophils, monocytes/macrophages, dendritic cells Fungal (yeast zymosan), bacterial (gram-positive bacterial peptidoglycan, lipoproteins), viral (lipoproteins)
TLR3 Intracellular: monocytes/macrophages, dendritic cells, T cells, NK cells, epithelial cells
Double-stranded RNA produced by many viruses
TLR4 Cell surface: granulocytes, monocytes/macrophages, dendritic cells, T cells, B cells, epithelial cells
Bacterial (primarily gram-negative bacterial LPS, lipoteichoic acids), viral (RSV F protein, hepatitis C)
TLR5 Cell surface: granulocytes, monocytes/macrophages, dendritic cells, NK cells, epithelial cells
Bacterial (flagellin); forms heterodimer with TLR4
TLR6 Cell surface: monocytes/macrophages, dendritic cells, B cells, NK cells Fungal, bacterial, viral; forms heterodimer with TLR2 (see TLR2 recognition)
TLR7 Intracellular: monocytes/macrophages, dendritic cells, B cells Natural ligand uncertain; may bind viral single-strand RNA TLR8 Cell surface: monocytes/macrophages, dendritic cells, NK cells Natural ligand uncertain; may bind fungal PAMPs or viral single-
stranded RNA TLR9 Intracellular: monocytes/macrophages, dendritic cells, B cells Bacterial (unmethylated DNA [CpG dinucleotides]) TLR10 Cell surface: monocytes/macrophages, dendritic cells, B cells Natural ligand uncertain; may form heterodimers with TLR2 TLR11 TLR11 gene does not code a full-length protein in humans No known immune response
Complement receptors are found on many cells of the innate and acquired immune responses (e.g., granulocytes, monocytes/macrophages, lymphocytes, mast cells, erythrocytes, platelets), as well as some epithelial cells. They recognize several fragments produced through activation of the complement system, particularly C3a, C5a, and C3b. Scavenger receptors are primarily expressed on macrophages and facilitate
recognition and phagocytosis of bacterial pathogens, as well as damaged cells and altered soluble lipoproteins associated with vascular damage (e.g., high-density lipoprotein [HDL], acetylated low-density lipoprotein [LDL], oxidized LDL).9 More than eight receptors have been identified. Some scavenger receptors (e.g., SR- PSOX) recognize the cell membrane phospholipid phosphatidylserine (PS). PS is normally sequestered on the cytoplasmic surface of the cell membrane, but is externalized under a very limited variety of conditions, including erythrocyte senescence and cellular apoptosis. Thus macrophages, through this receptor, can identify and remove old red blood cells and cells undergoing apoptosis. NOD-like receptors (NLRs) are cytoplasmic receptors that recognize products
of microbes and damaged cells. At least 22 NLRs have been identified in humans. NOD-1 and NOD-2 are cytoplasmic and recognize fragments of peptidoglycans from intracellular bacteria and initiate production of proinflammatory mediators, such as tumor necrosis factor (TNF) and interleukin-6 (IL-6).10 Other NLRs associate with intracellular multiprotein complexes called inflammasomes. Inflammasomes primarily bind cellular stress-related molecules, a type of DAMP, and control the production of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18.11
Cellular Products To elicit an effective inflammatory (or adaptive immune) response, intercellular communication and cooperation are necessary. Cytokines constitute a large family of small-molecular-weight soluble intercellular-signaling molecules that are secreted, bind to specific cell membrane receptors, and regulate innate or adaptive immunity (Figure 6-6). Cytokines may be either proinflammatory or anti- inflammatory in nature, depending on whether they tend to induce or inhibit the inflammatory response. These molecules usually diffuse over short distances, but some effects occur over long distances, such as the systemic induction of fever by some cytokines (i.e., endogenous pyrogens) that are produced at an inflammatory site. Binding of cytokines to a target cell often induces synthesis of additional cellular products. For example, binding of the cytokine TNF-α to a cell may result in synthesis and release of IL-1.
FIGURE 6-6 Principal Mediators of Inflammatory Processes. C3b, Large fragment produced from complement component C3; C5a, small fragment produced from complement component C5; ECF-A, eosinophil chemotactic factor of anaphylaxis; FGF, fibroblast growth factor; IFN, interferon; IgG, immunoglobulin G (predominant class of antibody in the blood); IL, interleukin;
MCF, monocyte chemotactic factor; NCF, neutrophil chemotactic factor; PAF, platelet-activating factor; TGF, T-cell growth factor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth
factor.
A large number of cytokines have been described and are classified into several families.12 The terms lymphokines and monokines refer respectively to cytokines secreted from lymphocytes or monocytes, although cytokines are secreted by many different types of cells. Chemokines are members of a special family of cytokines that are chemotactic and primarily attract leukocytes to sites of inflammation.13 Chemokines are synthesized by many cell types, including macrophages, fibroblasts, and endothelial cells, in response to proinflammatory cytokines, such as TNF-α. To date, more than 50 different human chemokines have been described.
Examples include those that primarily attract macrophages (e.g., monocyte/macrophage chemotactic proteins [MCP-1, MCP-2, and MCP-3]), macrophage inflammatory proteins ([MIP-α and MIP-1β]), or neutrophils (e.g., interleukin-8 [IL-8]). Interleukins (ILs) are produced predominantly by macrophages and lymphocytes
in response to stimulation of PRRs or by other cytokines.14 More than 30 interleukins have been identified. Their effects include the following:
1. Alteration of adhesion molecule expression on many types of cells
2. Attraction of leukocytes to a site of inflammation (chemotaxis)
3. Induction of proliferation and maturation of leukocytes in the bone marrow
4. General enhancement or suppression of inflammation
5. Development of the acquired immune response
Two major proinflammatory ILs are interleukin-1 and interleukin-6, which cooperate closely with another cytokine, tumor necrosis factor-alpha. Interleukin-1 (IL-1) is produced in two forms, IL-1α and IL-1β, mainly by macrophages.15 IL-1 activates monocytes, other macrophages, and lymphocytes, thereby enhancing both innate and acquired immunity, and acts as a growth factor for many cells. It has several effects on neutrophils, including induction of proliferation (resulting in an increase in the number of circulating neutrophils), attraction to an inflammatory site (chemotaxis), and increased cellular respiration and lysosomal enzyme activity (both effects resulting in increased cellular killing of bacteria). IL-1 is an endogenous pyrogen (i.e., fever-causing cytokine) that reacts with receptors on cells of the hypothalamus and affects the body's thermostat, resulting in fever. Interleukin-6 (IL-6) is produced by macrophages, lymphocytes, fibroblasts, and
other cells. IL-6 directly induces hepatocytes (liver cells) to produce many of the proteins needed in inflammation (acute-phase reactants, discussed later in this chapter). IL-6 also stimulates growth and differentiation of blood cells in the bone marrow and the growth of fibroblasts (required for wound healing). Although not classified as an interleukin, tumor necrosis factor-alpha (TNF-α)
is secreted by macrophages and other cells (e.g., mast cells) in response to stimulation of TLRs. TNF-α induces a multitude of proinflammatory effects, particularly on the vascular endothelium and macrophages. When secreted in large amounts, TNF-α has systemic effects that include the following:
1. Inducing fever by acting as an endogenous pyrogen
2. Causing increased synthesis of inflammation-related serum proteins by the liver
3. Causing muscle wasting (cachexia) and intravascular thrombosis in cases of severe infection and cancer.
Very high levels of TNF-α can be lethal and are probably responsible for fatalities from shock caused by gram-negative bacterial infections. Some cytokines are anti-inflammatory and diminish the inflammatory response.
The most important are interleukin-10 and transforming growth factor-beta. Interleukin-10 (IL-10) is primarily produced by lymphocytes and suppresses the growth of other lymphocytes and the production of proinflammatory cytokines by macrophages, leading to down-regulation of both inflammatory and acquired immune responses. Transforming growth factors, including transforming growth factor-beta (TGF-β), are produced by many cells in response to inflammation and induce cell division and differentiation of other cell types, such as immature blood cells. Interferons (IFNs) are members of a family of cytokines that protect against
viral infections and modulate the inflammatory response. (Mechanisms of viral infection are described in Chapter 8.) Type I interferons (primarily IFN-α, IFN-β) are produced and released by virally infected cells in response to viral double- stranded RNA and other viral PAMPs. These IFNs do not kill viruses directly but instead are secreted and induce antiviral proteins and protection in neighboring healthy cells. Type II interferon (IFN-γ) is produced primarily by lymphocytes; it activates macrophages, resulting in increased capacity to kill infectious agents (including viruses and bacteria), and enhances the development of acquired immune responses against viruses.
Mast Cells and Basophils The mast cell is probably the most important cellular activator of the inflammatory response. Mast cells are filled with granules and located in the loose connective tissues close to blood vessels near the body's outer surfaces (i.e., in the skin and lining the gastrointestinal and respiratory tracts). Basophils are found in the blood and probably function in the same way as tissue mast cells.16 A great number of stimuli activate mast cells to release potent soluble inducers of inflammation. These are released by (1) degranulation (the release of the contents of mast cell granules) and (2) synthesis (the new production and release of mediators in response to a stimulus) (Figure 6-7).
FIGURE 6-7 Mast Cell and Mast Cell Degranulation and Synthesis of Biologic Mediators During Inflammation. A, Colorized photomicrograph of mast cell; dense red granules contain histamine and other biologically active substances. Among these are histamine, which is a major initiator of vascular changes, and a variety of chemotactic factors. B, Mast cell degranulation (left) and synthesis (right). Histamine and other biologically active substances are released immediately
after stimulation of mast cells. (A from Roitt IM et al: Immunology, ed 3, St Louis, 1993, Mosby.)
Degranulation. In response to a stimulus, biochemical mediators in the mast cell granules, including histamine, chemotactic factors, and cytokines (e.g., tumor necrosis factor- alpha [TNF-α], IL-4), are released within seconds and exert their effects immediately. Histamine is a small-molecular-weight molecule with potent effects on many other cells, particularly those that control the circulation. Histamine, along with serotonin (found in many cells, but not human mast cells), is called a vasoactive amine. These molecules cause temporary, rapid constriction of smooth muscle and dilation of the postcapillary venules, which results in increased blood flow into the microcirculation. Histamine also causes increased vascular permeability resulting from retraction of endothelial cells lining the capillaries and increased adherence of leukocytes to the endothelium. Histamine affects cells by binding to histamine H1 and H2 receptors on the target cell surface (Figure 6-8). Antihistamines are drugs that block the binding of histamine to its receptors, resulting in decreased inflammation.
FIGURE 6-8 Effects of Histamine Through H1 and H2 Receptors. The effects depend on (1) the density and affinity of H1 or H2 receptors on the target cell and (2) the identity of the target cell. ATP, Adenosine triphosphate; cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine
monophosphate; GTP, guanosine triphosphate.
Binding of histamine to the H1 receptor is essentially proinflammatory; that is, it promotes inflammation. On the other hand, binding to the H2 receptor is generally anti-inflammatory because it results in suppression of leukocyte function. The H1
receptor is present on smooth muscle cells, especially those of the bronchi, and causes bronchial smooth muscle to contract (bronchoconstriction) when stimulated. Both types of receptors are distributed among many different cells and are often present on the same cells and may act in an antagonistic fashion. For instance, stimulation of H1 receptors on neutrophils results in augmentation of neutrophil chemotaxis, whereas H2 receptor stimulation results in its inhibition. The H2 receptor is especially abundant on parietal cells of the stomach mucosa and induces the secretion of gastric acid as part of the normal physiology of the stomach. The role of histamine receptors and hypersensitivity is discussed in Chapter 8. Mast cell granules also contain chemotactic factors, two of which are neutrophil
chemotactic factor (NCF) and eosinophil chemotactic factor of anaphylaxis (ECF-A). Chemotaxis is directional movement of cells along a chemical gradient formed by a chemotactic factor. Neutrophils are the predominant cell needed to kill bacteria in the early stages of inflammation. Eosinophils help regulate the inflammatory response. Both cells are discussed in more detail later in this chapter.
Synthesis of mediators. Activated mast cells initiate synthesis of other mediators of inflammation. These include leukotrienes, prostaglandins, and platelet-activating factor, which are produced from lipids (arachidonic acid) in the plasma membrane. Leukotrienes (slow-reacting substances of anaphylaxis [SRS-A]) are sulfur-containing lipids produced by lipoxygenase that initiate histamine-like effects: smooth muscle contraction and increased vascular permeability. Leukotrienes appear to be important in the later stages of the inflammatory response because they stimulate slower and more prolonged inflammatory responses than does histamine. Prostaglandins cause increased vascular permeability, neutrophil chemotaxis,
and pain by direct effects on nerves. They are long-chain, unsaturated fatty acids produced by the action of the enzyme cyclooxygenase (COX) on arachidonic acid; prostaglandins are classified into groups (E, D, A, F, and B) according to their structure with numeral subscripts designating the number of double bonds. Prostaglandins E1 and E2 cause increased vascular permeability and smooth muscle contraction. COX exists in two different forms: COX-1 is found in most tissues and COX-2 is associated with inflammation. Aspirin and other nonsteroidal anti- inflammatory drugs inhibit both COX-1 and COX-2, but inhibition of COX-1 causes complications, such as gastrointestinal toxicity. Selective COX-2 inhibitors are now available. Platelet-activating factor (PAF) is produced by removal of a fatty acid from the
plasma membrane phospholipids by phospholipase A2. Although mast cells are a
major source of PAF, this molecule also can be produced by neutrophils, monocytes, endothelial cells, and platelets. The biologic activity of PAF is virtually identical to that of leukotrienes, namely, causing endothelial cell retraction to increase vascular permeability, leukocyte adhesion to endothelial cells, and platelet activation.
Endothelium The lining of blood vessels consists of a layer of endothelial cells that adhere to an underlying matrix of connective tissue that contains a variety of proteins, including collagen, fibronectin, and laminins. Endothelial cells regulate circulating components of the inflammatory system and maintain normal blood flow by preventing spontaneous activation of platelets and members of the clotting system. Nitric oxide (NO) produced from arginine and prostacyclin (PGI2) from arachidonic acid maintain blood flow and pressure and inhibit platelet activation. PGI2 and NO are synergistic. NO is released continually to relax vascular smooth muscle and suppress the effects of low levels of cytokines, thus maintaining vascular tone. PGI2 production varies a great deal and is increased when additional regulation is needed. Damage to the endothelial cell lining of the vessel exposes the subendothelial
connective tissue matrix, which is prothrombogenic and initiates platelet activation and formation of clots (the contact activation [intrinsic] clotting pathway). Proinflammatory mediators (e.g., histamine, prostacyclin, and many others) affect the endothelium, resulting in adherence of leukocytes to the vessel surface, invasion of leukocytes into the tissue, and efflux of plasma from the vessel.
Platelets Platelets are anucleate cytoplasmic fragments formed from megakaryocytes. They circulate in the bloodstream until vascular injury occurs resulting in platelet activation by many products of tissue destruction and inflammation, including collagen, thrombin, and platelet-activating factor. Activated platelets (1) interact with components of the coagulation cascade to stop bleeding; (2) degranulate, releasing biochemical mediators such as serotonin, which has vascular effects similar to those of histamine; and (3) synthesize thromboxane A2 (TXA2) from prostaglandin H2. TXA2 is a potent vasoconstrictor and inducer of platelet aggregation. Prolonged use of low-dose aspirin preferentially suppresses production of TXA2 without interfering with the production of anti-inflammatory PGI2 by the endothelium. Platelets also release growth factors that promote wound
healing. (Platelet function is described in detail in Chapter 20.)
Phagocytes The primary role of most granulocytes (neutrophils, eosinophils, basophils) and monocytes/macrophages is phagocytosis—the process by which a cell ingests and disposes of damaged cells and foreign material, including microorganisms.
Neutrophils. The neutrophil, or polymorphonuclear neutrophil (PMN), is a member of the granulocytic series of white blood cells and is named for the characteristic staining pattern of its granules as well as its multilobed nucleus. Neutrophils are the predominant phagocytes in the early inflammatory site, arriving within 6 to 12 hours after the initial injury. Several inflammatory mediators (e.g., some bacterial proteins, complement fragments C3a and C5a, and mast cell neutrophil chemotactic factor) specifically and rapidly attract neutrophils from the circulation and activate them.17 Because the neutrophil is a mature cell that is incapable of division and sensitive
to acidic environments, it is short lived at the inflammatory site and becomes a component of the purulent exudate, or pus, which is removed from the body through the epithelium or drained from the infected site via the lymphatic system. (The lymphatic system is described in Chapter 23.) The primary roles of the neutrophil are removal of debris and dead cells in sterile lesions, such as burns, and destruction of bacteria in nonsterile lesions.
Eosinophils. Another population of granulocytes is the eosinophil. Although eosinophils are only mildly phagocytic, they have two specific functions: (1) serve as the body's primary defense against parasites, and (2) help regulate vascular mediators released from mast cells. The role of eosinophils in resistance to parasites occurs in collaboration with specific antibodies produced by the acquired immune system (discussed in Chapter 7).18 Regulation of mast cell–derived inflammatory mediators is critical to control
inflammation. The acute inflammatory response is needed only in a circumscribed area and for a limited time. Therefore, control mechanisms are necessary to prevent biochemical mediators from evoking more inflammation than necessary. Mast cell eosinophil chemotactic factor–A (ECF-A) attracts eosinophils to the site of inflammation. Eosinophil lysosomal granules contain enzymes that degrade vasoactive molecules, thereby controlling the vascular effects of inflammation.
Histaminase degrades histamine, and arylsulfatase B degrades leukotrienes.
Basophils. The basophil is the least prevalent granulocyte in the blood. It is very similar to mast cells in the content of its granules and, in addition, is an important source of the cytokine IL-4, which is a key regulator of the adaptive immune response. Although often associated with allergies and asthma, its primary role is yet unknown.
Monocytes and macrophages. Monocytes are the largest normal blood cells (14 to 20 µm in diameter). Monocytes are produced in the bone marrow, enter the circulation, and migrate to the inflammatory site where they develop into macrophages. Monocytes also appear to be the precursors of macrophages that are found in tissues (tissue macrophages) including Kupffer cells in the liver, alveolar macrophages in the lungs, and microglia in the brain. Macrophages are generally larger (20 to 40 µm) and are more active as phagocytes than their monocytic precursors. Macrophages, particularly those residing in the tissues, are often important cellular initiators of the inflammatory response. Monocyte-derived macrophages from the circulation may appear at the
inflammatory site as soon as 24 hours after the initial neutrophil infiltration, but usually arrive 3 to 7 days later. Neutrophils and monocytes/macrophages differ chiefly in the following ways:
1. Speed: Neutrophils arrive at the injury site first, whereas macrophages move more sluggishly.
2. Active life span: Macrophages survive and divide in the acidic inflammatory site, whereas neutrophils cannot.
3. Chemotactic factors: Neutrophils and macrophages are not attracted by the same factors, such as macrophage chemotactic factor, which is released by neutrophils.
4. Enzymatic content of their lysosomes, or digestive vacuoles: Neutrophils have a more active NADPH oxidase and produce more hydrogen peroxide; macrophage phagolysosomes are more acidic, favoring the activity of acidic proteases and other enzymes.
5. Role in the immune response: Macrophages, but not neutrophils, are involved in
activation of the adaptive immune system.
6. Role in wound repair: Macrophages are the primary cells that infiltrate tissue in wounds, remove cells and cellular debris, promote angiogenesis, and produce cytokines and growth factors that suppress further inflammation and initiate healing by promoting epithelial cell division, activating fibroblasts, and promoting synthesis of extracellular matrix and collagen.
The bactericidal activity of macrophages can increase markedly with the help of inflammatory cytokines produced by cells of the acquired immune system (subsets of T lymphocytes) or cells activated through Toll-like receptors (TLRs). Macrophage activation results in two subpopulations of cells.19 M1 macrophages are activated through TLRs by substances found in sites of inflammation and have greater bacterial killing capacity. M2 macrophages are activated by lymphocyte- produced cytokines and are primarily involved in healing and repair.20 Several bacteria are resistant to killing by granulocytes and can even survive
inside macrophages. Microorganisms, such as Mycobacterium tuberculosis (tuberculosis), Mycobacterium leprae (leprosy), Salmonella typhi (typhoid fever), Brucella abortus (brucellosis), and Listeria monocytogenes (listeriosis), can remain dormant or multiply inside the phagolysosomes of macrophages.
Dendritic cells. Dendritic cells provide one of the major links between the innate and acquired immune responses. They are the primary phagocytic cells located in the peripheral organs and skin, where molecules released from infectious agents are encountered, recognized through PRRs, and internalized through phagocytosis. Dendritic cells then migrate through the lymphatic vessels to lymphoid tissue, such as lymph nodes, and interact with T lymphocytes to generate an acquired immune response.21 Through the production of a family of cytokines, they guide development of a subset of T cells (helper cells) that coordinate the development of functional B and T cells (discussed in Chapter 7).
Phagocytosis. The two most important phagocytes are neutrophils and macrophages. Both cells are circulating in the blood and must first leave the circulation and migrate to the site of inflammation before initiating phagocytosis (Figure 6-9). Many products of inflammation affect expression of surface molecules involved in cell-to-cell adherence. Both leukocytes and endothelial cells begin expressing molecules
(selectins and integrins) that increase adhesion, or stickiness, causing the leukocytes to adhere more avidly to the endothelial cells in the walls of the capillaries and venules in a process called margination, or pavementing. Leukocyte-endothelial interactions lead to diapedesis, or emigration of the cells through the inter- endothelial junctions that have loosened in response to inflammatory mediators.22
FIGURE 6-9 Process of Phagocytosis. The process that results in phagocytosis is characterized by three interrelated steps: adherence and diapedesis, tissue invasion by
chemotaxis, and phagocytosis. A, Adherence, margination, diapedesis, and chemotaxis. The primary phagocyte in the blood is the neutrophil, which usually moves freely within the vessel (1). At sites of inflammation, the neutrophil progressively develops increased adherence to the endothelium, leading to accumulation along the vessel wall (margination or pavementing) (2). At sites of endothelial cell retraction the neutrophil exits the blood by means of diapedesis (3).
Chemotaxis. In the tissues, the neutrophil detects chemotactic factor gradients through surface receptors (1) and migrates towards higher concentrations of the factors (2). The high
concentration of chemotactic factors at the site of inflammation immobilizes the neutrophil (3). B, Specific receptors for recognition and attachment. C, Phagocytosis. Opsonized
microorganisms bind to the surface of a phagocyte through specific receptors (1). The microorganism is ingested into a phagocytic vacuole, or phagosome (2). Lysosomes fuse with the phagosome, resulting in the formation of a phagolysosome (3). During this process the microorganism is exposed to products of the lysosomes, including a variety of enzymes and
products of the hexose-monophosphate shunt (e.g., H2O2, O2 −). The microorganism is killed and
digested (4). Ab, Antibody; AbR, antibody receptor; C3b, complement component C3b; C3bR, complement C3b receptor; PAMP, pathogen-associated molecular pattern; PRR, pattern
recognition receptor.
Once inside the tissue, leukocytes undergo a process of directed migration (chemotaxis) by which they are attracted to the inflammatory site by chemotactic
factors.23 The primary chemotactic factors include many bacterial products, neutrophil chemotactic factor produced by mast cells, the chemokine IL-8, complement fragments C3a and C5a, and products of the clotting and kinin systems. Red blood cells cannot repair themselves and are phagocytized by macrophages at the end of their lifespan (Figure 6-10).
FIGURE 6-10 Phagocytosis of Red Blood Cell. This scanning electron micrographs shows the progressive steps in phagocytosis. A, Red blood cells (R) attach to the surface of a macrophage (M). B, Part of the macrophage (M) membrane starts to enclose the red cell (R). C, The red blood cells are almost totally engulfed by the macrophage. (Modified from King DW et al: General pathology: principles and
dynamics, Philadelphia, 1983, Lea & Febiger.)
At the inflammatory site, the process of phagocytosis involves five steps: (1) recognition and adherence of the phagocyte to its target, (2) engulfment (ingestion or endocytosis), (3) formation of a phagosome, (4) fusion of the phagosome with lysosomal granules within the phagocyte, and (5) destruction of the target. Throughout the process, both the target and the digestive enzymes are isolated within membrane-bound vesicles. Isolation protects the phagocyte itself from the harmful effects of the target microorganisms, as well as its own enzymes. Most phagocytes can trap and engulf bacteria using PRRs, although the process is
relatively slow. Opsonization greatly enhances adherence by acting as a glue to tighten the affinity of adherence between the phagocyte and the target cell. The most efficient opsonins are antibodies and C3b produced by the complement system. Antibodies are made against antigens on the surface of bacteria and are highly specific to that particular microorganism. Certain bacterial and fungal polysaccharide coatings activate the alternative and lectin pathways of complement activation, which deposits C3b on the bacterial surface and increases phagocytosis. The surface of phagocytes contains a variety of specific receptors that will strongly bind to opsonins. These include complement receptors that bind to C3b and Fc receptors that bind to a site on antibody molecules. Engulfment (endocytosis) is carried out by small pseudopods that extend from the
plasma membrane and surround the adherent microorganism, forming an intracellular phagocytic vacuole, or phagosome (see Figures 6-9 and 6-10). After the formation of the phagosome, lysosomes converge, fuse with the phagosome, and discharge their contents, creating a phagolysosome. Destruction of the bacterium takes place within the phagolysosome and is accomplished by both oxygen-dependent and oxygen-independent mechanisms. Oxygen-dependent killing mechanisms result from the production of toxic oxygen
species. Phagocytosis is accompanied by a burst of oxygen uptake by the phagocyte; this is termed the respiratory burst and results from a shift in much of the cell's glucose metabolism to the hexose-monophosphate shunt, which produces nicotinamide adenine dinucleotide phosphate (NADPH). A membrane-associated enzyme, NADPH oxidase, uses NADPH to generate superoxide (O2
−), hydrogen peroxide (H2O2), and other reactive oxygen species that can be highly damaging to bacteria. Hydrogen peroxide also can collaborate with the lysosomal enzyme myeloperoxidase and halide anions (Cl− and Br−) to form acids that kill bacteria and fungi. Oxygen-independent mechanisms of microbial killing include (1) the acidic pH
(3.5 to 4.0) of the phagolysosome, (2) cationic proteins that bind to and damage target cell membranes, (3) enzymatic attack of the microorganism's cell wall by lysozyme and other enzymes, and (4) inhibition of bacterial growth by lactoferrin
binding of iron. When a phagocyte dies at an inflammatory site, it frequently lyses (breaks open)
and releases its cytoplasmic contents into the tissue. For instance, contents of neutrophil primary granules (lysozyme, hydrolases, neutral proteases) and secondary granules (lysozyme, collagenase, gelatinase) can digest the connective tissue matrix, causing much of the tissue destruction associated with inflammation.24 The destructive effects of many enzymes and reactive oxygen molecules released by dying phagocytes are minimized by natural inhibitors found in the blood, such as superoxide dismutase (breaks down superoxide), catalase (breaks down hydrogen peroxide), and the antiproteinases α1-antitrypsin and α2-macroglobulin (both produced by the liver). An inherited deficiency of α1-antitrypsin often leads to chronic lung damage and emphysema as a result of inflammation. (The pulmonary effects of α1-antitrypsin deficiency are described in Chapter 27.)
Natural Killer Cells and Lymphocytes The main function of natural killer (NK) cells is recognition and elimination of cells infected with viruses, although they also are somewhat effective at elimination of other abnormal cells, specifically cancer cells.25 NK cells seem to be more efficient in this role when they encounter an infected cell within the circulatory system as opposed to within tissues. NK cells have inhibitory and activating receptors that allow differentiation between infected or tumor cells and normal cells. If the NK cell binds to a target cell through activating receptors, it produces several cytokines and toxic molecules that can kill the target.26 NK cells and lymphocytes, which are the principal cells of the adaptive immune response, will be discussed in much more detail in Chapter 7.
Quick Check 6-4
1. What are pattern recognition receptors?
2. What are cytokines? How do cytokines promote inflammation?
3. What products do the mast cells release during inflammation, and what are their effects?
4. What phagocytic cell types are involved in the acute inflammatory response? What is the role of each?
5. What are the four steps in the process of phagocytosis?
Acute and Chronic Inflammation Inflammation can be divided into phases of acute and chronic inflammation. The acute inflammatory response is self-limiting—that is, it continues only until the threat to the host is eliminated. This usually takes 8 to 10 days from onset to healing. If the acute inflammatory response proves inadequate, a chronic inflammation may develop and persist for weeks or months. If healing has not been initiated, inflammation may progress to a granulomatous response that is designed to contain the cause of tissue damage so it no longer poses any harm to the individual. The characteristics of the early (i.e., acute) inflammatory response differ from those of the later (i.e., chronic) response, and each phase involves different biochemical mediators and cells that function together. Depending on the successful containment of tissue damage and infection, the acute and chronic phases may lead to healing without progression to the next phase.
Local Manifestations of Acute Inflammation The cells and plasma protein systems of the inflammatory response interact to produce all the characteristics of inflammation, whether local or systemic (discussed in the next section), as well as determine the duration of inflammation, either acute or chronic. All the local characteristics of acute inflammation (i.e., swelling, pain, heat, and redness [erythema]) result from vascular changes and the subsequent leakage of circulating components into the tissue. The exudate of inflammation results from increased vascular permeability and
varies in composition, depending on the stage of the inflammatory response and, to some extent, the injurious stimulus. In early or mild inflammation, the exudate may be watery (serous exudate) with very few plasma proteins or leukocytes, such as the fluid in a blister. In more severe or advanced inflammation, the exudate may be thick and clotted (fibrinous exudate), such as in the lungs of individuals with pneumonia. If a large number of leukocytes accumulate, as in persistent bacterial infections, the exudate consists of pus and is called a purulent (suppurative) exudate. Purulent exudate is characteristic of walled-off lesions (cysts or abscesses). If bleeding occurs, the exudate is filled with erythrocytes and is described as a hemorrhagic exudate.
Systemic Manifestations of Acute Inflammation The three primary systemic changes associated with the acute inflammatory response are fever, leukocytosis (a transient increase in the levels of circulating
leukocytes), and increased levels of circulating plasma proteins.
Fever Fever is partially induced by specific cytokines (e.g., IL-1, released from neutrophils and macrophages). These are known as endogenous pyrogens to differentiate them from pathogen-produced exogenous pyrogens. Pyrogens act directly on the hypothalamus, the portion of the brain that controls the body's thermostat. (Mechanisms of temperature regulation and fever are discussed in Chapter 14.) A fever can be beneficial because some microorganisms (e.g., those that cause syphilis or gonococcal urethritis) are highly sensitive to small increases in body temperature. On the other hand, fever may have harmful side effects because it may enhance the host's susceptibility to the effects of endotoxins associated with gram-negative bacterial infections (bacterial toxins are described in Chapter 8).
Leukocytosis Leukocytosis is an increase in the number of circulating white blood cells (greater than 11,000/ml3 in adults). During many infections, leukocytosis may be accompanied by a left shift in the ratio of immature to mature neutrophils, so that the more immature forms of neutrophils, such as band cells, metamyelocytes, and occasionally myelocytes, are present in relatively greater than normal proportions. (Chapter 20 contains a more complete discussion of the development and maturation of blood cells.) Production of immature leukocytes increases primarily from proliferation and release of granulocyte and monocyte precursors in the bone marrow, which is stimulated by several products of inflammation.
Plasma Protein Synthesis The synthesis of many plasma proteins, mostly products of the liver, is increased during inflammation. These proteins, which can be either proinflammatory or anti- inflammatory in nature, are referred to as acute-phase reactants (Table 6-4). Acute-phase reactants reach maximal circulating levels within 10 to 40 hours after the start of inflammation. IL-1 is indirectly responsible for the synthesis of acute- phase reactants through the induction of IL-6, which directly stimulates liver cells to synthesize most of these proteins.
TABLE 6-4 Circulating Levels of Acute-Phase Reactants During Inflammation
Function Increased Decreased Coagulation components Fibrinogen None
Prothrombin Factor VIII Plasminogen
Protease inhibitors α1-Antitrypsin Inter-α1-antitrypsin α1-Antichymotrypsin
Transport proteins Haptoglobin Transferrin Hemopexin Ceruloplasmin Ferritin
Complement components C1s, C2, C3, C4, C5, C9, factor B, C1 inhibitor Properdin Miscellaneous proteins α1-Acid glycoprotein Albumin
Fibronectin Prealbumin Serum amyloid A (SAA) α1-Lipoprotein C-reactive protein (CRP) β-Lipoprotein
Common laboratory tests for inflammation measure levels of acute-phase reactants. For example, an increase in blood levels of acute-phase reactants, primarily fibrinogen, is associated with an increased adhesion among erythrocytes and a corresponding increase in the sedimentation rate. The erythrocyte sedimentation rate is a measurement of the rate at which red blood cells sediment in a tube over a prescribed time span (usually an hour). Although increased erythrocyte sedimentation is a nonspecific reaction, it is considered a good indicator of an acute inflammatory response.
Chronic Inflammation Superficially, the difference between acute and chronic inflammation is duration; chronic inflammation lasts 2 weeks or longer, regardless of cause. Chronic inflammation is sometimes preceded by an unsuccessful acute inflammatory response (Figure 6-11). For example, if bacterial contamination or foreign objects (e.g., dirt, wood splinter, silica, and glass) persist in a wound, an acute response may be prolonged beyond 2 weeks. Pus formation, suppuration (purulent discharge), and incomplete wound healing may characterize this type of chronic inflammation.
FIGURE 6-11 The Chronic Inflammatory Response. Inflammation usually becomes chronic because of the persistence of an infection, an antigen, or a foreign body in the wound. Chronic
inflammation is characterized by the persistence of many of the processes of acute inflammation. In addition, large amounts of neutrophil degranulation and death, the activation of lymphocytes, and the concurrent activation of fibroblasts result in the release of mediators that induce the infiltration of more lymphocytes and monocytes/macrophages and the beginning of wound healing and tissue repair. For more detailed information on each portion of the response,
see the figures referenced in this illustration.
Chronic inflammation can occur also as a distinct process without previous acute inflammation. Some microorganisms (e.g., mycobacteria that cause tuberculosis) have cell walls with a very high lipid and wax content, making them relatively insensitive to breakdown by phagocytes. Other microorganisms (e.g., those that cause leprosy, syphilis, and brucellosis) can survive within the macrophage and avoid removal by the acute inflammatory response. Other microorganisms produce toxins that damage tissue and cause persistent inflammation even after the organism is killed. Finally, chemicals, particulate matter, or physical irritants (e.g., inhaled dusts, wood splinters, and suture material) can cause a prolonged inflammatory response. Chronic inflammation is characterized by a dense infiltration of lymphocytes and
macrophages. If macrophages are unable to protect the host from tissue damage, the body attempts to wall off and isolate the infected area, thus forming a granuloma (Figure 6-12). For example, infections caused by some bacteria (listeriosis, brucellosis), fungi (histoplasmosis, coccidioidomycosis), and parasites (leishmaniasis, schistosomiasis, toxoplasmosis) can result in granuloma formation. TNF-α primarily drives granuloma formation.27 Some macrophages differentiate into large epithelioid cells, which specialize in taking up debris and other small
particles. Other macrophages fuse into multinucleated giant cells, which are active phagocytes that can engulf very large particles—larger than those that can be engulfed by a single macrophage. These two types of specialized cells form the center of the granuloma, which is surrounded by a wall of lymphocytes. The granuloma itself is often encapsulated by fibrous deposits of collagen and may become cartilaginous or possibly calcified by deposits of calcium carbonate and calcium phosphate.
FIGURE 6-12 Tuberculous Granuloma. A central area of amorphous caseous necrosis (C) is surrounded by a zone of lymphocytes (L) and enlarged epithelioid cells (E). Activated
macrophages frequently fuse to form multinucleated cells (Langhans giant cells). In tuberculoid granulomas the nuclei of the giant cells move to the cellular margins in a horseshoe-like
formation.
The classic granuloma associated with tuberculosis is characterized by a wall of epithelioid cells surrounding a cheeselike proteinaceous center derived from dead and decaying tissue (caseous necrosis) and mycobacteria.28 Decay of cells within the granuloma results in the release of acids and the enzymatic contents of lysosomes from dead phagocytes. In this inhospitable environment, the cellular debris is broken down into its basic constituents, and a clear fluid may remain (liquefaction necrosis). Eventually, this fluid diffuses out and leaves a hollow, thick-walled structure that has replaced normal tissue and reduced the function of the lung.
Quick check 6-5
1. Describe how acute inflammation differs from chronic inflammation. What characteristics do they share?
2. List the types of exudate produced in inflammation.
Wound Healing The conclusion of inflammation is healing and repair. The most favorable outcome is a return to normal structure and function if damage is minor, no complications occur, and destroyed tissues are capable of regeneration (replacement of damaged tissue with healthy tissue, such as occurs in the epithelia of the skin and intestines and in some organs, such as the liver) (Figure 6-13). This restoration is called resolution and may take up to 2 years, and local production of IL-10 appears to play a critical role.29 Resolution may not be possible if extensive damage is present, the tissue is not capable of regeneration, infection results in abscess or granuloma formation, or fibrin persists in the lesion. In those cases, repair takes place instead of resolution. Repair is the replacement of destroyed tissue with scar tissue. Scar tissue is composed primarily of collagen that fills in the lesion and restores strength but cannot carry out the physiologic functions of destroyed tissue, resulting in loss of function.
FIGURE 6-13 Wound Healing by Primary and Secondary Intention and Phases of Wound Healing. Phases of wound healing (coagulation, inflammation, proliferation, remodeling, and maturation) and steps in wound healing by primary intention (left) and secondary intention
(right). Note large amounts of granulation tissue and wound contraction in healing by secondary intention. (From Roberts JR, Custalow CB: Roberts and Hedges' clinical procedures in emergency medicine, ed 6, Philadelphia, 2013,
Saunders.)
Wound healing involves processes that (1) fill in, (2) seal, and (3) shrink the wound. These characteristics of healing vary in importance and duration among different types of wounds. A clean incision, such as a paper cut or a sutured surgical wound, heals primarily through the process of collagen synthesis. Because this type of wound has minimal tissue loss and close apposition of the wound edges, very little sealing (epithelialization) and shrinkage (contraction) are required. Wounds that heal under conditions of minimal tissue loss are said to heal by primary intention (see Figure 6-13).
Other wounds do not heal as easily. Healing of an open wound, such as a stage IV pressure ulcer (decubitus ulcer), requires a great deal of tissue replacement so that epithelialization, scar formation, and contraction take longer and healing occurs through secondary intention (see Figure 6-13). Healing by either primary or secondary intention may occur at different rates for different types of tissue injury. Epidermal wounds that heal by secondary intention and unsutured internal lesions
are not completely restored by healing. At best, repaired tissue regains 80% of its original tensile strength. Only epithelial, hepatic (liver), and bone marrow cells are capable of the complete mitotic regeneration of the normal tissue known as compensatory hyperplasia. In fibrous connective tissue, such as joints and ligaments, normal healing results in replacement of the original tissue with new tissue that does not have exactly the same structure or function as that of the original. Some tissues heal without replacement of cells. For example, damage resulting from myocardial infarction heals with a scar composed of fibrous tissue rather than with cardiac muscle. Wound healing occurs in three overlapping phases: inflammation, proliferation
and new tissue formation, and remodeling and maturation.
Phase I: Inflammation The early phase of wound healing, the transition from acute inflammation to healing, begins almost immediately. The inflammatory phase includes coagulation or hemostasis and the infiltration of cells that participate in wound healing, including platelets, neutrophils, and macrophages (Figure 6-14). The fibrin mesh of the blood clot acts as a scaffold for cells that participate in healing. Platelets contribute to clot formation and, as they degranulate, release growth factors that initiate proliferation of undamaged cells. Neutrophils clear the wound of debris and bacteria and are later replaced by macrophages. Macrophages are essential to wound healing because they clear debris, release wound healing mediators and growth factors, recruit fibroblasts, and help promote formation of a new blood supply (angiogenesis) during the proliferative phase of wound healing.
FIGURE 6-14 Time Course of Cells Infiltrating a Wound. Neutrophils and macrophages are the predominant cells that infiltrate a wound during inflammation. Lymphocytes appear later and peak at day 7. Fibroblasts are the predominant cells during the proliferative and remodeling phases of the healing process. (Adapted from Townsend CM et al, editors: Sabiston textbook of surgery, ed 19, St Louis, 2012,
Elsevier.)
Phase II: Proliferation and New Tissue Formation The proliferative phase begins 3 to 4 days after the injury and continues for as long as 2 weeks. The wound is sealed and the fibrin clot is replaced by normal tissue or scar tissue during this phase. The proliferative phase is characterized by macrophage invasion of the dissolving clot and recruitment and proliferation of fibroblasts (connective tissue cells), followed by fibroblast collagen synthesis, epithelialization, contraction of the wound, and cellular differentiation. Macrophages secrete a variety of biochemical mediators that promote healing, including:
1. Transforming growth factor-beta (TGF-β) stimulates fibroblasts entering the lesion to synthesize and secrete the collagen precursor procollagen.
2. Angiogenesis factors, such as vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2), stimulate vascular endothelial cells to form capillary buds that grow into the lesion; decreased pH and decreased wound oxygen tension also promote angiogenesis.30
3. Matrix metalloproteinases (MMPs) degrade and remodel extracellular matrix proteins (e.g., collagen and fibrin) at the site of injury.31
Granulation tissue grows into the wound from surrounding healthy connective tissue and consists of invasive cells, new lymphatic vessels, and new capillaries derived from capillaries in the surrounding tissue, giving the granulation tissue a red, granular appearance. During this process the healing wound must be protected. Epithelialization is the process by which epithelial cells grow into the wound from surrounding healthy tissue.32 Epithelial cells migrate under the clot or scab using MMPs to unravel collagen. Migrating epithelial cells contact similar cells from all sides of the wound and seal it. The epithelial cells remain active, undergoing differentiation to give rise to the various epidermal layers (see Chapter 41). Epithelialization of a skin wound can be hastened if the wound is kept moist, preventing the fibrin clot from becoming a scab. Fibroblasts are important cells during healing because they secrete collagen and
other connective tissue proteins. Fibroblasts are stimulated by macrophage-derived TGF-β to proliferate, enter the lesion, and deposit connective tissue proteins in débrided areas about 6 days after the fibroblasts have entered the lesion. Collagen is the most abundant protein in the body.33 It contains high concentrations of the amino acids glycine, proline, and lysine, many of which are enzymatically modified. Modification of proline and lysine requires several cofactors that are absolutely necessary for proper collagen polymerization and function. These include iron, ascorbic acid (vitamin C), and molecular oxygen (O2); absence of any of these results in impaired wound healing. As healing progresses, collagen molecules are cross-linked by intermolecular covalent bonds to form collagen fibrils that are further cross-linked to form collagen fibers. The complete process takes several months. In granulation tissue, TGF-β induces some fibroblasts to transition into
myofibroblasts, specialized cells responsible for wound contraction.34 Myofibroblasts have features of both smooth muscle cells and fibroblasts. They appear microscopically similar to fibroblasts but differ in that their cytoplasm contains bundles of parallel fibers similar to those found in smooth muscle cells. Wound contraction occurs as extensions from the plasma membrane of myofibroblasts establish connections between neighboring cells, contract their
fibers, and exert tension on the neighboring cells while anchoring themselves to the wound bed. Wound contraction is necessary for closure of all wounds, especially those that heal by secondary intention. Contraction is noticeable 6 to 12 days after injury.
Phase III: Remodeling and Maturation Tissue remodeling and maturation begins several weeks after injury and is normally complete within 2 years. During this phase, there is continuation of cellular differentiation, scar formation, and scar remodeling. The fibroblast is the major cell of tissue remodeling with the deposition of collagen into an organized matrix. Tissue regeneration and wound contraction continue in the remodeling and maturation phase—a phase for recovering normal tissue structure that can persist for years. For wounds that heal by scarring, scar tissue is remodeled and capillaries disappear, leaving the scar avascular. Within 2 to 3 weeks after maturation has begun, the scar tissue has gained about two thirds of its eventual maximal strength.
Dysfunctional Wound Healing Dysfunctional wound healing and impaired epithelialization may occur during any phase of the healing process. The cause of dysfunctional wound healing includes ischemia, excessive bleeding, excessive fibrin deposition, a predisposing disorder such as diabetes mellitus, obesity, wound infection, inadequate nutrients, numerous drugs, and tobacco smoke.35 Oxygen-deprived (ischemic) tissue is susceptible to cellular death and infection,
which prolongs inflammation and delays healing. Ischemia reduces energy production and impairs collagen synthesis and the tensile strength of regenerating connective tissue. Healing is prolonged if there is excessive bleeding. Large clots increase the
amount of space that granulation tissue must fill and serve as mechanical barriers to oxygen diffusion. Accumulated blood is an excellent culture medium for bacteria and promotes infection, thereby prolonging inflammation by increasing exudation and pus formation. Decreased blood volume also inhibits inflammation because of vessel constriction rather than the dilation required to deliver inflammatory cells, nutrients, and oxygen to the site of injury. Obesity delays wound healing because of impaired leukocyte function and
predisposition to infection, decreases in the number of growth factors, and increases in the levels of proinflammatory cytokines. Additionally, there is dysregulation in collagen synthesis and a decrease in angiogenesis.36
Excessive fibrin deposition is detrimental to healing. Fibrin released in response to injury must eventually be reabsorbed to prevent organization into fibrous adhesions. Adhesions formed in the pleural, pericardial, or abdominal cavities can bind organs together by fibrous bands and distort or strangulate the affected organ. Persons with diabetes are at risk for prolonged wound healing. Wounds are often
ischemic because of the potential for small-vessel diseases that impair the microcirculation and alter (glycosylated) hemoglobin, which has an increased affinity for oxygen and thus does not readily release oxygen in tissues. Consequences of hyperglycemia also include suppression of macrophages and increased risk for wound infection. Wound infection is caused by the infiltration of pathogens. Pathogens damage
cells, stimulate the continued release of inflammatory mediators, consume nutrients, and delay wound healing. Optimal nutrition is important during all phases of healing because metabolic
needs increase. Leukocytes need glucose to produce adenosine 5′-triphosphate (5′- ATP) necessary for chemotaxis, phagocytosis, intercellular killing, and initiation of healing; therefore the wounds of persons with diabetes who receive insufficient insulin heal poorly. Hypoproteinemia impairs fibroblast proliferation and collagen synthesis. Prolonged lack of vitamins A and C results in poorly formed connective tissue and greatly impaired healing because they are cofactors required for collagen synthesis.37 Other nutrients, including iron, zinc, manganese, and copper, are also required as cofactors for collagen synthesis. Malnutrition increases risk for wound infection, delays healing, and reduces wound tensile strength. Medications, including antineoplastic (anticancer) agents, nonsteroidal anti-
inflammatory drugs (NSAIDs), and steroids, delay wound healing. Antineoplastic agents slow cell division and inhibit angiogenesis. Although NSAIDs inhibit prostaglandin production and suppress acute inflammation and relieve pain, they also can delay wound healing, particularly bone formation, and may contribute to the formation of excessive scarring. Steroids prevent macrophages from migrating to the site of injury and inhibit release of collagenase and plasminogen activator. Steroids also inhibit fibroblast migration into the wound during the proliferative phase and delay epithelialization. Toxic agents in tobacco smoke (i.e., nicotine, carbon monoxide, and hydrogen cyanide) delay wound healing and increase the risk for wound infection. Dysfunctional collagen synthesis may involve excessive production of collagen,
leading to a hypertrophic scar or keloid.38 A hypertrophic scar is raised but remains within the original boundaries of the wound and tends to regress over time (Figure 6-15, A). A keloid is a raised scar that extends beyond the original boundaries of the wound, invades surrounding tissue, and is likely to recur after
surgical removal (Figure 6-15, B). A familial tendency to keloid formation has been observed, with a greater incidence in blacks than whites.
FIGURE 6-15 Hypertrophic Scar and Keloid Scar Formation. Hypertrophic scar (A) and keloid scar (B) caused by excessive synthesis of collagen at suture sites. (A from Flint PW et al: Cummings
otolaryngology: head & neck surgery, ed 6, Philadelphia, 2015, Mosby; B from Damjanov I, Linder J: Anderson's pathology, ed 10, St Louis, 1996, Mosby.)
Wound Disruption A potential complication of wounds that are sutured closed is dehiscence, in which the wound pulls apart at the suture line. Dehiscence generally occurs 5 to 12 days after suturing, when collagen synthesis is at its peak. Approximately half of dehiscence occurrences are associated with wound infection, but they also may be the result of sutures breaking because of excessive strain. Obesity increases the risk for dehiscence because adipose tissue is difficult to suture. Wound dehiscence
usually is heralded by increased serous drainage from the wound and a patient's perception that “something gave way.” Prompt surgical attention is required.
Impaired Contraction Wound contraction, although necessary for healing, may become excessive, resulting in a deformity or contracture of scar tissue. Burns of the skin are especially susceptible to contracture development, particularly at joints, resulting in loss of movement around the joints. Internal contractures include duodenal strictures caused by dysfunctional healing of a peptic ulcer; esophageal strictures caused by chemical burns, such as lye ingestion; or abdominal adhesions caused by surgery, infection, or radiation. Contracture may occur in cirrhosis of the liver, constricting vascular flow and contributing to the development of portal hypertension and esophageal varices. Proper positioning, range-of-motion exercises, and surgery are among the physical means used to overcome excessive skin contractures. Surgery is performed to release internal contractures.
Quick check 6-6
1. How does regeneration of tissue differ from repair of tissue?
2. What does it mean to heal by primary intention?
3. What is the role of fibroblasts in wound healing?
4. Describe various ways wound healing may be dysfunctional.
Pediatric Considerations Age-Related Factors Affecting Innate Immunity in the Newborn Child
• Newborn physiologic immunity acquired from mother through placenta and breast milk.
• Newborns have transiently depressed inflammatory responses.
• Neutrophils are incapable of chemotaxis, lacking fluidity in the plasma membrane.
• Complement levels are diminished, especially components of the alternative pathways (e.g., factor B), particularly in premature newborns.
• Monocyte/macrophage numbers are normal but chemotaxis of monocytes is delayed.
• There is a tendency for infections associated with chemotactic defects, for example, cutaneous abscesses caused by staphylococci and cutaneous candidiasis.
• There are diminished oxidative and bacterial responses in those stressed by in utero infection or respiratory insufficiency.
• There is a tendency to develop severe overwhelming sepsis and meningitis when infected by bacteria against which no maternal antibodies are present.
• The establishment of the gut microbiome is facilitated by breast milk.
• Cesarean delivered newborns have reduced gut microbial diversity.
Geriatric Considerations Age-Related Factors Affecting Innate Immunity in the Elderly
• Normal numbers of cells of innate immunity but possible diminished function (e.g., decreased phagocytic activity, decreased antibody production, and altered cytokine synthesis)
• Increased incidence of chronic inflammation, possibly related to increased production of proinflammatory mediators
• At risk for impaired healing and infection associated with chronic illness (e.g., diabetes mellitus, peripheral vascular disease, or cardiovascular disease) and decreased phagocytosis.
• Use of medications interfering with healing (e.g., anti-inflammatory steroids)
• Loss of subcutaneous fat, diminishing layers of protection against injury
• Atrophied epidermis, including underlying capillaries, which decreases perfusion and increases risk of hypoxia in wound bed
• Aging of the immune system, diminishing the effectiveness of vaccines
Did You Understand? Innate Immunity 1. Neonates often have transiently depressed inflammatory function, particularly neutrophil chemotaxis and alternative complement pathway activity.
2. Elderly persons are at risk for impaired wound healing, usually because of chronic illnesses.
3. There are three layers of human defense: barriers; innate immunity, which includes the inflammatory response; and adaptive (acquired) immunity.
4. Physical barriers are the first lines of defense that prevent damage to the individual and prevent invasion by pathogens; these include the skin and mucous membranes.
5. Antibacterial peptides (cathelicidins, defensins, collectins, and mannose-binding lectin) in mucous secretions, perspiration, saliva, tears, and other secretions provide a biochemical barrier against pathogenic microorganisms.
6. The skin and mucous membranes are colonized by commensal or mutualistic microorganisms that provide protection by releasing chemicals that facilitate immune responses, prevent colonization by pathogens, and facilitate digestion in the gastrointestinal tract.
7. The second line of defense is the inflammatory response, a rapid and nonspecific protective response to cellular injury from any cause. It can occur only in vascularized tissue.
8. The macroscopic hallmarks of inflammation are redness, swelling, heat, pain, and loss of function of the inflamed tissues.
9. The microscopic hallmarks of inflammation are vasodilation, increased capillary permeability, and an accumulation of fluid and cells at the inflammatory site.
10. Inflammation is mediated by three key plasma protein systems: the complement system, the clotting system, and the kinin system. The components of all three systems are a series of inactive proteins that are activated sequentially.
11. The complement system can be activated by antigen-antibody reactions (through the classical pathway) or by other products, especially bacterial polysaccharides (through the lectin pathway or the alternative pathway), resulting in the production of biologically active fragments that recruit phagocytes, activate mast cells, and destroy pathogens.
12. The most biologically potent products of the complement system are C3b (opsonin), C3a (anaphylatoxin), and C5a (anaphylatoxin, chemotactic factor).
13. The clotting system stops bleeding, localizes microorganisms, and provides a meshwork for repair and healing.
14. Bradykinin is the most important product of the kinin system and causes vascular permeability, smooth muscle contraction, and pain.
15. Control of inflammation regulates inflammatory cells and enzymes and localizes the inflammatory response to the area of injury or infection.
16. Carboxypeptidase, histaminase, and C1 esterase inhibitor are inactivating enzymes, and the fibrinolytic system and plasmin facilitate clot degradation after bleeding is stopped.
17. Many different types of cells are involved in the inflammatory process including mast cells, endothelial cells, platelets, phagocytes (neutrophils, eosinophils, monocytes and macrophages, dendritic cells), natural killer (NK) cells, and lymphocytes.
18. Most cells express plasma membrane pattern recognition receptors (PRRs) that recognize molecules produced by infectious microorganisms (pathogen-associated molecular patterns, or PAMPs), or products of cellular damage (damage-associated molecular patterns, or DAMPs). Toll-like receptors (TLRs) and NOD-like receptors are expressed on many inflammatory cells, recognize PAMPs and DAMPs, and promote release of cytokines and inflammatory mediators that eliminate damaged cells and protect against invasion by microbes.
19. The cells of the innate immune system secrete many biochemical mediators (cytokines) that are responsible for activating other cells and regulating the inflammatory response; these cytokines include chemokines, interleukins, interferons, and other molecules.
20. Chemokines induce chemotaxis of leukocytes, fibroblasts, and other cells to promote phagocytosis and wound healing.
21. Interleukins are produced primarily by lymphocytes and macrophages and promote or inhibit inflammation by activating growth and differentiation of leukocytes and lymphocytes.
22. The most important proinflammatory interleukins are interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α). Interleukins 6 and 10 down-regulate the inflammatory response.
23. Interferons are produced by cells that are infected by viruses. Once released from infected cells, interferons can stimulate neighboring healthy cells to produce substances that prevent viral infection.
24. The most important activator of the inflammatory response is the mast cell, which is located in connective tissue near capillaries and initiates inflammation by releasing biochemical mediators (histamine, chemotactic factors) from preformed cytoplasmic granules and synthesizing other mediators (prostaglandins, leukotrienes, and platelet-activating factor) in response to a stimulus. Basophils are found in the blood and function similar to mast cells.
25. Histamine is the major vasoactive amine released from mast cells. It causes dilation of capillaries and retraction of endothelial cells lining the capillaries, which increases vascular permeability.
26. The endothelial cells lining the circulatory system (vascular endothelium) normally regulate circulating components of the inflammatory system and maintain normal blood flow by preventing spontaneous activation of platelets and members of the clotting system.
27. During inflammation the endothelium expresses receptors that help leukocytes leave the vessel and retract to allow fluid to pass into the tissues.
28. Platelets interact with the coagulation cascade to stop bleeding and release a number of mediators that promote and control inflammation.
29. The polymorphonuclear neutrophil (PMN), the predominant phagocytic cell in the early inflammatory response, exits the circulation by diapedesis through the retracted endothelial cell junctions and moves to the inflammatory site by
chemotaxis.
30. Eosinophils release products that control the inflammatory response and are the principal cell that kills parasitic organisms.
31. The macrophage, the predominant phagocytic cell in the late inflammatory response, is highly phagocytic, is responsive to cytokines, and promotes wound healing.
32. Dendritic cells connect the innate and acquired immune systems by collecting antigens at the site of inflammation and transporting them to sites, such as the lymph nodes, where immunocompetent B and T cells reside and are transformed into functional cells.
33. Phagocytosis is a multistep cellular process for the elimination of pathogens and foreign debris. The steps include recognition and attachment, engulfment, formation of a phagosome and phagolysosome, and destruction of pathogens or foreign debris. Phagocytic cells engulf microorganisms and enclose them in phagocytic vacuoles (phagolysosomes), within which toxic products (especially metabolites of oxygen) and degradative lysosomal enzymes kill and digest the microorganisms.
34. Opsonins, such as antibody and complement component C3b, coat microorganisms and make them more susceptible to phagocytosis by binding them more tightly to the phagocyte.
Acute and Chronic Inflammation 1. Acute inflammation is self-limiting and usually resolves within 8 to 10 days.
2. Local manifestations of inflammation are the result of the vascular changes associated with the inflammatory process, including vasodilation and increased capillary permeability. The symptoms include redness, heat, swelling, and pain.
3. The principal systemic effects of inflammation are fever and increases in levels of circulating leukocytes (leukocytosis) and plasma proteins (acute-phase reactants [i.e., IL-1 and IL-6]).
4. Chronic inflammation can be a continuation of acute inflammation that lasts 2 weeks or longer. It also can occur as a distinct process without much preceding acute inflammation.
5. Chronic inflammation is characterized by a dense infiltration of lymphocytes and macrophages. The body may wall off and isolate the infection to protect against tissue damage by formation of a granuloma.
Wound Healing 1. Resolution (regeneration) is the return of tissue to nearly normal structure and function. Repair is healing by scar tissue formation.
2. Damaged tissue proceeds to resolution (restoration of the original tissue structure and function) if little tissue has been lost or if injured tissue is capable of regeneration. This is called healing by primary intention.
3. Tissues that sustained extensive damage or those incapable of regeneration heal by the process of repair resulting in the formation of a scar. This is called healing by secondary intention.
4. Resolution and repair occur in two separate phases: the reconstructive phase in which the wound begins to heal and the maturation phase in which the healed wound is remodeled.
5. Dysfunctional wound healing can be related to ischemia, excessive bleeding, excessive fibrin deposition, a predisposing disorder (such as diabetes mellitus), wound infection, inadequate nutrients, numerous drugs, or altered collagen synthesis.
6. Dehiscence is a disruption in which the wound pulls apart at the suture line.
7. A contracture is a deformity caused by the excessive shortening of collagen in scar tissue.
Key Terms Abscess, 149
Acute inflammation, 149
Acute-phase reactant, 149
Adaptive immunity, 134
Alternative pathway, 139
Anaphylatoxin, 138
Angiogenesis factor, 152
Antimicrobial peptide, 135
α1-Antitrypsin, 147
Basophil, 144
Blood clot, 139
Bradykinin, 141
C1 esterase inhibitor (C1 inh), 141
C1 inh deficiency, 141
Carboxypeptidase, 141
Cathelicidin, 135
Chemokine, 143
Chemotactic factor, 138
Chemotaxis, 145, 147
Chronic inflammation, 149
Classical pathway, 139
Clotting (coagulation) system, 139
Collagen, 153
Collectin, 135
Complement receptor, 142
Complement system, 140
Contact activation (intrinsic) pathway, 140
Contraction, 152
Contracture of scar tissue, 154
Cyst, 149
Cytokine, 143
Damage-associated molecular pattern (DAMP), 142
Defensin, 135
Degranulation, 144
Dehiscence, 154
Dendritic cell, 147
Diapedesis, 147
Endogenous pyrogen, 149
Endothelial cell, 146
Eosinophil, 146
Eosinophil chemotactic factor of anaphylaxis (ECF-A), 145
Epithelialization, 152
Epithelioid cell, 150
Exudate, 149
Fc receptor, 147
Fever, 149
Fibrinolytic system, 141
Fibrinous exudate, 149
Fibroblast, 153
Giant cell, 150
Granulation tissue, 153
Granuloma, 150
Hageman factor (factor XII), 140
Hemorrhagic exudate, 149
Hereditary angioedema, 141
Hexose-monophosphate shunt, 147
Histaminase, 141
Histamine, 145
Hypertrophic scar, 153
Inflammasomes, 142
Inflammation, 137
Inflammatory phase, 152
Inflammatory response, 134, 137
Innate immunity, 134
Interferon (IFN), 144
Interleukin (IL), 143
Interleukin-1 (IL-1), 144
Interleukin-6 (IL-6), 144
Interleukin-10 (IL-10), 144
Keloid, 154
Kinin system, 140
Lectin pathway, 139
Leukocytosis, 149
Leukotriene (slow-reacting substance of anaphylaxis [SRS-A]), 145
Lymphocyte, 141
Lysozyme, 135
Macrophage, 146
Mannose-binding lectin (MBL), 135
Margination (pavementing), 147
Mast cell, 144
Matrix metalloproteinase (MMP), 153
Monocyte, 146
Myofibroblast, 153
Natural killer (NK) cells, 147
Neutrophil (polymorphonuclear neutrophil [PMN]), 146
Neutrophil chemotactic factor (NCF), 145
Nitric oxide (NO), 146
NOD-like receptors (NLRs), 142
Normal flora, 135
Normal microbiome, 135
Opportunistic microorganism, 137
Opsonin, 138
Opsonization, 147
Pathogen-associated molecular pattern (PAMP), 142
Pattern recognition receptor (PRR), 142
Phagocyte, 138
Phagocytosis, 146
Phagolysosome, 147
Phagosome, 147
Plasma protein system, 138
Plasmin, 141
Plasminogen, 141
Platelet, 146
Platelet-activating factor (PAF), 145
Primary intention, 152
Proliferative phase, 152
Prostacyclin (PGI2), 146
Prostaglandin, 145
Purulent (suppurative) exudate, 149
Pyrogen, 149
Regeneration, 151
Repair, 152
Resolution, 151
Scar tissue, 152
Scavenger receptors, 142
Secondary intention, 152
Serous exudate, 149
T lymphocyte, 147
Tissue factor (extrinsic) pathway, 140
Tissue factor (TF; tissue thromboplastin), 140
Toll-like receptor (TLR), 142
Transforming growth factor, 144
Transforming growth factor-beta (TGF-β), 144
Tumor necrosis factor-alpha (TNF-α), 144
Wound contraction, 153
References 1. Peterson LW, Artis D. Intestinal epithelial cells: regulators of barrier function and immune homeostasis. Nat Rev Immunol. 2014;14(3):141–153.
2. Human Microbiome Project Consortium. Structure, function and diversity of the healthy human microbiome. Nature. 2012;486(7402):207–214.
3. Mayer EA, et al. Gut/brain axis and the microbiota. J Clin Invest. 2015;125(3):926–938.
4. Moal VL-L, Servin AL. Anti-infective activities of Lactobacillus strains in the human intestinal microbiota: from probiotics to gastrointestinal anti- infectious biotherapeutic agents. Clin Microbiol Rev. 2014;27(2):167–199.
5. Ricklin D, Lambris JD. Complement in immune and inflammatory disorders: therapeutic interventions. J Immunol. 2013;190(8):3839–3847.
6. Sekine H, et al. The role of MASP-1/3 in complement activation. Adv Exp Med Biol. 2013;735:41–53.
7. O'Neil LA, et al. The history of Toll-like receptors—redefining innate immunity. Nat Rev Immunol. 2013;13(6):453–460.
8. Qian C, Cao X. Regulation of Toll-like receptor signaling pathways in innate immune responses. Ann N Y Acad Sci. 2013;1283(2013):67–74.
9. Canton J, et al. Scavenger receptors in homeostasis and immunity. Nat Rev Immunol. 2013;13(9):621–634.
10. Philpott DJ, et al. NOD proteins: regulators of inflammation in health and disease. Nat Rev Immunol. 2014;14(1):9–23.
11. Saxena M, Yeretssian G. NOD-like receptors: master regulators of inflammation and cancer. Front Immunol. 2014;5:327.
12. Schett G, et al. Toward a cytokine-based disease taxonomy. Nat Med. 2013;19(7):822–824.
13. Martins-Green M, et al. Chemokines and their receptors are key players in the orchestra that regulates wound healing. Adv Wound Care (New Rochelle). 2013;2(7):327–347.
14. Akdis M, et al. Interleukins, from 1 to 37 and interferon-γ: receptors, functions, and roles in diseases. J Allergy Clin Immunol. 2011;127(3):701– 721.
15. Ivashkiv LB, Donlin LT. Regulation of type I interferon responses. Nat Rev Immunol. 2014;14(1):36–49.
16. Cromheecke JL, et al. Emerging role of human basophil biology in health and disease. Curr Allergy Asthma Rep. 2014;14(1):408 [1-9].
17. Kolaczkowska E, Kubes P. Neutrophil recruitment and function in health and inflammation. Nat Rev Immunol. 2013;13(3):159–175.
18. Melo RC, et al. Eosinophil-derived cytokines in health and disease: unraveling novel mechanisms of selective secretion. Allergy. 2013;68(3):274–284.
19. Wynn TA, et al. Macrophage biology in development, homeostasis and disease. Nature. 2013;496(7446):445–455.
20. Van Dyken SJ, Locksley RM. Interleukin-4- and interleukin-13-mediated alternatively activated macrophages: roles in homeostasis and disease. Annu Rev Immunol. 2013;31(2013):317–343.
21. Platt AM, Randolph GJ. Dendritic cell migration through the lymphatic vasculature to lymph nodes. Adv Immunol. 2013;120(2013):51–68.
22. Herter J, Zarbock A. Integrin regulation during leukocyte recruitment. J Immunol. 2013;190(9):4451–4457.
23. Weninger W, et al. Leukocyte migration in the interstitial space of non- lymphoid organs. Nat Rev Immunol. 2014;14(4):232–248.
24. Wilgus TA, et al. Neutrophils and wound repair: positive actions and negative reactions. Adv Wound Care (New Rochelle). 2013;2(7):379–388.
25. Campbell KS, Hasegawa J. Natural killer cell biology: an update and future directions. J Allergy Clin Immunol. 2013;132(3):536–544.
26. Long EO, et al. Controlling natural killer cell responses: integration of signals for activation and inhibition. Annu Rev Immunol. 2013;31(2013):227–258.
27. Dorhoi A, Kaufmann SH. Tumor necrosis factor alpha in mycobacterial infection. Semin Immunol. 2014 May 9 [Epub ahead of print].
28. O'Garra A, et al. The immune response to tuberculosis. Annu Rev Immunol. 2013;31(2013):475–527.
29. King A, et al. Regenerative wound healing: the role of interleukin-10. Adv Wound Care (New Rochelle). 2014;3(4):315–323.
30. Raju R, et al. A network map of EGF-1/FGFR signaling system. J Signal Transduct. 2014;962962.
31. Khokha R, et al. Metalloproteinases and their natural inhibitors in inflammation and immunity. Nat Rev Immunol. 2013;13(9):649–665.
32. Longmate WM, Dipersio CM. Integrin regulation of epidermal function in wounds. Adv Wound Care (New Rochelle). 2014;3(3):229–246.
33. Mienaltowski MJ, Birk DE. Structure, physiology, and biochemistry of collagens. Adv Exp Med Biol. 2014;802(2014):5–29.
34. Yang X, et al. Reversal of myofibroblast differentiation: a review. Eur J Pharmacol. 2014;734:83–90 [Jul 5].
35. Pierpont YN, et al. Obesity and surgical wound healing: a current review. ISRN Obes. 2014;2014:638936.
36. Pence BD, Woods JA. Exercise, obesity, and cutaneous wound healing: evidence from rodent and human studies. Adv Wound Care (New Rochelle). 2014;3(1):71–79.
37. Moores J. Vitamin C: a wound healing perspective. Br J Community Nurs. 2013;Suppl:S6 [S8-S11].
38. Monstrey S, et al. Updated scar management practical guidelines: non- invasive and invasive measures. J Plast Reconstr Aesthet Surg. 2014;67(8):1017–1025.
7
Adaptive Immunity Neal S. Rote
CHAPTER OUTLINE
Third Line of Defense: Adaptive Immunity, 158 Antigens and Immunogens, 160 Antibodies, 161
Classes of Immunoglobulins, 161 Antigen-Antibody Binding, 162 Function of Antibodies, 162
Immune Response: Collaboration of B Cells and T Cells, 166
Generation of Clonal Diversity, 166 Clonal Selection, 167
Cell-Mediated Immunity, 172
T-Lympohocyte Function, 172 PEDIATRIC CONSIDERATIONS: Age-Related Factors Affecting Mechanisms of Self-Defense in the Newborn Child, 173 GERIATRIC CONSIDERATIONS: Age-Related Factors Affecting Mechanisms of Self-Defense in the Elderly, 173
The third line of defense in the human body is adaptive (acquired) immunity, often called the immune response or immunity, and consists of lymphocytes (Figure 7-1) and serum proteins called antibodies. Once external barriers have been compromised and inflammation (innate immunity, see Chapter 6) has been activated, the adaptive immune response is called into action. Inflammation is the “first responder” that contains the initial injury and slows the spread of infection, whereas adaptive immunity slowly augments the initial defenses against infection and provides long-term security against reinfection.
FIGURE 7-1 Lymphocytes. A scanning electron micrograph showing lymphocytes (yellow, like cotton candy), red blood cells, and platelets. (Copyright Dennis Kunkel Microscopy, Inc.)
Third Line of Defense: Adaptive Immunity Inflammation and adaptive immunity differ in several key ways. First, the components of inflammation are activated immediately after tissue damage. Adaptive immunity is inducible; the effectors of the immune response, lymphocytes and antibodies, do not preexist but must be produced in response to infection. Thus, adaptive immunity develops more slowly than inflammation. Second, the inflammatory response is similar regardless of differences in the cause of tissue damage or whether the inflammatory site is sterile or contaminated with infectious microorganisms. The immune response is exquisitely specific. The lymphocytes and antibodies induced in response to infection are extremely specific to the infecting microbe. Third, the residual mediators of inflammation must be removed quickly to limit damage to surrounding healthy tissue and allow healing. The effectors of the immune response are long-lived and systemic, providing long-term protection against specific infections. Finally, the inflammatory response to both recurrent tissue damage and infection is identical. The immune response has memory. If reinfected with the same microbe, protective lymphocytes and antibody are produced immediately, thus providing permanent long-term protection against infection. Despite the differences, the innate and adaptive immune systems are highly
interactive and complementary. Many components of innate resistance are necessary for the development of the adaptive immune response. Conversely, products of the adaptive immune response activate components of innate resistance. Thus, both systems are essential for complete protection against infectious disease. The mechanisms underlying the immune response will be discussed in this
chapter. As with Chapter 6, a complete description of all the important components and processes of an effective immune response would require far more space than available. Therefore, this chapter will focus on the basic concepts and the most important, or well-studied, mediators of the immune response. The adaptive immune response has its own vocabulary (Figure 7-2). Antigens are
the molecular targets of antibodies and lymphocytes. Antigens are generally small molecules, usually within proteins, carbohydrates, or lipids, found on the surface of microbes or infected cells, although this definition will be expanded as we discuss immunologic diseases in Chapter 8. In the fetus, well before being exposed to any infectious microorganisms, lymphocytes have undergone extensive differentiation. Some lymphoid stem cells enter the thymus and differentiate into T lymphocytes (T cells, T indicates thymus derived), whereas others enter specific regions of the bone marrow and differentiate into B lymphocytes (B cells, B indicates bone marrow derived). Each type of cell develops origin-specific cell surface proteins that
identify them as T or B cells. Both B and T cells also develop cell surface antigen receptors. The receptors are remarkable because an individual lymphocyte is programmed to recognize only one specific antigen before having encountered that antigen. It is estimated that before birth each individual has produced a population of B and T lymphocytes capable of recognizing at least 108 different antigens. This process is called generation of clonal diversity and refers to the process by which the extensive diversity of antigen receptors on B and T cells is established (see Figure 7-2).
FIGURE 7-2 Overview of the Immune Response. The immune response can be separated into two phases: the generation of clonal diversity and clonal selection. During the generation of clonal diversity, lymphoid stem cells from the bone marrow migrate to the central lymphoid organs (the thymus or regions of the bone marrow), where they undergo a series of cellular
division and differentiation stages resulting in either immunocompetent T cells from the thymus or immunocompetent B cells from the bone marrow. These cells are still naïve in that they have
never encountered foreign antigen. The immunocompetent cells enter the circulation and migrate to the secondary lymphoid organs (e.g., spleen and lymph nodes), where they establish residence in B- and T-cell–rich areas. The clonal selection phase is initiated by exposure to foreign antigen. The antigen is usually processed by antigen-presenting cells (APCs) for
presentation to T-helper cells (Th cells). The intercellular cooperation among APCs, Th cells, and immunocompetent T and B cells results in a second stage of cellular proliferation and
differentiation. Because antigen has “selected” those T and B cells with compatible antigen receptors, only a small population of T and B cells undergo this process at one time. The result is an active cellular immunity or humoral immunity, or both. Cellular immunity is mediated by a population of effector T cells that can kill targets (T-cytotoxic cells) or regulate the immune
response (T-regulatory cells), as well as a population of memory cells (T-memory cells) that can respond more quickly to a second challenge with the same antigen. Humoral immunity is
mediated by a population of soluble proteins (antibodies) produced by plasma cells and by a population of memory B cells that can produce more antibody rapidly to a second challenge with
the same antigen.
Lymphocytes leave the primary lymphoid organs (bone marrow and thymus) as immunocompetent, but naïve, B and T cells. The cells are immunocompetent in that they have the capacity to respond to antigens, but they are naïve in that they have not yet encountered antigen. These cells enter the blood and lymphatic vessels and migrate to the secondary lymphoid organs (e.g., lymph nodes, spleen) of the systemic immune system (Figure 7-3). Some take up residence in B cell and T cell rich areas of those organs, and others reenter the circulation. Approximately 60% to 70% of circulating lymphocytes are immunocompetent T cells, and 10% to 20% are
immunocompetent B cells.
FIGURE 7-3 Lymphoid Tissues: Sites of B-Cell and T-Cell Differentiation. Immature lymphocytes migrate through central (primary) lymphoid tissues: the bone marrow (central lymphoid tissue
for B lymphocytes) and the thymus (central lymphoid tissue for T lymphocytes). Mature lymphocytes later reside in the T- and B-lymphocyte–rich areas of the peripheral (secondary)
lymphoid tissues.
A second process, clonal selection, is initiated when an infection occurs. This process requires the cooperation among a variety of cells in the secondary lymphoid organs; antigen needs to be processed by phagocytic cells, primarily dendritic cells, which also express the processed antigen on their surfaces and present the antigen to lymphocytes. Thus begins a symphony of cellular interactions, referred to as clonal selection, involving several subsets of B and T cells, intercellular adhesion through antigen receptors and specific intercellular adhesion molecules, the production and response to multiple cytokines, and eventual
differentiation of immunocompetent B and T cells into highly specialized effector cells. B cells develop into plasma cells that become factories for the production of antibody. T cells develop into several subsets that can identify and kill a target cell (T-cytotoxic cells, Tc cells), regulate the immune response by helping the clonal selection process (T-helper cells, Th cells), or suppress inappropriate immune responses (T-regulatory cells, Treg cells). Both B and T cells also differentiate into very long-lived memory cells that exist for decades or, in some cases, for the life of the individual. Memory cells are rapidly activated if a second infection occurs with the same microbe. Antibodies circulate in the blood and defend against extracellular microbes and
microbial toxins. This is referred to as the humoral immune response, or humoral immunity. Effector T cells are found in the blood and tissues and defend against intracellular pathogens (e.g., viruses) and cancer cells. This is referred to as the cellular immune response, or cellular immunity (also cell-mediated immunity). The preceding overview describes what is termed active immunity (active
acquired immunity), which develops in response to antigen. In certain clinical situations, preformed antibody or lymphocytes may be administered to an individual, termed passive immunity (passive acquired immunity). Examples include individuals exposed to an infectious agent without having a preexisting vaccine-induced immunity (e.g., hepatitis A virus or rabies virus) (Table 7-1). Passive immunization with specific T cells has been used to treat several forms of cancer. Whereas active acquired immunity is long lived, passive immunity is only temporary because the donor's antibodies or T cells are eventually destroyed.
TABLE 7-1 Clinical Use of Antigen or Antibody
USE OF ANTIGEN OR ANTIBODY Antigen Source
Protection: Combat Active Disease Protection: Vaccination Diagnosis Therapy
Infectious agents
Neutralize or destroy pathogenic microorganisms (e.g., antibody response against viral infections)
Induce safe and protective immune response (e.g., recommended childhood vaccines)
Measure circulating antigen from infectious agent or antibody (e.g., diagnosis of hepatitis B infection)
Passive treatment with antibody to treat or prevent infection (e.g., administration of antibody against hepatitis A)
Cancers Prevent tumor growth or spread (e.g., immune surveillance to prevent early cancers)
Prevent cancer growth or spread (e.g., vaccination with cancer antigens)
Measure circulating antigen (e.g., circulating PSA for diagnosis of prostate cancer)
Immunotherapy (e.g., treatment of cancer with antibodies against cancer antigens)
Environmental substances
Prevent entrance into body (e.g., secretory IgA limits systemic exposure to potential allergens)
No clear example Measure circulating antigen or antibody (e.g., diagnosis of allergy by measuring circulating IgE)
Immunotherapy (e.g., administration of antigen for desensitization of individuals with severe allergies)
Self-antigens Immune system tolerance to self- antigens, which may be altered by an infectious agent leading to autoimmune disease (see Chapter 8)
Some cases of vaccination alter tolerance to self- antigens, leading to autoimmune disease
Measure circulating antibody against self-antigen for diagnosis of autoimmune disease (see Chapter 8)
Oral administration of self- antigens to diminish production of autoimmune disease–associated autoantibodies
PSA, Prostate-specific antigen.
Antigens and Immunogens We need to initially understand the molecules against which an immune response is directed. Although the terms antigen and immunogen are commonly used as synonyms, there are clinically important differences between the two. Antigen is commonly used to describe a molecule that can bind with antibodies or antigen receptors on B and T cells. A molecule that will induce an immune response is an immunogen. Thus all immunogens are antigens but not all antigens are immunogens. For instance, immunogenicity is frequently related to the size of the antigen. In general, large molecules (those greater than 10,000 daltons), such as proteins and polysaccharides, are most immunogenic. Many low-molecular-weight molecules can function as haptens; they are too small to be immunogens by themselves but become immunogenic after combining with larger molecules that function as carriers for the hapten. Poison ivy contains an oily sap called urushiol (molecular weight approximately 1500 daltons), which upon contact with the skin is chemically altered, binds to large proteins in the skin, and becomes immunogenic, resulting in a T-cell response and onset of a classic poison ivy rash. Similar conditions will be discussed in Chapter 8.
Quick Check 7-1
1. Define acquired immunity.
2. Distinguish between innate and acquired immunity.
3. Distinguish between humoral and cell-mediated immunity.
4. What are the differences among antigens, immunogens, and haptens?
Antibodies A basic understanding of antibodies and how they react with antigen provides a foundation for more complex topics, such as the B-cell and T-cell antigen receptors, the generation of clonal diversity, and intercellular collaborations during clonal selection, which are discussed later in this chapter. The terms antibody and immunoglobulin (Ig) are frequently used interchangeably. In general, immunoglobulin is frequently used as a generic description of a general group of antibodies, whereas antibody commonly denotes one particular set of immunoglobulins known to have specificity for a particular antigen.
Classes of Immunoglobulins There are five classes of immunoglobulins (IgG, IgA, IgM, IgE, and IgD), which are characterized by differences in structure and function (Figure 7-4). Both IgG and IgA have subclasses (Table 7-2).
FIGURE 7-4 Structures of Different Immunoglobulins. Secretory IgA, IgD, IgE, IgG, and IgM. The black circles attached to each molecule represent carbohydrate residues.
TABLE 7-2 Properties of Immunoglobulins
Class Subclass Adult Serum Levels (mg/dl)
Present in Secretions
Complement Activation
Opsonin Agglutinin Mast Cell Activation
Placental Transfer
IgG IgG1 800-900 + ++ ++ + − +++ IgG2 280-300 + + − + − + IgG3 90-100 + +++ ++ + − +++ IgG4 50 − − − + + ++
IgM 120-150 + ++++ − ++++ − − IgA IgA1 280-300 + − − + − −
IgA2 50 + − − + − − sIgA 5 ++++ − − + − −
IgD 3 − − − − − − IgE 0.03 + − − − +++ −
sIgA, Secretory immunoglobulin A; − indicates lack of activity; + to ++++ indicate relative activity or concentration.
IgG is the most abundant class of immunoglobulins, constituting 80% to 85% of the immunoglobulins in the blood and accounting for most of the protective activity against infections. During pregnancy maternal IgG is transported across the placenta and protects the newborn child during the first 6 months of life. IgA is found in the blood and in bodily secretions as secretory IgA (subclass
IgA2). Secretory IgA is a dimer consisting of two IgA2 molecules held together through a J chain and secretory piece. The secretory piece is attached to dimeric IgA during transportation through mucosal epithelial cells to protect against degradation by enzymes also found in secretions. IgM is the largest immunoglobulin and usually exists as a pentamer (a molecule
consisting of five identical smaller molecules) that is stabilized by a J chain. It is the first antibody produced during the initial, or primary, response to antigens. IgM is usually synthesized early in neonatal life, but may be increased as a response to infection in utero. IgD is found in low concentrations in the blood. Its primary function is as an
antigen receptor on the surface of early B cells. IgE is normally at low concentrations in the circulation. It has very specialized
functions as a mediator of many common allergic responses (see Chapter 8) and in the defense against parasitic infections.1
Molecular Structure There are three parts to an antibody molecule (Figure 7-5). Two identical fragments have the ability to bind antigen and are termed antigen-binding fragments (Fab). The third fragment is termed the crystalline fragment (Fc). The Fab portions contain the recognition sites (receptors) for antigens and confer the molecule’s specificity toward a particular antigen. The Fc portion is responsible for most of the
biologic functions of antibodies.
FIGURE 7-5 Antigen-Antibody Binding. CH, Constant region heavy chain; VL, Variable region light chain; VH, Variable region heavy chain; CL, Constant region light chain; Fab, Fragment antigen
binding; Fc, Crystalline fragment; CDR's, Complementary determining regions; FR's, Framework regions; Red lines are disulfide linkages.
An immunoglobulin molecule consists of four polypeptide chains: two identical light (L) chains and two identical heavy (H) chains. The class of antibody is determined by different amino acid sequences in the heavy chains. The light and heavy chains are held together by noncovalent bonds and covalent disulfide linkages. A set of disulfide bridges between the heavy chains occurs in the hinge region and, in some instances, lends a degree of flexibility at that site. Each L and H chain is further subdivided structurally into constant (C) and
variable (V) regions. The constant regions have relatively stable amino acid sequences within a particular immunoglobulin class.Conversely, among different antibodies, the sequences of the variable regions have a large number of amino acid differences and these are called complementary determining regions (CDR). They determine the specificity of an antibody for a particular antigen. The regions
between the CDR’s are called framework regions (FR) and they have more stable amino acid sequences (see Figure 7-5).
Antigen-Antibody Binding Because antigens are relative small, a large molecule (e.g., protein, polysaccharide, nucleic acid) usually contains multiple and diverse antigens. The precise area of the antigen that is recognized by an antibody is called its antigenic determinant, or epitope. The matching portion on the antibody is sometimes referred to as the antigen-binding site, or paratope. The antigen fits into the antigen binding site of the antibody with the specificity of a key into a lock and is held there by noncovalent chemical interactions.
Function of Antibodies The chief function of antibodies is to protect against infection. The mechanism can be either direct—through the action of antibody alone or indirect—requiring activation of other components of the innate immune response (Figure 7-6). Directly, antibodies can affect infectious agents or their toxic products by neutralization (inactivating or blocking the binding of antigens to receptors), agglutination (clumping insoluble particles that are in suspension), or precipitation (making a soluble antigen into an insoluble precipitate). For instance, many pathogens initiate infection by attaching to specific receptors on cells. Viruses that cause the common cold or the influenza virus must attach to specific receptors on respiratory tract epithelial cells. Some bacteria, such as Neisseria gonorrhoeae that causes gonorrhea, must attach to specific sites on urogenital epithelial cells. Antibodies may protect the host by covering sites on the microorganism that are needed for attachment, thereby preventing infection. Many viral infections can be prevented by vaccination with inactivated or attenuated (weakened) viruses designed to induce neutralizing antibody production at the site of the entrance of the virus into the body. Vaccination against influenza using an inhaled vaccine particularly induces protective IgA in the respiratory tract.
FIGURE 7-6 Direct and Indirect Functions of Antibody. Protective activities of antibodies can be direct (through the action of antibody alone) or indirect (requiring activation of other
components of the innate immune response, usually through the Fc region). Direct means include neutralization of viruses or bacterial toxins before they bind to receptors on the surface
of the host's cells. Indirect means include activation of the classical complement pathway through C1, resulting in formation of the membrane-attack complex (MAC), or increased phagocytosis of bacteria opsonized with antibody and complement components bound to
appropriate surface receptors (FcR and C3bR).
Some bacteria secrete toxins that harm individuals. For instance, specific bacterial toxins cause the symptoms of tetanus or diphtheria. Most toxins are proteins that bind to surface molecules on cells and damage those cells. Protective antibodies produced against the toxin (referred to as antitoxins) can bind to the toxins, prevent their interaction with host cells, and neutralize their biologic effects (see Chapter 8). Indirectly, through the Fc portion, antibodies activate components of innate
resistance, including complement and phagocytes (Figure 7-7). Through the classical pathway, complement component C1 will be activated by binding simultaneously to the Fc regions of two adjacent antibodies bound to a microbe, resulting in activation of the entire cascade. Phagocytic cells express receptors that bind the Fc portion of antibody; thus antibody is an opsonin that facilitates phagocytosis of bacteria.3 IgM is the best complement-activating antibody, and IgG is the best opsonin. Some antibodies are more protective than others. It is now a
common procedure to clone the “best” antibodies (monoclonal antibodies) for use in diagnostic tests and for therapy (Box 7-1).
Box 7-1 Monoclonal Antibodies Most humoral immune responses are polyclonal—that is, a mixture of antibodies produced from multiple B lymphocytes. Most antigenic molecules have multiple antigenic determinants, each of which induces a different group of antibodies. Thus, a polyclonal response is a mixture of antibody classes, specificities, and function, some of which are more protective than others. Monoclonal antibody is produced in the laboratory from one B cell that has been
cloned; thus the entire antibody is of the same class, specificity, and function. The advantages of monoclonal antibodies are that (1) a single antibody of known antigenic specificity is generated rather than a mixture of different antibodies; (2) monoclonal antibodies have a single, constant binding affinity; (3) monoclonal antibodies can be diluted to a constant titer (concentration in fluid) because the actual antibody concentration is known; and (4) the antibody can be easily purified. Thus, a highly concentrated antibody with optimal function has been used to develop extremely specific and sensitive laboratory tests (e.g., home and laboratory pregnancy tests) and therapies (e.g., for certain infectious diseases or several experimental therapies for cancer).
IgE IgE is a special class of antibody that protects the individual from infection with large parasitic worms (helminths).4 However, when IgE is produced against relatively innocuous environmental antigens, it is also the primary cause of common allergies (e.g., hay fever, dust allergies, bee stings). The role of IgE in allergies is discussed in Chapter 8. Large multicellular parasites usually invade mucosal tissues. Many antigens from
the parasites induce IgE, as well as other antibody classes. IgG, IgM, and IgA bind to the surface of parasites, activate complement, generate chemotactic factors for neutrophils and macrophages, and serve as opsonins for those phagocytic cells. This response, however, does not greatly damage parasites. The only inflammatory cell that can adequately damage a parasite is the eosinophil because of the special contents of its granules, including major basic protein, eosinophil cationic protein, eosinophil peroxidase, and eosinophil neurotoxin, each of which can damage infectious worms. Thus, IgE is designed to specifically initiate an inflammatory
reaction that preferentially attracts eosinophils to the site of parasitic infection. Mast cells in the tissues have Fc receptors that specifically and with high affinity
bind IgE. IgE antibodies against antigens of the parasite are rapidly bound to the mast cell surface. Soluble parasite molecules with multiple antigenic determinants diffuse to neighboring mast cells and simultaneously bind to multiple IgE molecules. This reaction initiates a cascade of effects that can ultimately kill the parasite. The steps of the cascade are presented in Figure 7-7.
FIGURE 7-7 IgE Function. (1) Soluble antigens from a parasitic infection cause production of IgE antibody by B cells. (2) Secreted IgE binds to IgE-specific receptors on the mast cell. (3)
Additional soluble parasite antigen cross-links the IgE on the mast cell surface, (4) leading to mast cell degranulation and release of many proinflammatory products, including eosinophil chemotactic factor of anaphylaxis (ECF-A). (5) ECF-A attracts eosinophils from the circulation.
(6) The eosinophil attaches to the surface of the parasite and releases potent lysosomal enzymes that damage microorganisms.
Secretory Immune System Immunocompetent lymphocytes migrate among secondary lymphoid organs and tissue as part of the systemic immune system. Another, partially independent, immune system protects the external surfaces of the body through lacrimal and salivary glands and a network of lymphoid tissues residing in the breasts, bronchi, intestines, and genitourinary tract. This system is called the secretory (mucosal)
immune system (Figure 7-8). Plasma cells in those sites secrete antibodies in bodily secretions such as tears, sweat, saliva, mucus, and breast milk to prevent pathogenic microorganism from infecting the body's surfaces and possibly penetrating to cause systemic disease.5 Alternatively, the microorganisms may reside in the membranes without causing disease, be shed, and cause infection for other individuals. Thus, an individual may become a carrier for a particular infectious organism. For instance, in the 1950s two vaccines were developed to prevent infection with poliovirus, which enters through the gastrointestinal tract. The Sabin vaccine was administered orally as an attenuated (i.e., inactivated so as to render relatively harmless) live virus. This route caused a transient, limited infection and induced effective systemic and secretory immunity that prevented both the disease and the establishment of a carrier state. The Salk vaccine, on the other hand, consisted of killed viruses administered by injection in the skin. It induced adequate systemic protection but did not generally prevent an intestinal carrier state. Thus, recipients of the Salk vaccine were protected from disease but could still shed the virus and infect others.
FIGURE 7-8 Secretory Immune System. A, Lymphocytes from the mucosal-associated lymphoid tissues circulate throughout the body in a pattern separate from other lymphocytes. For example, lymphocytes from the gut-associated lymphoid tissue circulate through the
regional lymph nodes, the thoracic duct, and the blood and return to other mucosal-associated lymphoid tissues rather than to lymphoid tissue of the systemic immune system. B, Lymphoid tissue associated with mucous membranes is called mucosal-associated lymphoid tissue
(MALT).
IgA is the dominant secretory immunoglobulin, although IgM and IgG also are present in secretions. The primary role of IgA is to prevent the attachment and invasion of pathogens through mucosal membranes, such as those of the gastrointestinal, pulmonary, and genitourinary tracts. Dimeric IgA antibodies containing the J chain are produced by plasma cells of the mucosa. Mucosal epithelium expresses a cell surface immunoglobulin receptor that binds and internalizes IgA. The IgA, along with the epithelial receptor (secretory piece), is secreted as secretory IgA (sIgA). The lymphoid tissues of the secretory immune system are connected; thus many
foreign antigens in a mother's gastrointestinal tract (e.g., polio virus) induce secretion of specific antibodies into the breast milk. Colostral antibodies (i.e., those found in the colostrum of breast milk) may protect the nursing newborn against infectious disease agents that enter through the gastrointestinal tract. Although colostral antibodies provide the newborn with passive immunity against gastrointestinal infections, they do not provide systemic immunity because transport across the newborn's gut into the bloodstream is discontinued after the first 24 hours of life. Maternal antibodies that pass across the placenta into the fetus before birth provide passive systemic immunity.
Immune Response: Collaboration of B Cells and T Cells Generation of Clonal Diversity The immune response occurs in two phases: generation of clonal diversity and clonal selection (Table 7-3 and see Figure 7-2). Clonal diversity is the production of a large population of B cells and T cells before birth that have the capacity to recognize almost any foreign antigen found in the environment. This process mostly occurs in specialized lymphoid organs (the primary [central] lymphoid organs): the bone marrow for B cells and the thymus for T cells.6 The result is the differentiation of lymphoid stem cells into B and T lymphocytes with the ability to react against almost any antigen that will be encountered throughout life. It is estimated that B and T cells can collectively recognize more than 108 different antigenic determinants. Lymphocytes are released from these organs into the circulation as immunocompetent cells that have the capacity to react with antigens and migrate to the circulation and other (secondary) lymphoid organs in the body.
TABLE 7-3 Generation of Clonal Diversity vs. Clonal Selection
Generation of Clonal Diversity Clonal Selection Purpose? To produce large numbers of T and B lymphocytes with
maximum diversity of antigen receptors Select, expand, and differentiate clones of T and B cells against specific antigen
When does it occur?
Primarily in fetus Primarily after birth and throughout life
Where does it occur?
Central lymphoid organs: thymus for T cells, bone marrow for B cells
Peripheral lymphoid organs, including lymph nodes, spleen, and other lymphoid tissues
Is foreign antigen involved?
No Yes, antigen determines which clones of cells will be selected
What hormones or cytokines are involved?
Thymic hormones, IL-7, others Many cytokines produced by Th cells and APCs
Final product? Immunocompetent T and B cells that can react with antigen, but have not seen antigen, and migrate to secondary lymphoid organs
Plasma cells that produce antibody, effector T cells that help (Th cells), kill targets (Tc cells), or regulate immune responses (Treg cells); memory B and T cells
APCs, Antigen-presenting cells; Tc, T-cytotoxic cells; Th, T-helper cells; Treg cells, T-regulatory cells.
Development of B Lymphocytes Lymphocytes destined to become B cells circulate through the specialized regions of the bone marrow, where they are exposed to hormones and cytokines that induce proliferation and differentiation into B cells (see Figure 7-2). Lymphoid stem cells in the bone marrow interact with stromal cells through a variety of intercellular adhesion molecules. As the stem cell begins to mature, it progressively develops a
variety of necessary surface markers important for the further differentiation and proliferation of the B cell.7 The next stage in development is formation of the B-cell receptor (BCR). The B-cell receptor (BCR) is a complex of antibody bound to the cell surface
and other molecules involved in intracellular signaling (Figure 7-9). Its role is to recognize an antigen and communicate that information to the cell's nucleus. The BCRs in immunocompetent cells are membrane-associated IgM (mIgM) and IgD (mIgD) immunoglobulins that have identical specificities for antigen. The mIgM is a monomer rather than the pentamer primarily found in the blood.
FIGURE 7-9 B-cell Antigen Receptor and T-cell Antigen Receptor. A, The antigen receptor on the surface of B cells (BCR complex) is a monomeric (single) antibody with a structure similar to that of circulating antibody, with an additional transmembrane region (TM) that anchors the
molecule to the cell surface. The active BCR complex contains molecules (Igα and Igβ) that are responsible for intracellular signaling after the receptor has bound antigen. B, The T-cell
receptor (TCR) consists of an α- and a β-chain joined by a disulfide bond. Each chain consists of a constant region (Cα and Cβ) and a variable region (Vα and Vβ). Each variable region contains
CDRs and FRs in a structure similar to that of antibody. The active TCR is associated with several molecules that are responsible for intracellular signaling after antigen binding. These include the CD3, which is a complex of γ (gamma), ξ (epsilon), and δ (delta) subunits and a complex of two ζ (zeta) molecules. The ζ molecules are attached to a cytoplasmic protein
kinase (ZAP70) that is critical to intracellular signaling.
As described previously, the variable regions of antibodies, as well as the BCR, contain CDR areas. The diversity of these CDRs is responsible for the variety of antigens that can be recognized by immunocompetent B cells.8 The enormous repertoire of specificities is made possible by rearrangement of existing DNA during B-cell development in the primary lymphoid organs, a process known as somatic recombination. Multiple loci in the DNA that encode for the variable regions of immunoglobulins are recombined to generate receptors that collectively
can recognize and bind to any possible antigen.8 To create the variable region of a light chain, different regions are rearranged using enzymes encoded by recombination activating genes (RAG-1, RAG-2). The DNA is cut and spliced (repaired) so that after this manipulation, the progeny of a single lymphocyte will synthesize immunoglobulins with identical variable regions. Those variable regions, however, are cut and spliced differently from those of another lymphocyte, making each cell unique and therefore able to react with different antigens. The gene for the H chain undergoes similar rearrangement. Somatic rearrangement of the variable regions will frequently result in a BCR
that recognizes the individual's own antigens, which may result in inadvertent attack on “self” antigens expressed on various tissue and organs causing autoimmune disease or hypersensitivites. Many of these “autoreactive” B cells are eliminated in the bone marrow. It is estimated that more than 90% of developing B cells are induced to undergo apoptosis. This process is referred to as central tolerance, so that resultant immunocompetent B cells are against foreign antigens and “tolerant” to self-antigens. The process of peripheral tolerance is discussed on p. 173. B-cell differentiation also is characterized by the development of a variety of
important surface molecules that are markers for B cells. These include CD21 (a complement receptor) and CD40 (adhesion molecule required for later interactions with T cells).
Development of T Lymphocytes The process of T-cell proliferation and differentiation is similar to that for B cells (see Figure 7-2). The primary lymphoid organ for T-cell development is the thymus.9 Lymphoid stem cells journey through the thymus, where, under influence of thymic hormones and the cytokine IL-7, they are driven to undergo cell division and simultaneously produce receptors (T-cell receptors [TCRs]) against the diversity of antigens the individual will encounter throughout life. They exit the thymus through the blood vessels and lymphatics as mature (immunocompetent) T cells with antigen-specific receptors on the cell surface and establish residence in secondary lymphoid organs. Production of the TCR proceeds in a manner very similar to that described
earlier for B cells. The most common TCR resembles an antibody Fab region and consists of two protein chains, α- and β-chains, each of which has a variable region and a constant region (see Figure 7-9). The variable regions also undergo somatic recombination. As with the BCR, a set of intracellular signaling molecules co- assemble in the membrane with the TCR. The complex of these signaling molecules is called CD3.10 Thus, all immunocompetent T cells can be identified by the
presence of CD3 on the surface. Differentiation of T cells in the thymus also results in expression in a variety of
other important surface molecules. Initially, proteins called CD4 and CD8 are concurrently expressed on the developing cells. CD4 cells develop into T-helper cells (Th cells), whereas CD8 cells become T-cytotoxic cells (Tc cells). Approximately 60% of immunocompetent T cells in the circulation express CD4 and 40% express CD8. Central tolerance also occurs in the thymus where more than 95% of developing
T cells are deleted. Like B-cells, T-cells can also become autoreactive.
Quick Check 7-2
1. What are the major functions of antibodies?
2. What is the difference between the secretory and systemic immune systems?
3. What are the different types of T cells, and what function does each have?
Clonal Selection Antigens initiate the second phase of the immune response, clonal selection. Clonal selection is the processing of antigen for a specific immune response. This process involves a complex interaction among cells in the secondary lymphoid organs (see Figure 7-2). To initiate an effective immune response, most antigens must be processed because they cannot react directly with most cells of the immune system and must be shown or presented to the immune cells in a specific manner. This is the job of antigen-processing (antigen-presenting) cells (usually dendritic cells, macrophages, or similar cells), generally referred to as APCs. The interaction among APCs, subpopulations of T cells that facilitate immune responses (T-helper [Th] cells), and immunocompetent B or T cells results in differentiation of B cells into active antibody-producing cells (plasma cells) and T cells into effector cells, such as T-cytotoxic cells. Both lines also develop into memory cells that respond even faster when that antigen enters the body again. Thus, activation of the immune system produces a long-lasting protection against specific antigens (see Figure 7-2). Defects in any aspect of cellular collaboration will lead to defects in cell-mediated immunity, humoral immunity, or both and, depending on the particular defect, potentially the individual's death from infection (see Chapter 8).
Primary and Secondary Immune Responses
The immune response to antigen has classically been divided into two phases—the primary and secondary responses—that are most easily demonstrated by measuring concentrations of circulating antibodies over time (Figure 7-10). After a single initial exposure to most antigens, there is a latent period, or lag phase, during which clonal selection occurs. After approximately 5 to 7 days, IgM antibody is detected in the circulation. This is the primary immune response, characterized typically by initial production of IgM followed by production of IgG against the same antigen. The quantity of IgG may be about equal to or less than the amount of IgM. The amount of antibody in a serum sample is frequently referred to as the titer; a higher titer indicates more antibodies. If no further exposure to the antigen occurs, the circulating antibody is catabolized (broken down) and measurable quantities fall. The individual's immune system, however, has been primed.
FIGURE 7-10 Primary and Secondary Immune Responses. The initial administration of antigen induces a primary response during which IgM is initially produced, followed by IgG. Another administration of the antigen induces the secondary response in which IgM is transiently
produced and larger amounts of IgG are produced over a longer period of time.
A second challenge by the same antigen results in the secondary immune response, which is characterized by the more rapid production of a larger amount of antibody than the primary response. The rapidity of the secondary immune response is the result of memory cells that require less further differentiation. IgM may be transiently produced in the secondary response, but IgG production is
increased considerably, making it the predominant antibody class. Natural infection (e.g., rubella) may result in measurable levels of protective IgG for the life of the individual. Some vaccines (e.g., polio) also may produce extremely long-lived protection, although most vaccines require boosters at specified intervals.
Antigen Processing and Presentation For most antigens, the first step in clonal selection is processing and presentation by APCs. Antigens are usually expressed on large molecules found on microbes, which undergo phagocytosis and destruction by dendritic cells and macrophages. These are referred to as exogenous antigens. Other antigens, endogenous antigens, originate within a cell that has been infected by a virus or has become cancerous. Processing results in the release of small antigenic determinants, which are
presented on the surface of APCs by specialized molecules, molecules of the major histocompatibility complex (MHC). MHC molecules in humans also are called human leukocyte antigens (HLA) (discussed in more detail in Chapter 8) and are related to their role in transplantation. Major histocompatibility complex (MHC) molecules are glycoproteins found on the surface of all human cells except red blood cells. They are divided into two general classes, class I and class II, based on their molecular structure, distribution among cell populations, and function in antigen presentation. MHC class I molecules are composed of a large alpha (α) chain along with a smaller chain called β2-microglobulin. MHC class II molecules are composed of α- and β-chains that differ from the ones used for MHC class I. The α- and β-chains of the MHC molecules are encoded from different genetic loci located as a large complex of genes on human chromosome 6 (Figure 7-11). MHC genes are probably the most polymorphic of any human genes; therefore, no two individuals, except identical twins, will have a complete set of identical MHC molecules.
FIGURE 7-11 Antigen-Presenting Molecules.
MHC class I molecules present endogenous antigens, which are primarily recognized by T-cytotoxic (Tc) cells. Because MHC class I molecules are expressed on all cells, except red blood cells, any change in that cell caused by viral infection or malignancy may result in foreign antigens being presented. MHC class II molecules present exogenous antigens (Figure 7-12). Antigen presented by MHC class II molecules is preferentially recognized by T-helper (Th) cells. Thus, antigen presentation to Tc cells is MHC class I restricted and presentation to Th cells is MHC class II restricted. MHC class II molecules are co-expressed with MHC class I molecules on a limited number of cells that have APC function, including macrophages, dendritic cells, and B lymphocytes.
FIGURE 7-12 Antigen Processing. Antigen processing and presentation are required for initiation of most immune responses. Foreign antigen may be either endogenous (cytoplasmic protein) or exogenous (e.g., bacterium). Endogenous antigenic peptides are transported into the
endoplasmic reticulum (ER) (1), where the MHC molecules are being assembled. In the ER, antigenic peptides bind to the α-chains of the MHC class I molecule (2), and the complex is
transported to the cell surface (3). The α- and β-chains of the MHC class II molecules are also being assembled in the endoplasmic reticulum (4), but the antigen-binding site is blocked by a small molecule (invariant chain) to prevent interactions with endogenous antigenic peptides.
The MHC class II–invariant chain complex is transported to phagolysosomes (5), where exogenous antigenic fragments have been produced as a result of phagocytosis (6). In the
phagolysosomes, the invariant chain is digested and replaced by exogenous antigenic peptides (7), after which the MHC class II–antigen complex is inserted into the cell membrane (8).
Thus, the term antigen processing relates to the process by which large exogenous and endogenous antigens are cut up by enzymes into small antigenic fragments that are linked with the appropriate MHC molecules and inserted into the membrane of the APC.11 Lipid antigens are frequently presented by a molecule unrelated to the MHC, CD1, which is not discussed here.
Cellular Interactions in the Immune Response The second step in clonal selection is a finally tuned set of intercellular collaborations that result in the production of effector cells (plasma cells, Th cells, Tc cells) and memory cells.12 Each collaboration requires three complementary intracellular signaling events: antigen-specific recognition through the TCR complex, activation of intercellular adhesion molecules, and the response to specific groups of cytokines. Without each signaling event, a protective immune response will not be produced.
T-helper lymphocytes. Regardless of whether an antigen primarily induces a cellular or humoral immune response, APCs usually must present antigens to T-helper cells (Th cells). The APC presents antigen held by the polymorphic regions (α1 and β1) of the α- and β-chains of MHC class II molecules.13 The antigen also binds to the TCR on the Th cell (see Figure 7-9). The strength of the intercellular antigen binding is increased by CD4 on the Th cell, which binds to a nonpolymorphic region of the β2 region of the MHC class II molecule. The cytoplasmic portions of CD3 and CD4 interact to activate intracellular signaling pathways. A second co-stimulatory signal results from the interaction of a variety of adhesion molecules; the most critical being B7 on the APC and CD28 on the Th cell. The third signal occurs through Th-cell cytokine receptors. In the early stages of
Th-cell differentiation, IL-1 secreted by the APC provides this signal through the IL- 1 receptor on the Th cell (Figure 7-13). The initial differentiation response by the Th cell includes the production of the cytokine IL-2 and up-regulation of IL-2 receptors. IL-2 is secreted and acts in an autocrine (self-stimulating) fashion to induce further maturation and proliferation of the Th cell. Without IL-2 production, the Th cell cannot efficiently mature into a functional helper cell.
FIGURE 7-13 Development of T-Cell Subsets. The most important step in clonal selection is the production of populations of T-helper (Th) cells (Th1, Th2, and Th17) and T-regulatory (Treg) cells
that are necessary for the development of cellular and humoral immune responses. In this model, APCs (1) (probably multiple populations) may influence whether a precursor Th cell (Thp cell) (2) will differentiate into a Th1, Th2, Th17, or Treg cell (3). Differentiation of the Thp cell is
initiated by three signaling events. The antigen signal is produced by the interaction of the T-cell receptor (TCR) and CD4 with antigen presented by MHC class II molecules. A set of co-
stimulatory signals is produced from interactions between adhesion molecules (not shown). A third signal is produced by the interactions of cytokines (particularly interleukin-1 [IL-1]) with
appropriate cytokine receptors (IL-1R) on the Thp cell. The Thp cell up-regulates IL-2 production and expression of the IL-2 receptor (IL-2R), which acts in an autocrine fashion to accelerate Thp cell differentiation and proliferation. Commitment to a particular phenotype results from the relative concentrations of other cytokines. IL-12 and IFN-γ produced by some populations of APCs favor differentiation into the Th1 cell phenotype; IL-4, which is produced by a variety of cells, favors differentiation into the Th2 cell phenotype; IL-6 and TGF-β (T-cell growth factor)
facilitate differentiation into Th17 cells; IL-2 and TGF-β induce differentiation into Treg cells. The Th1 cell is characterized by the production of cytokines that assist in the differentiation of T- cytotoxic (Tc) cells, leading to cellular immunity, whereas the Th2 cell produces cytokines that favor B-cell differentiation and humoral immunity. Th1 and Th2 cells affect each other through the production of inhibitory cytokines: IFN-γ will inhibit development of Th2 cells, and IL-4 will inhibit the development of Th1 cells. Th17 cells produce cytokines that affect phagocytes and increase inflammation. Treg cells produce immunosuppressive cytokines that prevent the
immune response from being excessive. APC, Antigen-presenting cell; IFN, interferon; MHC, major histocompatibility complex; TGF, transforming growth factor.
At this point and depending on the predominant cytokines in the immediate environment, Th cells undergo differentiation into one of several subsets: Th1, Th2, Th17, or Treg cells.14 These subsets have different functions: Th1 cells preferentially provide help in developing Tc cells (cell-mediated immunity), Th2 cells provide more help for developing B cells (humoral immunity), Th17 cells are lymphokine-secreting cells that activate macrophages, and Treg cells limit the
immune response (these will discussed later in this chapter).15 The Th subsets differ considerably in the spectrum of cytokines they produce. Additionally, Th1 and Th2 cells may suppress each other so that the immune response may favor either antibody formation, with suppression of a cell-mediated response, or the opposite. For example, antigens derived from viral or bacterial pathogens and those derived from cancer cells seem to induce a greater number of Th1 cells relative to Th2 cells, whereas antigens derived from multicellular parasites and allergens may result in production of more Th2 cells. Many antigens (e.g., tetanus vaccine), however, will produce excellent humoral and cell-mediated responses simultaneously. Th cells are necessary for development of most humoral and cellular immune responses; therefore the virus that causes acquired immune deficiency syndrome (AIDS) results in life-threatening infections because it specifically infects and destroys Th cells (see Chapter 8).
Superantigens. Several pathogenic microorganisms, particularly viruses and bacteria, manipulate the normal interaction between APCs and Th cells to the detriment of the individual and the benefit of the microbe. A group of microbial molecules are called superantigens (SAGs). SAGs bind to the portion of the TCR outside of its normal antigen-specific binding site, as well as to MHC class II molecules outside of their antigen-presentation sites (Figure 7-14). Some SAGs also react with CD28 on the Th cells and provide a co-stimulatory signal. Thus, SAGs are not processed by an APC to be presented to an immune cell. This binding, which is independent of antigen recognition, provides a signal for Th-cell activation, proliferation, and cytokine production. The normal antigen-specific recognition between Th cells and APCs results in activation of relatively few cells—only those cells with specific TCRs against that antigen. SAGs activate a large population of Th cells, regardless of antigen specificity, and induce excessive production of cytokines, including IL-2, interferon gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α). The overproduction of inflammatory cytokines results in symptoms of a systemic inflammatory reaction, including fever, low blood pressure, and, potentially, fatal shock. Some examples of SAGs are the bacterial toxins produced by Staphylococcus aureus and Streptococcus pyogenes (SAGs that cause toxic shock syndrome and food poisoning).16
FIGURE 7-14 Superantigens. The T-cell receptor (TCR) and major histocompatibility complex (MHC) class II molecule are normally held together by processed antigen. Superantigens, such as some bacterial exotoxins, bind directly to the variable region of the TCR β-chain and the MHC class II molecule. Each superantigen activates sets of Vβ chains independently of the antigen
specificity of the TCR.
T-cytotoxic lymphocytes. The differentiation of immunocompetent T cells into effector T-cytotoxic cells (Tc cells) requires similar intercellular communications as described for Th cells, with some very important differences. Rather than interacting with an APC, the immunocompetent Tc cell recognizes antigen presented by MHC class I molecules on the surface of a virus-infected cell or cancerous cell (Figure 7-15). The Tc cell expresses CD8, rather than CD4. CD8 binds to the MHC class I molecule and, as with Th cell differentiation, the proximity of the CD3 and CD8 cytoplasmic portions activates intercellular signaling pathways. Cytokine signals, especially IL-2, are produced by Th1 cells and activate cytokine receptors on the Tc cells.
FIGURE 7-15 Tc-Cell Clonal Selection. The immunocompetent Tc cell can react with antigen but cannot yet kill target cells. During clonal selection, this cell reacts with antigen presented by MHC class I molecules on the surface of a virally infected or cancerous abnormal cell. (1) The antigen–MHC class I complex is recognized simultaneously by the T-cell receptor (TCR), which binds to antigen, and CD8, which binds to the MHC class I molecule. (2) A separate signal is provided by cytokines, particularly IL-2 from Th1 cells. (3) In response to these signals, the Tc
cell develops into an effector Tc cell with the ability to kill abnormal cells.
B-cell clonal selection. A further sequence of cellular interactions is required to produce an effective antibody response. The immunocompetent B cell is also an APC and expresses surface mIgM and mIgD B-cell receptors (BCRs) (Figure 7-16). Unlike the T-cell receptor that can only see processed and presented antigens, the BCR can react with soluble antigens that have not been processed. B cells also express surface CD21, which is a receptor for opsonins produced by complement activation. Antigen binding through the BCR and CD21 activates the B cell, resulting in internalization, processing, and presentation of antigen fragments by MHC class II molecules.17 The antigen presented on the B-cell surface is recognized by a Th2 cell through the TCR and CD4. The intercellular bridges created through antigen and other intercellular adhesion molecules induce the Th2 cell to secrete cytokines (particularly IL-4) that initiate B-cell proliferation and maturation into plasma cells.18
FIGURE 7-16 B-Cell Clonal Selection. Immunocompetent B cells undergo proliferation and differentiation into antibody-secreting plasma cells. Multiple signals are necessary (1). The B cell itself can directly bind soluble antigen through the B-cell receptor (BCR) and act as an
antigen-processing cell. Antigen is internalized, processed (2), and presented (3) to the TCR on a Th2 cell by MHC class II molecules (4). A cytokine signal is provided by the Th2 cell cytokines (e.g., IL-4) that react with the B cell (5). The B cell differentiates into plasma cells that secrete
antibody (6).
A major component of B-cell maturation is class switch, the process that results in the change in antibody production from one class to another (e.g., IgM to IgG during the primary immune response). Before exposure to antigens and Th2 cells, the B cell produces IgM and IgD, which are used as cell membrane receptors. During the clonal selection process, a B cell proliferates and develops into antibody-secreting plasma cells, and each B cell has the option of becoming a secretor of IgM or changing the class of antibody to a secreted form of IgG, IgA, or IgE. Class switch occurs by another round of somatic recombination with the variable region of the antibody heavy chain being combined with a different constant region of the heavy chain. Because the variable region is conserved and the light chain remains unchanged, the antigenic specificity of the antibody also remains unchanged. The particular constant region chosen by each cell during class switch appears to be, at least partially, under the control of specific Th2 cytokines. For instance, IL-4 and IL-13 appear to preferentially stimulate switch to IgE secretion, and transforming growth factor-beta (TGF-β) and IL-5 appear to play major roles in class switch to IgA secretion. Thus, during clonal selection, a B cell may produce a population of plasma cells that are capable of producing many different classes of antibodies against the same antigen. Although most antigens require B cells to interact with Th cells, a few antigens
can bypass the need for cellular interactions and can directly stimulate B-cell maturation and proliferation. These are called T-cell–independent antigens (Figure
7-17). They are mostly bacterial products that are large and are likely to have repeating identical antigenic determinants that bind and cross-link several BCRs. The accumulated intracellular signal is adequate to induce differentiation into a plasma cell but is not adequate to induce a change in the class of antibody that will be produced. Therefore, T-cell–independent antigens usually induce relatively pure IgM primary and secondary immune responses.
FIGURE 7-17 Activation of a B Cell by a T-Cell–Independent Antigen. Molecules containing repeating identical antigenic determinants may interact simultaneously with several receptors on the surface of the B cell and induce the proliferation and production of immunoglobulins. Because Th2 cells do not participate, class switch does not occur and the resultant antibody
response is IgM.
Memory cells. During the clonal selection process, both B cells and T cells differentiate and proliferate into an extremely large population of long-lived memory cells.19 Memory cells remain inactive until subsequent exposure to the same antigen. Upon reexposure, these memory cells do not require much further differentiation and will therefore rapidly become new plasma cells or effector T cells without the cellular interactions described previously.
Cell-Mediated Immunity The rather straightforward function of antibodies has been discussed earlier in this chapter. The function of effector T cells is more complex and utilizes the principles of intercellular recognition necessary for clonal selection.
T-Lymphocyte Function The clonal selection process produces several subsets of effector T cells. Th cells and T memory cells have already been discussed. Other effector T cells include T- cytotoxic (Tc) cells that attack and destroy cells expressing antigens from intracellular (endogenous) origins, T-regulatory cells (Treg) that limit (suppress) the immune response, and T-lymphokine producing cells that secrete cytokines that activate other cells.
T-Cytotoxic Lymphocytes T-cytotoxic (Tc) cells are responsible for the cell-mediated destruction of tumor cells or cells infected with viruses. In a fashion similar to intercellular recognition during the clonal selection process, the Tc cell must directly adhere to the target cell through antigen presented by MHC class I molecules and CD8 (Figure 7-18). Because of the broad cellular distribution of MHC class I molecules, Tc cells can recognize antigens on the surface of almost any type of cell that has been infected by a virus or has become cancerous. Unlike clonal selection, the roles of co- stimulatory signals through adhesion molecules and cytokines are of less importance here. Attachment to a target cell activates multiple killing mechanisms through which the Tc cell induces the target cell to undergo apoptosis.
FIGURE 7-18 Cellular Killing Mechanisms. Several cells have the capacity to kill abnormal (e.g., virally infected, cancerous) target cells. (1) T-cytotoxic (Tc) cells recognize endogenous antigen presented by MHC class I molecules. The Tc cell mobilizes multiple killing mechanisms that induce apoptosis of the target cell. (2) Natural killer (NK) cells identify and kill target cells
through receptors that recognize abnormal surface changes. NK cells specifically kill targets that do not express surface MHC class I molecules. (3) Several cells, including macrophages and NK cells, can kill by antibody-dependent cellular cytotoxicity (ADCC). IgG antibodies bind to foreign antigen on the target cell, and cells involved in ADCC bind IgG through Fc receptors (FcR) and initiate killing. The insert is a scanning electron microscopic view of Tc cells (L) attacking a
much larger tumor cell (Tu). (Insert from Abbas A, Lichtman A: Cellular and molecular immunology, ed 5, Philadelphia, 2003, Saunders.)
Various other cells kill targets in a fashion similar to Tc lymphocytes. Prominent among these cells are natural killer cells. Natural killer (NK) cells are a special group of lymphoid cells that are similar to T cells but lack antigen-specific receptors. Instead, they express a variety of cell surface activation receptors (similar to pattern recognition receptors, see Chapter 6) that identify protein changes on the surface of cells infected with viruses or that have become cancerous. After attachment, the NK cell kills its target in a manner similar to that of Tc cells. NK cells also have receptors for MHC class I. However, NK cells lack CD8; therefore binding to MHC class I molecules results in inactivation of the NK cell. Thus, NK cells complement the effects of Tc cells. In some instances, a virus-infected or
cancerous cell will “protect” itself by down-regulating MHC class I molecule expression. Without surface MHC class I molecules a cell becomes resistant to Tc- cell recognition and killing. NK cells primarily kill target cells that have suppressed the expression of MHC class I. NK cells, as well as some macrophages, can specifically kill targets through use
of antibodies. NK cells express Fc receptors for IgG. If antigens on the infected or cancerous cell bind IgG, the NK cell can attach through Fc receptors and activate its normal killing mechanisms. This is referred to as antibody-dependent cellular cytotoxicity (ADCC).
Lymphokine-Secreting T Cells Two subsets of Th cells amplify inflammation. Th1 cells, in addition to assisting Tc- cell clonal selection, secrete cytokines that activate M1 macrophages to increase phagocytic and microbial killing functions (described in Chapter 6). The most important cytokine for macrophage activation is interferon-γ (IFN-γ). Th2 cells, in addition to assisting B-cell clonal selection, secrete cytokines (e.g., IL-4, IL-13) that activate M2 macrophages for healing and repair of damaged tissue (described in Chapter 6). Th17 cells secrete a set of cytokines (e.g., IL-17, IL-22, chemokines) that recruit phagocytic cells to a site of inflammation.20 Th17-cell cytokines also may activate cells, particularly epithelial cells, to produce antimicrobial proteins in defense against certain bacterial and fungal pathogens.
T-Regulatory Lymphocytes T-regulatory (Treg) cells are a diverse group of T cells that control the immune response, usually suppressing the response and maintaining tolerance against self- antigens.21 This process occurs in the secondary lymphoid organs and other tissues, known as peripheral tolerance, in contrast to the process of central tolerance described earlier. This population of Treg cells that differentiate from the Th-cell population expresses CD4 and binds to antigens presented by MHC class II molecules. Unlike other Th cells, however, Treg cells express consistently high levels of CD25 (the IL-2 receptor). Differentiation from the Th precursor cell is controlled, primarily by TGF-β and IL-2. Treg cells produce very high levels of immunosuppressive cytokines TGF-β and IL-10, which generally decrease Th1 and Th2 activity by suppressing antigen recognition and Th-cell proliferation.
Quick Check 7-3
1. What are antigen-presenting cells?
2. Define BCR and TCR.
3. What is the role of T-helper cells?
4. Why are cytokines important to the immune response?
5. What is the difference between central tolerance and peripheral tolerance?
Age-related mechanisms of self-defense in the newborn child and in the elderly are listed in the Pediatric Considerations and Geriatric Considerations boxes.
Pediatric Considerations Age-Related Factors Affecting Mechanisms of Self-Defense in the Newborn Child
Normal human newborns are immunologically immature; they have deficient antibody production, phagocytic activity, and complement activity, especially components of alternative pathways (e.g., factor B).
The newborn cannot produce all classes of antibody; IgM is produced by the newborn (develops in the last trimester) to in utero infections (e.g., cytomegalovirus, rubella virus, and Toxoplasma gondii); only limited amounts of IgA are produced in the newborn; IgG production begins after birth and rises steadily throughout the first year of life.
Maternal antibodies provide protection within the newborn's circulation (see figure below).
Deficits in specific maternal transplacental antibody may lead to a tendency to develop severe, overwhelming sepsis and meningitis in the newborn.
Antibody Levels in Umbilical Cord Blood and in Neonatal Circulation. Early in gestation, maternal IgG begins active transport across the placenta and enters the fetal circulation. At birth, the fetal circulation may contain nearly adult levels of IgG, which is almost exclusively from the
maternal source. The fetal immune system has the capacity to produce IgM and small amounts of IgA before birth (not shown). After delivery, maternal IgG is rapidly destroyed and neonatal IgG
production increases.
Geriatric Considerations Age-Related Factors Affecting Mechanisms of Self-Defense in the Elderly
Immune function decreases with age; diminished T-cell function and reduced antibody responses to antigenic challenge occur with age.
The thymus reaches maximum size at sexual maturity and then undergoes involution until it is a vestigial remnant by middle age; by 45 to 50 years of age, the thymus is only 15% of its maximum size.
With age there is a decrease in thymic hormone production and the organ's ability to mediate T-cell differentiation.
Did You Understand? Third Line of Defense: Adaptive Immunity 1. Adaptive immunity is a state of protection, primarily against infectious agents, that differs from inflammation by being slower to develop, being more specific, and having memory that makes it much longer lived.
2. The adaptive immune response is most often initiated by cells of the innate system. These cells process and present portions of invading pathogens (i.e., antigens) to lymphocytes in peripheral lymphoid tissue.
3. The adaptive immune response is mediated by two different types of lymphocytes —B lymphocytes and T lymphocytes. Each has distinct functions. B cells are responsible for humoral immunity that is mediated by circulating antibodies (immunoglobulins), whereas T cells are responsible for cell-mediated immunity, in which they kill targets directly or stimulate the activity of other leukocytes.
4. Adaptive immunity can be either active or passive depending on whether immune response components originated in the host or came from a donor.
Antigens and Immunogens 1. Antigens are molecules that bind and react with components of the immune response, such as antibodies and receptors on B and T cells. Most antigens can induce an immune response, and these antigens are called immunogens.
2. All immunogens are antigens but not all antigens are immunogens.
3. Some pathogens are successful because they mimic “self” antigens but avoid inducing an immune response.
4. Large molecules, such as proteins, polysaccharides, and nucleic acids, are most immunogenic. Thus molecular size is an important factor for antigen immunogenicity.
5. Haptens are antigens too small to be immunogens by themselves but become immunogenic after combining with larger molecules.
6. The antigenic determinant, or epitope, is the precise chemical structure with
which an antibody or B-cell/T-cell receptor reacts.
7. Self-antigens are antigens on an individual's own cells. The individual's immune system does not normally recognize self-antigens as immunogenic, a condition known as tolerance.
8. The response to antigen can be divided into two phases: the primary and secondary responses. The primary response of humoral immunity is usually dominated by IgM, with lesser amounts of IgG. The secondary immune response has a more rapid production of a larger amount of antibodies, predominantly IgG.
Antibodies 1. The humoral immune response consists of molecules (antibodies) produced by B cells. B cells are lymphocytes.
2. Antibodies are plasma glycoproteins that can be classified by chemical structure and biologic activity as IgG, IgM, IgA, IgE, or IgD.
3. A typical antibody molecule is constructed of two identical heavy chains and two identical light chains (either κ or λ) and has two Fab portions that bind antigen and an Fc portion that interacts with complement or receptors on cells.
4. The protective effects of antibodies may be direct through the action of antibody alone or indirect requiring activation of other components of the innate immune response.
5. IgE is a special class of antibody produced against environmental antigens that are the primary cause of common allergies. It also protects the individual from infection by large parasitic worms (helminthes).
6. The secretory immune system protects the external surfaces of the body through secretion of antibodies in bodily secretions, such as tears, sweat, saliva, mucus, and breast milk. IgA is the dominant secretory immunoglobulin.
Immune Response: Collaboration of B Cells and T Cells 1. The generation of clonal diversity results in production of B and T lymphocytes
with receptors against millions of antigens that possibly will be encountered in an individual's lifetime occurs in the fetus in the primary lymphoid organs: the thymus for T cells and portions of the bone marrow for B cells..
2. The generation of clonal diversity is the differentiation of lymphoid stem cells into B and T lymphocytes. Lymphoid stem cells interact with stromal cells through a variety of adhesion factors. As the stem cell matures it develops a variety of surface markers or receptors, one of the earliest is IL-7 receptor. IL-7, produced by stromal cells is critical for driving differentiation and proliferation of the B cell.
3. The next stage in development is formation of the B-cell receptor (BCR). The role of the BCR is to recognize antigen and communicate that information to the cell's nucleus.
4. The variable regions of antibodies, as well as the BCR, contain CDR areas. The diversity of these CDRs is responsible for the variety of antigens recognized by immunocompetent B cells. The enormous repertoire of antibody specificities is made possible by rearrangement of existing DNA during B-cell development in the primary lymphoid organs, a process called somatic recombination.
5. Somatic rearrangement of the antibody variable regions will frequently result in a BCR that recognizes the individual's own antigens, which may result in attack on “self” antigens expressed on various tissue and organs. Many of these “autoreactive” B cells are eliminated in the bone marrow. Most of the developing B cells undergo apoptosis. This entire process is referred to as central tolerance.
6. The process of T-cell proliferation and differentiation is similar to that for B cells. The primary lymphoid organ for T-cell development is the thymus. Lymphoid stem cells travel through the thymus, where thymic hormones and the cytokine IL-7 promote lymphoid stem cell division and the production of receptors. They exit the thymus as mature immunocompetent T cells with antigen-specific receptors on the cell surface.
7. T cell receptor, or TCR, proceeds in a manner similar to BCR. Initially proteins called CD4 and CD8 are expressed on the developing cells. Eventually CD4 cells develop into T-helper cells (Th cells) and CD 8 cells become T-cytotoxic cells. Other mature T cells include T-regulatory cells (Treg) and memory cells.
8. The generation of clonal diversity concludes when immunocompetent T and B cells migrate from the primary lymphoid organs into the circulation and secondary
lymphoid organs to await antigen.
9. The induction of an immune response, or clonal selection, begins when antigen enters the individual's body.
10. Most antigens must first interact with antigen-presenting cells (APCs) (e.g., macrophages). Dendritic cells present in the skin, mucosa, and lymphoid tissues also present antigen.
11. Antigen is processed in the APCs and presented on the cell surface by molecules of the MHC. The particular MHC molecule (class I or class II) that presents antigen determines which cell will respond to that antigen. Th cells require that the antigen be presented in a complex with MHC class II molecules. Tc cells require that the antigen be presented by MHC class I molecules.
12. The T cell sees the presented antigen through the T-cell receptor (TCR) and accessory molecules: CD4 or CD8. CD4 is found on Th cells and reacts specifically with MHC class II. CD8 is found on Tc cells and reacts specifically with MHC class I.
13. Th cells consist of Th1 cells, which help Tc cells respond to antigen; Th2 cells, which help B cells develop into plasma cells; and Th17 cells, which help activate macrophages.
14. Tc cells bind to and kill cellular targets such as cells infected with viruses or cancer cells.
15. The natural killer (NK) cell has some characteristics of the Tc cells and is important for killing target cells in which viral infection or malignancy has resulted in the loss of cellular MHC molecules.
Pediatric Considerations: Age-Related Factors Affecting Mechanisms of Self-Defense in the Newborn Child 1. Neonates often have transiently depressed inflammatory function, particularly neutrophil chemotaxis and alternative complement pathway activity.
2. The T-cell–independent immune response is adequate in the fetus and neonate, but
the T-cell–dependent immune response develops slowly during the first 6 months of life.
3. Maternal IgG antibodies are transported across the placenta into the fetal blood and protect the neonate for the first 6 months, after which they are replaced by the child's own antibodies.
Geriatric Considerations: Age-Related Factors Affecting Mechanisms of Self-Defense in the Elderly 1. Elderly persons are at risk for impaired wound healing, usually because of chronic illnesses.
2. T-cell function and antibody production are somewhat deficient in elderly persons. Elderly individuals also tend to have increased levels of circulating autoantibodies (antibodies against self-antigens).
Key Terms Active immunity (active acquired immunity), 159
Adaptive (acquired) immunity, 158
Agglutination, 162
Antibody, 161
Antibody-dependent cellular cytotoxicity (ADCC), 172
Antigen, 158, 160
Antigen-binding fragment (Fab), 161
Antigen-binding site (paratope), 162
Antigen processing, 168
Antigen-processing (antigen-presenting) cell (APC), 167
Antigenic determinant (epitope), 162
B-cell receptor (BCR), 166
B lymphocyte (B cell), 158
CD3, 167
CD4, 167
CD8, 167
CDR, 167
Cellular immunity, 159
Central tolerance, 167
Class switch, 170
Cloncal diversity, 159, 166
Clonal selection, 159, 167
Complementary-determining region (CDR), 162
Crystalline fragment (Fc), 161
Dendritic cell, 167
Hapten, 160
Human leukocyte antigens (HLA), 168
Humoral immunity, 159
Immune response, 158
Immunity, 158
Immunocompetent, 159
Immunogen, 160
Immunoglobulin (Ig), 161
Lymphocyte, 158
Lymphoid stem cell, 166
Major histocompatibility complex (MHC), 168
Memory cell, 159
Natural killer (NK) cell, 172
Neutralization, 162
Passive immunity (passive acquired immunity), 159
Peripheral tolerance, 173
Plasma cell, 159
Precipitation, 162
Primary immune response, 167
Primary (central) lymphoid organ, 166
Secondary immune response, 167
Secondary lymphoid organ, 159
Secretory (mucosal) immune system, 164
Secretory immunoglobulin, 164
Somatic recombination, 167
Superantigen (SAG), 169
Systemic immune system, 164
T-cell receptor (TCR), 167
T-cytotoxic (Tc) cell, 159
T-helper (Th) cell, 159
T lymphocyte (T cell), 158
T-regulatory (Treg) cell, 159, 169, 172
Th1 cell, 168
Th2 cell, 169
Th17 cell, 169
Titer, 167
References 1. Wu LC, Zarrin AA. The production and regulation of IgE by the immune system. Nat Rev Immunol. 2014;14(4):247–259.
2. Sela-Culang I, et al. The structural basis of antibody-antigen recognition. Front Immunol. 2013;4:302.
3. Guilliams M, et al. The function of Fcγ receptors in dendritic cells and macrophages. Nat Rev Immunol. 2014;14(2):94–108.
4. Fitzsimmons CM, et al. Helminth allergens, parasite-specific IgE, and its protective role in human immunity. Front Immunol. 2014;5:61.
5. Rescigno M. Mucosal immunology and bacterial handling in the intestine. Best Pract Res Clin Gastroenterol. 2013;27(1):17–24.
6. Miyazaki K, et al. The establishment of B versus T cell identity. Trends Immunol. 2014;35(5):205–210.
7. Clark MR, et al. Orchestrating B cell lymphopoiesis through interplay of IL-7 receptor and pre-B cell receptor signalling. Nat Rev Immunol. 2014;14(2):69–89.
8. Shih H-Y, Krangel MS. Chromatin architecture, CCCTC-binding factor, and V(D)J recombination: managing long-distance relationships at antigen receptor loci. J Immunol. 2013;190(10):4915–4921.
9. Boehm T, Swann JB. Thymus involution and regeneration: two sides of the same coin? Nat Rev Immunol. 2013;13(11):831–838.
10. Brownlie RJ, Zamoyska R. T cell receptor signalling networks: branching, diversified and bounded. Nat Rev Immunol. 2013;13(4):257–269.
11. Blum JS, et al. Pathways of antigen processing. Annu Rev Immunol. 2013;31(2013):443–473.
12. Batista FD, Dustin ML. Cell:cell interactions in the immune system. Immunol Rev. 2013;251(1):7–12.
13. Fooksman DR. Organizing MHC class II presentation. Front Immunol. 2014;5:158.
14. Yamane H, Paul WE. Early signaling events that underlie fate decisions of naïve CD4+ T cells toward distinct T-helper cell subsets. Immunol Rev. 2013;252(1):12–23.
15. Jiang S, Dong C. A complex issue on CD4+ T-cell subsets. Immunol Rev. 2013;252(1):5–11.
16. Ramachandran G. Gram-positive and gram-negative bacterial toxins in sepsis. Virulence. 2014;5(1):213–218.
17. Avalos AM, Ploegh HL. Early BCR events and antigen capture, processing, and loading on MHC class II on B cells. Front Immunol. 2014;5:92.
18. Njau MN, Jacob J. The CD28/B7 pathway: a novel regulator of plasma cell function. Adv Exp Med Biol. 2013;785(2013):67–75.
19. Farber DL, et al. Human memory T cells: generation, compartmentalization and homeostasis. Nat Rev Immunol. 2014;14(1):24–35.
20. Annunziato F, et al. Main features of human T helper 17 cells. Ann N Y Acad Sci. 2013;1284(2013):66–70.
21. Singer BD, et al. Regulatory T cells as immunotherapy. Front Immunol. 2014;5:46.
8
Infection and Defects in Mechanisms of Defense Neal S. Rote
CHAPTER OUTLINE
Infection, 176
Microorganisms and Humans: A Dynamic Relationship, 176 Countermeasures Against Infectious Microorganisms, 187
Deficiencies in Immunity, 189
Initial Clinical Presentation, 189 Primary (Congenital) Immune Deficiencies, 190 Secondary (Acquired) Immune Deficiencies, 192 Evaluation and Care of Those with Immune Deficiency, 193 Replacement Therapies for Immune Deficiencies, 193 Acquired Immunodeficiency Syndrome (AIDS), 194
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity, 199
Mechanisms of Hypersensitivity, 199 Antigenic Targets of Hypersensitivity Reactions,
206
The defensive system protecting the body from infection is a finely tuned network, but it is not perfect. Sometimes infectious agents can inhibit or escape defense mechanisms or the system may break down, leading to inadequate protection or inappropriate activation. An inadequate response (commonly called an immune deficiency) may range from relatively mild defects to life-threatening severity. Inappropriate responses (hypersensitivity reactions) may be (1) exaggerated against noninfectious environmental substances (allergy); (2) misdirected against the body's own cells (autoimmunity); or (3) directed against beneficial foreign tissues, such as transfusions or transplants (alloimmunity). Several of these inappropriate responses can be serious or life-threatening. This chapter provides an overview of conditions under which our protective systems have failed.
Infection Modern health care has shown great progress in preventing and treating infectious diseases. In the United States, heart disease and malignancies greatly surpass infectious disease as major causes of death. However, endemic diseases, such as chronic hepatitis, human immunodeficiency virus (HIV), other sexually transmitted infections, and foodborne infections, remain major challenges.1 Most deaths related to infections occur in individuals whose protective systems are compromised (children, elderly, and those with chronic disease). Influenza/pneumonia (eighth leading cause of death) and sepsis (eleventh leading cause) accounted for more than 89,000 deaths (3.5% of the total number of deaths) in 2011.2 Other infections resulted in an additional 27,000 deaths. Infectious disease remains a significant threat to life in many parts of the world,
including India, Africa, and Southeast Asia.3 The advent of sanitary living conditions, clean water, uncontaminated food, vaccinations, and antimicrobial medications has improved the health of many; but inefficient healthcare systems, endemic poverty, political unrest, and other factors have slowed progress in some regions. As a result of these initiatives, smallpox has been eradicated from the globe (the last reported case was in 1975 in Somalia). Worldwide, polio has declined by more than 99% and eradicated from the Western hemisphere. Measles was decreased by 78% and was nearly eliminated in the Western hemisphere. Although vaccines and antimicrobials have diminished the frequency of some infectious diseases, the emergence of new diseases, such as West Nile virus, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome coronavirus (MERS-CoV), and Hantavirus, and the uncontrolled spread of diseases, such as Ebola virus infection, into new regions of Africa, as well as the continued development of many multiple drug–resistant microorganisms, are examples of the current intense challenges in the struggle to prevent and control infectious disease. Some tropical diseases are emerging for the first time in the United States, possibly a result of global warming.
Microorganisms and Humans: A Dynamic Relationship The increase in antibiotic resistance, in particular, places more importance on maintenance of an intact inflammatory and immune system. Individuals with immune deficiencies become easily infected with opportunistic microorganisms— those that normally would not cause disease but seize the opportunity provided by the person's decreased immune or inflammatory responses.
Unlike opportunistic infections, true pathogens have devised means to circumvent the normal controls provided by the innate and adaptive system. Several factors influence the capacity of a pathogen to cause disease. • Communicability: Ability to spread from one individual to others (e.g., measles and pertussis spread very easily; human immunodeficiency virus [HIV] is of lower communicability)
• Infectivity: Ability of the pathogen to invade and multiply in the host (e.g., herpes simplex virus can survive for long periods in a latent stage)
• Virulence: Capacity of a pathogen to cause severe disease (e.g., measles virus is of low virulence; rabies and Ebola viruses are highly virulent)
• Pathogenicity: Ability of an agent to produce disease—success depends on communicability, infectivity, extent of tissue damage, and virulence (e.g., HIV can kill T lymphocytes)
• Portal of entry: Route by which a pathogenic microorganism infects the host (e.g., direct contact, inhalation, ingestion, or bites of an animal or insect)
• Toxigenicity: Ability to produce soluble toxins or endotoxins, factors that greatly influence the pathogen's degree of virulence Infectivity is facilitated by the ability of pathogens to attach to cell surfaces,
release enzymes that dissolve protective barriers, multiply rapidly, escape the action of phagocytes, or resist the effect of low pH. After penetrating protective barriers (invasion), pathogens then multiply and spread through the lymph and blood to tissues and organs, where they continue multiplying and cause disease. In humans the route of entrance of many pathogenic microorganisms also becomes the site of shedding of new infectious agents to other individuals, completing a cycle of infection. Infectious disease can be caused by microorganisms that range in size from 20
nanometers (nm) (poliovirus) to 10 meters (m) (tapeworm). Classes of pathogenic microorganisms and their characteristics are summarized in Table 8-1. Some mechanisms of tissue damage caused by microorganisms are summarized in Table 8-2. The multiple layers of defense against infection are described in Chapters 6 and 7. Table 8-3 contains examples of microorganisms that defeat our protective systems.
TABLE 8-1 Classes of Microorganisms Infectious to Humans
Class Size Site of Reproduction Example Virus 20-300 nm Intracellular Poliomyelitis Chlamydiae 200-1000 nm Intracellular Urethritis Rickettsiae 300-1200 nm Intracellular Rocky Mountain spotted fever Mycoplasma 125-350 nm Extracellular Atypical pneumonia Bacteria 0.8-15 mcg Skin Staphylococcal wound infection
Mucous membranes Cholera Extracellular Streptococcal pneumonia Intracellular Tuberculosis
Fungi 2-200 mcg Skin Tinea pedis (athlete's foot) Mucous membranes Candidiasis (e.g., thrush) Extracellular Sporotrichosis Intracellular Histoplasmosis
Protozoa 1-50 mm Mucosal Giardiasis Extracellular Sleeping sickness
Helminths 3 mm to 10 m Intracellular Trichinosis Extracellular Filariasis
TABLE 8-2 Examples of Microorganisms That Cause Tissue Damage
PATHOGENS THAT DIRECTLY CAUSE TISSUE DAMAGE Produce Exotoxin Streptococcus pyogenes Tonsillitis, scarlet fever Staphylococcus aureus Boils, toxic shock syndrome, food poisoning Corynebacterium diphtheria Diphtheria Clostridium tetani Tetanus Vibrio cholerae Cholera Produce Endotoxin Escherichia coli Gram-negative sepsis Haemophilus influenzae Meningitis, pneumonia Salmonella typhi Typhoid Shigella Bacillary dysentery Pseudomonas aeruginosa Wound infection Yersinia pestis Plague Cause Direct Damage with Invasion Variola Smallpox Varicella-zoster Chickenpox, shingles Hepatitis B virus Hepatitis Poliovirus Poliomyelitis Measles virus Measles, subacute sclerosing panencephalitis Influenza virus Influenza Herpes simplex virus Cold sores PATHOGENS THAT INDIRECTLY CAUSE TISSUE DAMAGE Produce Immune Complexes Hepatitis B virus Kidney disease S. pyogenes Glomerulonephritis Treponema pallidum Kidney damage in secondary syphilis Most acute infections Transient renal deposits Cause Cell-Mediated Immunity Mycobacterium tuberculosis Tuberculosis Mycobacterium leprae Tuberculoid leprosy Lymphocytic choriomeningitis virus Aseptic meningitis Borrelia burgdorferi Lyme arthritis Herpes simplex virus Herpes stromal keratitis
Data modified from Janeway CA et al: Immunobiology: the system in health and disease, ed 5, New York, 2001, Garland.
TABLE 8-3 Examples of Mechanisms Used by Pathogens to Resist the Immune System
Mechanisms Effect on Immunity Example of Specific Microorganisms
Destroy or Block Component of Immune System Produce toxins Kills phagocyte or interferes with chemotaxis
Prevents phagocytosis by inhibiting fusion between phagosome and lysosomal granules Staphylococcus Streptococcus Mycobacterium tuberculosis
Produce antioxidants (e.g., catalase, superoxide dismutase) Produce protease to digest IgA
Prevents killing by O2-dependent mechanisms Promotes bacterial attachment
Mycobacterium sp. Salmonella typhi Neisseria gonorrhoeae (urinary tract infection), Haemophilus influenzae, and Streptococcus pneumoniae (pneumonia)
Produce surface molecules that mimic Fc receptors and bind antibody
Prevents activation of complement system Prevents antibody functioning as opsonin
Staphylococcus Herpes simplex virus
Mimic Self-Antigens Produce surface antigens (e.g., M protein, red blood cell antigens) that are similar to self-antigens
Pathogen resembles individual's own tissue; in some individuals, antibodies can be formed against self-antigen, leading to hypersensitivity disease (e.g., antibody to M protein also reacts with cardiac tissue, causing rheumatic heart disease; antibody to red blood cell antigens can cause anemia)
Group A Streptococcus (M protein) Mycoplasma pneumoniae (red cell antigens)
Change Antigenic Profile Undergo mutation of antigens or activate genes that change surface molecules
Immune response delayed because of failure to recognize new antigen Influenza HIV Some parasites
Bacterial Disease Bacteria are prokaryocytes (lacking a discrete nucleus) and are relatively small. They can be aerobic or anaerobic and motile or immotile. Spherical bacteria are called cocci, rodlike forms are called bacilli, and spiral forms are termed spirochetes. Gram stain differentiates the microorganisms as gram-positive or gram-negative bacteria. Examples of human diseases caused by specific bacteria are listed in Table 8-4. The general structure of bacteria is reviewed in Figure 8-1.
TABLE 8-4 Examples of Common Bacterial Infections
Microorganism Gram Stain Respiratory Pathway
Intracellular or Extracellular
Respiratory Tract Infections Upper Respiratory Tract Infections Corynebacterium diphtheriae (diphtheria) Gram + Facultative anaerobic Extracellular Haemophilus influenzae Gram − Facultative anaerobic Extracellular Streptococcus pyogenes (group A) Gram + Facultative anaerobic Extracellular Otitis Media Haemophilus influenzae Gram − Facultative anaerobic Extracellular Streptococcus pneumoniae Gram + Facultative anaerobic Extracellular
Lower Respiratory Tract Infections Bacillus anthracis (pulmonary anthrax) Gram + Facultative anaerobic Extracellular
Bordetella pertussis (whooping cough) Gram − Aerobic Extracellular Chlamydia pneumonia Not stainable Aerobic Obligate intracellular Escherichia coli Gram − Facultative anaerobic Extracellular Haemophilus influenzae Gram − Facultative anaerobic Extracellular Legionella pneumophila Gram − Aerobic Facultative intracellular Mycobacterium tuberculosis Gram +
(weakly) Aerobic Extracellular
Mycoplasma pneumoniae Not stainable Aerobic Extracellular Neisseria meningitidis (develops into meningitis) Gram − Aerobic Extracellular Pseudomonas aeruginosa Gram − Aerobic Extracellular Streptococcus agalactiae (group B; develops to meningitis) Gram + Facultative anaerobic Extracellular Streptococcus pneumoniae Gram + Facultative anaerobic Extracellular Yersinia pestis (plague) Gram − Facultative anaerobic Extracellular Gastrointestinal Infections Inflammatory Gastrointestinal Infections Bacillus anthracis (gastrointestinal anthrax) Gram + Facultative anaerobic Extracellular Clostridium difficile Gram + Anaerobic Extracellular Escherichia coli O157:H7 Gram − Facultative anaerobic Extracellular Vibrio cholerae Gram − Facultative anaerobic Extracellular Invasive Gastrointestinal Infections Brucella abortus (brucellosis, undulant fever, leading to sepsis, heart infection)
Gram − Aerobic Intracellular
Helicobacter pylori (gastritis and peptic ulcers) Gram − Microaerophilic Extracellular Listeria monocytogenes (leading to sepsis and meningitis) Gram + Aerobic Intracellular Salmonella typhi (typhoid fever) Gram − Anaerobic Extracellular Shigella sonnei Gram − Facultative anaerobic Extracellular Food Poisoning Bacillus cereus Gram + Facultative anaerobic Extracellular Clostridium botulinum Gram + Anaerobic Extracellular Clostridium perfringens Gram + Anaerobic Extracellular Staphylococcus aureus Gram + Facultative anaerobic Extracellular Sexually Transmitted Infections Chlamydia trachomatis (pelvic inflammatory disease) Not stainable Aerobic Intracellular Neisseria gonorrhoeae (urethritis) Gram − Aerobic Facultative intracellular Treponema pallidum (spirochete; syphilis) Gram − Aerobic Extracellular Skin and Wound Infections Bacillus anthracis (cutaneous anthrax) Gram + Facultative anaerobic Extracellular Borrelia burgdorferi (Lyme disease; spirochete) Gram − Aerobic Extracellular Clostridium tetani (tetanus) Gram + Anaerobic Extracellular Clostridium perfringens (gas gangrene) Gram + Anaerobic Extracellular Mycobaterium leprae (leprosy) Gram +
(weakly) Aerobic Extracellular
Pseudomonas aeruginosa Gram − Aerobic Extracellular Rickettsia prowazekii (rickettsia; typhus) Gram − Aerobic Obligate intracellular Staphylococcus aureus Gram + Facultative anaerobic Extracellular Streptococcus pyogenes (group A) Gram + Facultative anaerobic Extracellular Eye Infections Chlamydia trachomatis (conjunctivitis) Not stainable Aerobic Obligate intracellular Haemophilus aegyptius (pink eye) Gram − Facultative anaerobic Extracellular Zoonotic Infections Bacillus anthracis (anthrax) Gram + Facultative anaerobic Extracellular Brucella abortus (brucellosis, also called undulant fever) Gram − Aerobic Intracellular Borrelia burgdorferi (spirochete; Lyme disease) Gram − Aerobic Extracellular Listeria monocytogenes Gram + Aerobic Intracellular Rickettsia rickettsii (rickettsia; Rocky Mountain spotted fever) Gram − Aerobic Obligate intracellular Rickettsia prowazekii (rickettsia; typhus) Gram − Aerobic Obligate intracellular Yersinia pestis (plague) Gram − Facultative anaerobic Extracellular Nosocomial Infections Enterococcus faecalis Gram + Facultative anaerobic Extracellular Enterococcus faecium Gram + Facultative anaerobic Extracellular Escherichia coli (cystitis) Gram − Facultative anaerobic Extracellular Pseudomonas aeruginosa Gram − Obligate anaerobic Extracellular Staphylococcus aureus Gram + Facultative anaerobic Extracellular Staphylococcus epidermidis Gram + Facultative anaerobic Extracellular
FIGURE 8-1 General Structure of Bacteria. A, The structure of the bacterial cell wall determines its staining characteristics with Gram stain. A gram-positive bacterium has a thick layer of peptidoglycan (left). A gram-negative bacterium has a thick peptidoglycan layer and an outer
membrane (right). B, Example of a gram-positive (darkly stained microorganisms, arrow) group A Streptococcus. This microorganism consists of cocci that frequently form chains. C, Example of a gram-negative (pink microorganisms, arrow) Neisseria meningitides in cerebrospinal fluid. Neisseria form complexes of two cocci (diplococci). (A from Murray PR et al: Medical microbiology, ed 7, Philadelphia,
2013, Saunders; B, C from Murray PR et al: Medical microbiology, ed 4, St Louis, 2002, Mosby.)
Bacterial survival and growth depend on the effectiveness of the body's defense mechanisms and on the bacterium's ability to resist these defenses. A vast amount of
information has been published about bacterial pathogenesis. The main aspects of how bacteria cause disease may be illustrated in how one particular microorganism, Staphylococcus aureus, has adapted to become a life-threatening pathogen. Staphylococcus aureus has become a major cause of hospital-acquired
(nosocomial) infections and is now spreading throughout the community. This microorganism is a common commensal inhabitant of normal skin and nasal passages (estimates depict that from 30% to 80% of individuals may be nasal carriers) and can be transmitted by direct skin-to-skin contact or by contact with shared items or surfaces that have become contaminated by another person (e.g., towels, used bandages).4 Although a relatively benign commensal microorganism under normal
conditions, S. aureus is well equipped to act as a life-threatening pathogen when the opportunity arises; thus it is an opportunistic microorganism. Skin infections may occur at sites of trauma, such as cuts and abrasions, and at areas of the body covered by hair (e.g., back of neck, groin, buttock, armpit, beard area of men). Most infections are relatively mild and localized, appearing as red and swollen pustules on the skin, containing pus or other drainage. They can develop into abscesses, boils, carbuncles, cellulitis, or furunculosis. Invasive disease may originate from wound infections (e.g., trauma, surgical wounds, indwelling medical devices, prosthetic joints) and lead to fatal septicemia and abscesses in internal organs (e.g., lungs, kidney, bones, skeletal muscle, meninges, or heart) (Figure 8-2).
FIGURE 8-2 Staphylococcus aureus Infections. Different strains of S. aureus (gram-positive cocci in sputum from an individual with pneumonia [center photograph]) cause a variety of
infections. The particular infection may depend on the toxin produced: exfoliative toxin (scalded skin syndrome), enterotoxins A-G (food poisoning), or toxic shock syndrome toxin-1 (TSST- 1). (Toxic shock syndrome, carbuncle, impetigo, and wound infection photos from Cohen J, Powderly W G: Infectious diseases, ed 3, St Louis, 2010, Mosby; folliculitis photo from Goldman L, Ausiello D: Cecil medicine, ed 24, Philadelphia, 2012, Saunders; center photo and photos of food poisoning and endocarditis from Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 8, Philadelphia, 2010, Saunders; furuncle photo from Long S et al: Principles and practice of pediatric infectious diseases, ed 4, Philadelphia, 2012, Saunders; scalded skin syndrome and
pneumonia photos from Mandell G et al: Principles and practice of infectious diseases, ed 7, Philadelphia, 2010, Churchill Livingstone.)
Microscopically, staphylococci are gram-positive cocci that generally grow in grapelike clusters. However, this microorganism possesses a myriad of potential virulence factors that determine the severity, location, and clinical features of infection. It should be noted that individual strains of this opportunistic pathogen utilize only some of the entire array of virulence factors. Microorganisms frequently exist as part of complex multicellular masses called
biofilms. Biofilms consist of mixed species of microorganisms, including bacteria, fungi, and viruses. Growth of bacteria in biofilms offers survival advantage by protection from the host's responses and exposure to antibiotics. These structures are associated with otitis media; urinary tract infections secondary to indwelling catheters; foot ulcers in diabetic persons; infected burn wounds; vaginitis;
osteomyelitis; pneumonia secondary to cystic fibrosis; and diseases of the oral cavity related to dental plaque, such as dental caries and periodontitis. S. aureus biofilms are associated with persistent nasopharyngeal colonization and colonization of implanted devices.5 A variety of surface proteins mediate adherence among microorganisms in
biofilms and to connective tissue (laminin, fibrin, fibronectin) and endothelium. Attachment to collagen occurs in strains causing osteomyelitis and septic arthritis. The capsular polysaccharide mediates attachment to prosthetic devices and also protects against phagocytosis. One surface protein, protein A, binds IgG by the Fc portion so that the Fab regions are facing outward. Thus, the bacteria appear coated with a self-protein, and, with the Fc bound
directly to protein A, the IgG cannot activate complement or act as an opsonin.6 A coagulase that induces fibrin clotting on the bacterial surface also masks bacterial antigens under a surface of self-proteins. Staphylococcal protein A and also a protein called staphylococcal binder of immunoglobulin are secreted and bind and neutralize IgG. Staphylococcus produces proteins that inhibit complement activity, including activation of C3 and C5, preventing production of C3b, C3a, and C5a.7 Some strains of S. aureus are programmed to avoid innate immunity. They can
produce inhibitors of antimicrobial peptides and avoid recognition by Toll-like receptors.8 Even when engulfed by a phagocyte, S. aureus may resist intracellular oxidative killing by inactivating hydrogen peroxide and other reactive oxygen species. They also resist lysozyme by changing the chemistry of the cell wall.9 Many bacteria use toxins as virulence factors, including exotoxins and
endotoxins. Exotoxins are secreted molecules and are immunogenic eliciting production of antibodies known as antitoxins (important for vaccine development, see page 187). The most poisonous yet discovered is botulinum neurotoxin produced by Clostridium botulinum; less than 1 ng/kg is toxic to humans. Strains of S. aureus are capable of producing a wide array of secreted toxic molecules or exotoxins. These include those that damage the cell membrane (α-toxin, which forms pores in membranes; hemolysin, which destroys erythrocytes; β-toxin, which is a sphingomyelinase; δ-toxin, a detergent-like toxin; and leukocidin, which lyses phagocytes). Other toxins include coagulase, which causes blood clots; staphylokinase, which breaks down clots; exfoliative toxins, which cause separation of the epidermis resulting in scalded skin syndrome; lipase, which degrades lipids on the skin surface and facilitates abscess formation; enterotoxins, which cause food poisoning; and superantigens (discussed in Chapter 7).10 Each infectious strain of S. aureus may produce a few of these toxins so that strains differ in their capacities to cause particular diseases; thus, different strains may cause purulent dermal infections, food poisoning, or toxic shock syndrome.
Antibiotic resistance has become a major problem with S. aureus. For several decades pathogenic strains have commonly produced β-lactamase, an enzyme that destroys penicillin. More recently, staphylococci have developed resistance to broad-spectrum antibiotics, including methicillin-like antibiotics (methicillin- resistant Staphylococcus aureus [MRSA]), which were widely used to treat penicillin-resistant microorganisms. It is clear that S. aureus succeeds as an opportunistic pathogen because of a wide
array of virulence factors that neutralize important components of the innate and adaptive immune systems, destroy tissue, and resist much of our repertoire of antibiotics. The major remaining option is the development of an effective vaccine, a task that is sometimes difficult.11 As mentioned in the beginning of this section, S. aureus is only one of many bacteria that have developed similar characteristics. Gram-negative microbes produce an endotoxin (lipopolysaccharide [LPS]) that
is a structural portion of the cell wall and is released during growth, lysis, or destruction of the bacteria or during treatment with antibiotics. Therefore, antibiotics cannot prevent the toxic effects of the endotoxin. Bacteria that produce endotoxins are called pyrogenic bacteria because they activate the inflammatory process and produce fever. The innermost part of the lipopolysaccharide, lipid A, consists of polysaccharide and fatty acids and is responsible for the substance's toxic effects. Bacteremia occurs when bacteria are present in the blood. Gram-negative sepsis
(sepsis or septicemia) occurs when bacteria are growing in the blood and release large amounts of endotoxin, which can cause endotoxic shock with up to 50% mortality.12 Released endotoxin, as well as other bacterial products, reacts with pattern recognition receptors (PRRs) and induces the overproduction of proinflammatory cytokines, particularly tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6), which may secondarily be immunosuppressive.13 Endotoxin also is a potent activator of the complement and clotting systems, leading to a degree of capillary permeability sufficient to permit escape of large volumes of plasma into surrounding tissue, contributing to hypotension and, in severe cases, cardiovascular shock (see Chapter 24). Activation of the coagulation cascade leads to the syndrome of disseminated (or diffuse) intravascular coagulation (see Chapter 21).
Viral Disease Viral diseases are the most common afflictions of humans and range from the common cold, caused by many viruses, and the “cold sore” of herpes simplex virus to cancers and acquired immunodeficiency syndrome (AIDS). Examples of human
diseases caused by specific viruses are listed in Table 8-5. Viruses are very simple microorganisms consisting of nucleic acid protected from the environment by a layer or layers of proteins (capsid). The viral genome can be double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or single- stranded RNA (ssRNA). A select group of viruses (e.g., human immunodeficiency virus [HIV], herpesviruses, influenza virus) bud from the surface of an infected cell, retaining a portion of the cell's plasma membrane (envelope) as added protection. Viral replication depends totally on their ability to infect a permissive host cell—a cell that cannot resist viral invasion and replication. Thus, viruses are obligatory intracellular microbes. Transmission is usually from one infected individual to an uninfected individual by aerosols of respiratory tract fluids, contact with infected blood, sexual contact, or transmission from an animal reservoir (zoonotic infection) usually through a vector, such as mosquitoes.14
TABLE 8-5 Examples of Human Diseases Caused by Specific Viruses
Baltimore Classification
Family Virus Envelope Main Route of Transmission Disease
dsDNA Adenoviruses Adenovirus No Droplet contact Acute febrile pharyngitis Herpesviruses Herpes simplex type 1
(HSV-1) Yes Direct contact with saliva or
lesions Lesions in mouth, pharynx, conjunctivitis
Herpes simplex type 2 (HSV-2)
Yes Sexually, contact with lesions during birth
Sores on labia, meningitis in children
Herpes simplex type 8 (HSV-8)
Yes Sexually?, body fluids Kaposi sarcoma
Epstein-Barr virus (EBV) Yes Saliva Mononucleosis, Burkitt lymphoma Cytomegalovirus (CMV) Yes Body fluids, mother's milk,
transplacental Mononucleosis, congenital infection
Varicella-zoster virus (VZV)
Yes Droplet contact Chickenpox, shingles
ssDNA Papovaviruses Papillomavirus No Direct contact Warts, cervical carcinoma dsRNA Reoviruses Rotavirus No Fecal-oral Severe diarrhea ssRNA+ Picornaviruses Coxsackievirus No Fecal-oral, droplet contact Nonspecific febrile illness,
conjunctivitis, meningitis Hepatitis A virus No Fecal-oral Acute hepatitis Poliovirus No Fecal-oral Poliomyelitis Rhinovirus No Droplet contact Common cold
Flaviviruses Hepatitis C virus Yes Blood, sexually Acute or chronic hepatitis, hepatocellular carcinoma
Yellow fever virus Yes Mosquito vector Yellow fever Dengue virus Yes Mosquito vector Dengue fever West Nile virus Yes Mosquito vector Meningitis, encephalitis
Togaviruses Rubella virus Yes Droplet contact, transplacental Acute or congenital rubella Coronaviruses SARS Yes Droplets in aerosol or direct
contact Severe respiratory tract disease
Caliciviruses Norovirus No Fecal-oral Gastroenteritis ssRNA− Orthomyxoviruses Influenza virus Yes Droplet contact Influenza
Paramyxoviruses Measles virus Yes Droplet contact Measles Mumps virus Yes Droplet contact Mumps Parainfluenza virus Yes Droplet contact Croup, pneumonia, common cold Respiratory syncytial virus (RSV)
Yes Droplet contact, hand-to-mouth Pneumonia, influenza-like syndrome
Rhabdoviruses Rabies virus Yes Animal bite, droplet contact Rabies Bunyaviruses Hantavirus Yes Aerosolized animal fecal
material Viral hemorrhagic fever
Filoviruses Ebola virus Yes Direct contact with body fluids Viral hemorrhagic fever Marburg virus Yes Direct contact with body fluids Viral hemorrhagic fever
Arenavirus Lassa virus Yes Aerosolized animal fecal material
Viral hemorrhagic fever
ssRNA+ with RT Retroviruses HIV Yes Sexually, blood products AIDS dsDNA with RT Hepadnaviruses Hepatitis B virus Yes All body fluids Acute or chronic hepatitis,
hepatocellular carcinoma
To understand the basic concepts of viral pathogenicity, it may be best to look closely at a single virus. Influenza is a ssRNA virus with a segmented genome (eight pieces of ssRNA). It is transmitted through aerosols or body fluids and is highly infectious. Symptoms begin 1 to 4 days after infection and may include chills, fever, sore throat, muscle aches, severe headaches, coughing, weakness, generalized discomfort, nausea, and vomiting and may lead to pneumonia. It can be fatal, particularly in young children and older adults.15 The normal rate of infectivity is about 5% to 15%, with a mortality of about 0.1%, and in most cases recovery occurs
in 1 to 2 weeks. Yearly seasonal influenza outbreaks result in about 250,000 to 500,000 deaths worldwide. The life cycle of every virus is completely intracellular and involves several
steps, the first being attachment to a receptor on the target cell (Figure 8-3). The influenza virion expresses two surface proteins that are essential to virulence. The hemagglutinin (HA) protein is a glycoprotein that is necessary for entrance into cells by binding to glycan receptors on the surface of respiratory tract epithelium. The viral surface neuraminidase (NA) is an enzyme that is necessary for release of new virions from infected cells by cleaving cellular sialic acids (a common component of mammalian cell membranes). The specificity of this virus-receptor interaction (tropism) dictates the range of host cells that a particular virus will infect and, therefore, the clinical symptoms that reflect the alteration of the function of the infected cells. Other viruses also use specific receptors; for example, HIV attaches to CD4 on T-helper cells, Epstein-Barr virus (EBV, a cause of mononucleosis and Burkitt lymphoma) attaches to complement receptor 2 (CR2) on B lymphocytes, and Rhinovirus (a group of viruses that cause the common cold) attaches to intracellular adhesion molecule-1 (ICAM-1) on respiratory tract epithelium.
FIGURE 8-3 Stages of Viral Infection of a Host Cell. The virion (1) becomes attached to the cell's plasma membrane by absorption; (2) releases enzymes that weaken the membrane and allow it to penetrate the cell; (3) uncoats itself; (4) replicates; and (5) matures and escapes from the cell
by budding from the plasma membrane. The infection then can spread to other host cells.
Attachment is followed by penetration (entrance into the cell by endocytosis or membrane fusion), uncoating (release of viral nucleic acid from the viral capsid by viral or host enzymes), replication (synthesis of mRNA and viral proteins), assembly (formation of new virions), and release (exit from the cell by lysis or budding). The influenza virus enters the respiratory tract epithelial cells by endocytosis. Low pH leads to intermembrane fusion between the endosome and viral envelop and uncoating.16 The viral ssRNA is transported to the nucleus where transcription and replication occur using the viral RNA-dependent RNA polymerase.17 Viral proteins assemble in the cytoplasm to form the matrix around the viral genome, and the virion buds from the cell surface. Infected cells usually die as a direct effect of the virus. The severity of clinical symptoms is usually secondary to the level of cytokines produced by the infected cells or in response to death of the cells. The effects of virus on the infected cell vary greatly. Some viruses, such as
herpesviruses, will initiate a latency phase during which the host cell is transformed (i.e., herpes simplex viruses 1 and 2 establish latency in neurons). During this phase, the viral DNA may be integrated into the DNA of the host cell and become a permanent passenger in that cell and its progeny. In response to stimuli, such as stress, hormonal changes, or disease, the virus may exit latency and enter a
productive cycle. Herpesviruses 1 and 2 are released from the neurons and infect skin epithelium, where lesions in the skin are a result of the immune response against the infected epithelium. Cytopathic effects caused by other viruses include the following:
1. Cessation of DNA, RNA, and protein synthesis (e.g., herpesvirus)
2. Disruption of lysosomal membranes, resulting in release of digestive lysosomal enzymes that can kill the cell (e.g., herpesvirus)
3. Fusion of host cells, producing multinucleated giant cells (e.g., respiratory syncytial virus)
4. Alteration of the antigenic properties, or identity, of the infected cell, causing the individual's immune system to attack the cell as if it were foreign (e.g., hepatitis B virus)
5. Transformation of host cells into cancerous cells, resulting in uninhibited and unregulated growth (e.g., human papillomavirus)
6. Promotion of secondary bacterial infection in tissues damaged by viruses
The principal method by which influenza virus eludes the immune system is by changing viral surface antigens, a process known as antigenic variation. Antibodies against the HA and NA antigens are responsible for protection against influenza infection. Infections are seasonal and protection gained from the previous year's infection does not totally protect against influenza in the following year because the HA and NA antigens undergo yearly change. Usually antigenic variation is relatively minor (antigenic drift) and results from mutations. Individuals frequently have partial protection resulting from the previous year's infection, which lessens the clinical effects of the disease. Two groups of influenza virus, influenza A and influenza B, infect humans and the yearly vaccine against influenza is a trivalent mixture of inactivated proteins from two influenza A subtypes and one influenza B subtype. Influenza B almost exclusively infects humans and mutates at a much lower rate than influenza A. Influenza A has antigenically distinct subtypes based on HA (17 forms) and NA (10 forms) antigens. Currently, subtypes H1N1, H1N2, and H3N2 are the primary causes of influenza worldwide. Influenza A periodically undergoes major antigenic changes (antigenic shifts)
(Figure 8-4). Influenza A can infect birds and mammals and shifts occur in animals coinfected by a human and an avian strain of influenza. The genome is segmented
and the segments can undergo recombination, during which the human virus obtains a new HA or NA antigen. Without a shift occurring, clinical influenza is usually considered epidemic (the number of new infections exceeds the number usually observed at other times of the year). When major antigenic changes occur, previous protection may not exist, resulting in a major pandemic (an epidemic that spreads over a large area, such as a continent or worldwide) and much more severe disease.
FIGURE 8-4 Antigenic Shifts in Influenza Virus. One theory proposes that antigenic shifts occur when a human influenza virus (blue) and an avian influenza virus (red) coinfect a species that is
permissive for both. The eight ssRNA strands are co-expressed in the same infected cell, resulting in mixing of the strands so that a hybrid virus can be produced. The hybrid virus
indicated here contains all the genetic information of the original virus that infected humans, but contains a new hemagglutinin (HA)-containing stand from the avian virus. This virus expresses
a new HA antigen and will be less susceptible to residual immunity that normally provides partial protection against yearly influenza infections.
A major worry regards zoonotic influenza during which a lethal influenza virus that infects birds or other animals suddenly develops the capacity to infect humans.14 These infections are monitored closely by agencies, such as the Centers for Disease Control and Prevention (CDC) in Atlanta. The CDC is currently monitoring human
cases of several zoonotic influenza outbreaks, including swine influenza virus (H1N1), a pathogenic H5N1 avian influenza virus, and a new strain of avian influenza (H7N9) that recently appeared. Viral pathogens bypass many defense mechanisms by hiding within cells and
away from normal inflammatory or immune responses. Some viruses spread from cell-to-cell through the bloodstream (e.g., influenza, rubella) and are highly sensitive to neutralizing antibodies that block viral spread and eventually cure the infection; therefore the disease is described as self-limiting. Other viruses (e.g., measles, herpes) are inaccessible to antibodies after initial infection because they remain inside infected cells, spreading by direct cell-to-cell contact. Most viruses have developed additional defense mechanisms. For instance, influenza virus produces NS1 protein (viral non-structural protein-1) that blocks the antiviral effects of type I interferon.
Fungal Disease Fungi are relatively large eukaryotic microorganisms with thick walls that have two basic structures: single-celled yeasts (spheres) or multicellular molds (filaments or hyphae) (Figure 8-5). Some fungi can exist in either form and are called dimorphic fungi. The cell walls of fungi are rigid and multilayered and composed of polysaccharides different from the peptidoglycans of bacteria. The lack of peptidoglycans allows fungi to resist the action of bacterial cell wall inhibitors such as penicillin and cephalosporin. Molds are aerobic, and yeasts are facultative anaerobes, which adapt to, but do not require, anaerobic conditions. They usually reproduce by simple division or budding.
FIGURE 8-5 Morphology of Fungi. (A) Fungi may be either mold or yeast forms, or dimorphic. (B) Photograph showing Candida albicans with both the mycelial and the yeast forms. (C) Oral infection with C. albicans (candidiasis, i.e., thrush). (D) Gram stain of sputum showing that
clinical isolates of C. albicans present as chains of elongated budding yeasts (× 1000). (A, B from Goering R et al: Mims' medical microbiology, ed 5, London, 2013, Saunders. C from McPherson R, Pincus M: Henry's clinical diagnosis and
management by laboratory methods, ed 22, Philadelphia, 2012, Saunders; D courtesy Dr. Stephen Raffanti.)
Diseases caused by fungi are called mycoses. Mycoses can be superficial, deep, or opportunistic. Superficial mycoses occur on or near skin or mucous membranes and usually produce mild and superficial disease. Fungi that invade the skin, hair, or nails are known as dermatophytes. The diseases they produce are called tineas (ringworm), for example, tinea capitis (scalp), tinea pedis (feet), and tinea cruris (groin). Chapter 41 discusses the various skin disorders caused by fungi. Pathologic fungi cause disease by adapting to the host environment. Fungi that
colonize the skin can digest keratin. Other fungi can grow with wide temperature variations in lower oxygen environments. Still other fungi have the capacity to suppress host immune defenses. Phagocytes and T lymphocytes are important in controlling fungi. Low white blood cell counts promote fungal infection and infection control is particularly important for individuals who are immunosuppressed. Common pathologic fungi are summarized in Table 8-6.
TABLE 8-6 Common Pathogenic Fungi
Primary Site of Infection Fungus Disease (Primary) Symptoms Superficial (no tissue invasion, little inflammation)
Malassezia furfur Tinea versicolor, seborrheic dermatitis, dandruff
Red rash on body
Cutaneous (no tissue invasion, inflammatory response)
Dermatophytes Trichophyton mentagrophytes Trichophyton rubrum Microsporum canis
Tinea pedis (athlete's foot) Tinea cruris (jock itch) Tinea corporis (ringworm)
Scaling, fissures, pruritus Rash, pruritus Lesion, raised border, scaling
Candida albicans Cutaneous candidiasis Lesions in most areas of skin, mucous membranes, thrush, vaginal infection
Subcutaneous (tissue invasion) Sporothrix schenckii Sporotrichosis Ulcers or abscesses on skin and other organ systems Systemic (dimorphic; causes disease in healthy individuals)
Stachybotrys chartarum, or “black mold”
Black mold disease Rash, headaches, nausea, pain
Coccidioides immitis Coccidioidomycosis Valley fever, flulike symptoms Histoplasma capsulatum Histoplasmosis Lung, flulike symptoms, disseminates to multiple
organs, eye Blastomyces dermatitidis Blastomycosis Flulike symptoms, chest pains
Systemic (opportunistic) Aspergillus fumigatus, Aspergillus flavus
Aspergillosis Invasive to lungs and other organs
Pneumocystis jiroveci Pneumocystis pneumonia (PCP) Pneumonia Cryptococcus neoformans Cryptococcosis Pneumonia-like illness, skin lesions, disseminates to
brain, meningitis Candidia albicans Systemic candidiasis Sepsis, endocarditis, meningitis
AIDS, Acquired immunodeficiency syndrome; DNA, deoxyribonucleic acid; ds, double-stranded; HIV, human immunodeficiency virus; RNA, ribonucleic acid; RT, reverse transcriptase; SARS, severe acute respiratory syndrome; ss, single-stranded.
Candida albicans is the most common cause of fungal infections in humans. It is an opportunistic yeast that is a commensal inhabitant in the normal microbiome of many healthy individuals, residing in the skin, gastrointestinal tract, mouth (30% to 55% of healthy individuals), and vagina (20% of healthy women). Candida albicans is normally under the control of local defense mechanisms, including members of the bacterial microbiome that produce antifungal agents. In healthy individuals antibiotic therapy can diminish the microbiome (e.g., diminished levels of Lactobacillus in the gastrointestinal or vaginal microbiome). Candida overgrowth may occur, resulting in localized infection such as vaginitis or oropharyngeal infection (thrush). In immunocompromised individuals, particularly those with diminished levels of
neutrophils (neutropenia), disseminated infection may occur. Candida is the most common fungal infection in people with cancer (particularly acute leukemia and other hematologic cancers), transplantation (bone marrow and solid organ), and HIV/AIDS. Invasive candidiasis also may be secondary to indwelling catheters, intravenous lines, or peritoneal dialysis, which provides direct entrance into the bloodstream. Disseminated candidiasis may involve deep infections of several internal organs,
including abscesses in the kidney, brain, liver, and heart, and is characterized by
persistent or recurrent fever, gram-negative shock-like symptoms (hypotension, tachycardia), and disseminated intravascular coagulation (DIC). The death rates of septic or disseminated candidiasis are in the range of 30% to 40%.
Parasitic Disease Parasitic microorganisms establish a relationship in which the parasite benefits at the expense of the other species. Parasites range from a unicellular protozoan to large worms. Parasitic worms (helminths) include intestinal and tissue nematodes (e.g., hookworm, roundworm), flukes (e.g., liver fluke, lung fluke), and tapeworms. A protozoan is a eukaryotic, unicellular microorganism with a nucleus and cytoplasm. Pathogenic protozoa include malaria (Plasmodium), amoebae (e.g., Entamoeba histolytica, which causes amoebic dysentery), and flagellates (e.g., Giardia lamblia, which causes diarrhea; Trypanosoma, which causes sleeping sickness). Although less common in the United States, parasites and protozoa are common causes of infections worldwide, with a significant effect on the mortality and morbidity of individuals in developing countries. Important parasites of humans are listed in Table 8-7.
TABLE 8-7 Examples of Parasites That Are Important in Humans
Category Subgroup Species Disease Organs Affected/Symptoms Protozoa Ameboid Entamoeba histolytica Amebiasis Dysentary, liver abscess
Flagellate Giardia lamblia Giardiasis* Diarrhea Trichomonas vaginalis Trichomoniasis Inflammation of reproductive organs Trypanosoma cruzi, T. brucei Chagas disease: African sleeping
sickness Generalized, blood and lymph nodes, progressing to cardiac and CNS
Ciliate Balantidium coli Balantidiasis Small intestines, invasion of colon, diarrhea Sporozoa (nonmotile)
Cryptosporidium parvum, C. hominis
Cryptosporidiosis* Intestine, diarrhea
Plasmodium spp. Malaria Blood, liver Toxoplasma gondii Toxoplasmosis* Intestine, eyes, blood, heart, liver
Helminths Flukes (trematodes) Fasciola hepatica Fasciolosis Liver destruction Schistosoma mansoni Schistosomiasis Blood, diarrhea, bladder, generalized symptoms
Tapeworms (cestodes)
Taenia solium Pork tapeworm Encysts in muscle, brain, liver
Roundworms (nematodes)
Ascaris lumbricoides Ascariasis Intestinal obstruction, bile duct obstruction Necator americanus (hookworm)
Hookworm disease Intestinal parasite
Trichinella spiralis Trichinosis* Intestine, diarrhea, muscle, CNS, death Wuchereria bancrofti Filariasis, elephantiasis Lymphatics Enterobius vermicularis (pinworm)
Pinworm infection Intestines
Onchocerca volvulus Onchocerciasis Blindness, dermatitis
*Most common in the United States.
Malaria is one of the most common infections worldwide. In 2012, the World Health Organization (WHO) estimated that there were 207 million cases of malaria
with an estimated 627,000 deaths; 90% were in Africa where 82% of the deaths were children younger than age 5 years.18 Malaria is caused by Plasmodium falciparum, a protozoan (unicellular) parasite. Many protozoan parasites are transmitted through vectors or ingested. Vectors
include the tsetse fly (Trypanosoma cruzi, which causes Chagas disease in South America; Trypanosoma brucei, which causes sleeping sickness in Africa) and sand fleas (leishmaniasis). Water and food can be contaminated with protozoal parasites (e.g., E. histolytica, G. lamblia). Transmission of Plasmodium is through the bite of an infected female Anopheles mosquito, where the parasite grows in the salivary gland. The initial attachment to cells depends on the presence of the microorganism in
the bloodstream or gastrointestinal tract. Microorganisms in the bloodstream have surface proteins that allow them to attach to various receptors to infect macrophages, red blood cells, or organ cells such as the liver. For example, multiplication of Plasmodium occurs in erythrocytes and results in the release of additional parasites that infect other erythrocytes. Periodic (48 to 72 hours) lysis of the erythrocytes results in anemia and induction of cytokines (e.g., TNF-α, IFN-γ, IL-1) that provoke fever, chills, sweating, headache, muscle pains, and vomiting, Severe symptoms include anemia, pulmonary edema, and other complications causing death. Neurologic complications may result from infected red blood cells adhering to endothelium in capillaries of the brain.
Countermeasures Against Infectious Microorganisms The body's innate and adaptive responses against microorganisms are numerous and involve an interaction between the immune and inflammatory systems. Pathogenic microorganisms, however, have developed means of circumventing the individual's protective defenses. Therefore prophylactic or interventive procedures have been developed either to prevent the pathogen from initiating disease (vaccines, public health measures) or to destroy the pathogen once the disease process has started (antimicrobials). Most vaccine development has focused on preventing the most severe and common infections (Table 8-8). With the initial success of antibiotic therapy, there was no perceived need for vaccination against many common and non–life-threatening infections. The increasing problem of antibiotic-resistant pathogens, however, has forced a reappraisal of that strategy, and a greater emphasis now is being placed on the development of new vaccines.
TABLE 8-8 Reduction in Vaccine-Preventable Diseases in the United States as of 2009
Disease Baseline 20th Century Annual Cases* 2011* Cases % Reduction Diphtheria 175,885 0 100 Measles 503,282 212 99.9 Mumps 152,209 370 99.4 Pertussis 147,271 15,216 90.8 Smallpox 48,164 0 100 Polio 16,316 0 100 Rubella 47,745 4 99.9 Tetanus 1,314 9 99.9 Haemophilus influenzae type b, invasive 20,000 1,170 94.2
*Average number of reported cases over multiple years before initiation of vaccine.
From Centers for Disease Control and Prevention: *2012 data from provisional cases of selected notifiable diseases, MMWR Morb Mortal Wkly Rep 60(51):1762-1765, 2011. Available at: http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/G/cases-deaths.pdf.
Infection Control Measures Although effective means of safeguarding populations from exposure to infectious disease are well-known, lack of implementation or breakdowns in application of these initiatives has led to the reemergence of some infectious diseases, particularly in less developed countries. The following are some examples of environmental infection control measures:
1. Sanitary disposal of sewage, garbage, and animal waste
2. Provision of water treatment and prevention of water contamination
3. Maintenance of sanitation practices for the transport, preparation, and serving of food
4. Control of insect vectors by draining standing water and implementation of mosquito eradication programs
5. Support of research to develop safe agents for insecticide-resistant insect vectors
Antimicrobials Since initiation of the widespread use of penicillin during World War II, antibiotics have significantly prevented the spread of infections. Antibiotics are natural products of fungi, bacteria, and related microorganisms that affect the growth of
other microorganisms. Some antibacterial antibiotics are bactericidal (kill the microorganism), whereas others are bacteriostatic (inhibit growth until the microorganism is destroyed by the individual's own protective mechanisms). The mechanisms of action of most antibiotics are (1) inhibition of the function or production of the cell wall/membrane, (2) prevention of protein synthesis, (3) blockage of DNA replication, or (4) interference with folic acid metabolism (Table 8-9). Because viruses use the enzymes of the host's cells, there has been far less success in developing antiviral antibiotics.
TABLE 8-9 Chemicals or Antimicrobials Identified That Prevent Growth of or Destroy Microorganisms
Mechanism of Action Agents Inhibits synthesis of cell wall Penicillins, cephalosporins, monobactams, carbapenems, vancomycin, bacitracin, cycloserine,
fosfomycin Cell membrane inhibitors Amphotericin, ketoconazole, polymycin Damages cytoplasmic membrane Polymyxins, polyene antifungals, imidazoles Alters metabolism of nucleic acid Quinolones, rifampin, nitrofurans, nitroimidazoles Inhibits protein synthesis Aminoglycosides, tetracyclines, chloramphenicol, macrolides, clindamycin, spectinomycin Inhibits folic acid synthesis (needed for protein synthesis)
Sulfonamides, trimethoprim
Alters energy metabolism Trimethoprim, dapsone, isoniazid
Adapted from Brenner GM, Stevens CW: Pharmacology, ed 4, Philadelphia, 2013, Saunders.
Immediately after antibiotics became widely used, antibiotic-resistant microorganisms were observed. By 1944 an adequate supply of penicillin allowed its widespread use to treat infections. In 1946 a hospital in Britain reported that 14% of all Staphylococcus aureus infections were penicillin resistant, producing β- lactamase, an enzyme that destroys penicillin. The same hospital reported an increase to 59% by 1950 and to greater than 89% in the 1990s. More than 2 million individuals develop antibiotic-resistant infections yearly,
resulting in more than 23,000 deaths. Antibiotic resistance to a single antibiotic has rapidly progressed to multiple-antibiotic resistance. The CDC released a lengthy document on Antibiotic Resistance Threats in the United States, 2013, in which 18 pathogens were sorted into “Urgent Threats,” “Serious Threats,” and “Concerning Threats.”19 The most urgent threats are Clostridium difficile (C. difficile), carbapenem (an “antibiotic of last resort” against penicillin-resistant organisms) resistant Enterobacteriaceae species (i.e., Klebsiella and E. coli), and drug-resistant Neisseria gonorrhoeae (N. gonorrhoeae). Many other infections considered routine and easily treatable are now resistant to
almost all currently available antibiotics, including methicillin-resistant Staphylococcus aureus [MRSA] and Streptococcus pneumoniae, which causes
pneumonia, meningitis, and acute otitis media (middle ear infection), which were once routinely susceptible to penicillin. Additionally, there are major increases in resistant Salmonella typhi (typhoid fever), Shigella (bloody diarrhea), Acinetobacter (pneumonia), Campylobacter (bloody diarrhea), Enterococcus (sepsis, wound infection, urinary tract infection), Pseudomonas aeruginosa (burn infection, sepsis), and Mycobacterium tuberculosis (tuberculosis).20 Antibiotic-resistant fungi (e.g., fluconazole-resistant Candida albicans) have evolved and malarial parasites have recently developed broad drug resistance, including to chloroquine—the previous mainstay of the preventive and therapeutic arsenal of antimalarial drugs. Antibiotic resistance is usually a result of genetic mutations that can be
transmitted directly to neighboring microorganisms by plasmid exchange or incorporation of free DNA. Some microorganisms can inactivate antibiotics, penicillin resistance being the classic example. Other forms of resistance result from modification of the target molecule. Azidothymidine (AZT) is a family of antivirals that suppresses the enzymatic activity of reverse transcriptase, a viral- specific enzyme responsible for the replication of viral RNA and production of a DNA copy. HIV frequently mutates and produces an AZT-resistant reverse transcriptase. Multidrug transporters in the microorganism's membrane mediate a third mechanism of resistance. These transporters affect the rate of intracellular accumulation of the antimicrobial by preventing entrance or, more commonly, by increasing active efflux of the antibiotic. Antibiotic-resistant strains of M. tuberculosis are protected from aminoglycosides and tetracycline by a multidrug pump that increases efflux. Why have multiple antibiotic–resistant microorganisms appeared? Lack of
compliance in completing the therapeutic regimen with antibiotics allows the selective resurgence of microorganisms that are more relatively resistant to the antibiotic. Overuse of antibiotics can lead to the destruction of the normal microbiome, allowing the selective overgrowth of antibiotic-resistant strains or pathogens that had previously been controlled. There also is concern that overuse of antibiotics to promote growth in cattle results in ingestion of antibiotic-containing meat.21
Active Immunization Recovery from an infection generally results in the strongest resistance to a future infection with the same microbe. Vaccines are biologic preparations of antigens that when administered stimulate production of protective antibodies or cellular immunity against a specific pathogen without causing potentially life-threatening disease. The purpose of vaccination is to induce long-lasting protective immune
responses under safe conditions. The primary immune response from vaccination is generally short lived; therefore booster injections are used to push the immune response through multiple secondary responses that result in large numbers of memory cells and sustained protective levels of antibody or T cells, or both. Mass vaccination programs have been tremendously successful and have led to
major changes in the health of the world's population.22 In the early 1950s an estimated 50 million cases of smallpox occurred each year, with about 15 million deaths. The World Health Organization (WHO) conducted an aggressive immunization campaign from 1967 to 1977 that resulted in the global eradication of smallpox by 1979. Many vaccines are used in the United States and the Centers for Disease Control and Prevention (CDC) provides updated vaccine schedules at their website: www.cdc.gov/vaccines/recs/schedules/default.htm. Development of a successful vaccine is costly and depends on several factors.
These include identification of the protective immune response and the appropriate antigen to induce that response. For instance, individuals with ongoing HIV infection produce a great deal of antibody against several HIV antigens. But, for development of a successful vaccine, we must first understand which antibody, if any, will protect against an initial infection. Once a good candidate antigen is identified, it must be developed into an
effective, cost-efficient, stable, and safe vaccine. Most vaccines against viral infection (measles, mumps, rubella, varicella [chickenpox]) contain live viruses that are weakened (attenuated virus) so they continue to express appropriate antigens but establish only a limited and easily controlled infection. Limited replication of the virus appears to afford better long-term protection than using viral antigen. Current exceptions are the hepatitis B vaccine, which uses a recombinant viral protein, and the hepatitis A vaccine, which is an inactivated (killed) virus and normally should not cause an infection. Even attenuated viruses can establish life-threatening infections in individuals
whose immune systems are deficient or suppressed. The risk of infection by the vaccine strain of virus is extremely small, but it may affect the choice of recommended vaccines. For instance, the Sabin polio vaccine was an attenuated virus that was administered orally. It provided systemic protection and induced a secretory immune response to prevent growth of the poliovirus in the intestinal tract. Being a live virus, the vaccine could cause polio in some children who had unsuspected immune deficiencies (about 1 case in 2.4 million doses). The Salk vaccine was a completely inactivated virus administered by injection. It induced protective systemic immunity but did not provide adequate secretory immunity. Therefore even if the individual was protected from systemic infection by poliovirus, the virus could establish a limited infection in the individual's intestinal
mucosa, be shed, and infect others. When polio was epidemic, the oral vaccine was preferred. However, the live attenuated vaccine itself caused about eight cases of paralytic polio per year in the United States in individuals with inadequate immune systems. As a result, the current recommendation of the CDC is vaccination with the killed virus. Some common bacterial vaccines are killed microorganisms or extracts of
bacterial antigens. The vaccine against pneumococcal pneumonia consists of a mixture of capsular polysaccharides from 23 strains of Streptococcus pneumoniae. Of the more than 90 known strains of this microorganism, these 23 cause the most severe illnesses. However, the capsular vaccine is not very immunogenic in young children. A conjugated vaccine is available that contains capsular polysaccharides from 13 strains conjugated to carrier proteins in order to increase immunogenicity. A similar vaccine is available for Haemophilus influenzae type b (Hib). Some bacterial pathogens are not invasive, but colonize mucosal membranes or
wounds and release potent exotoxins that act locally or systemically. Vaccination against systemic exotoxins (e.g., diphtheria, tetanus, pertussis) has been achieved using toxoids—purified exotoxins that have been chemically detoxified without loss of immunogenicity. Pertussis (whooping cough) vaccine has been changed from a killed whole-cell vaccine to cellular extract (acellular) vaccine that contains the pertussis toxoid and additional bacterial antigens. This change has dramatically reduced adverse side effects (fever, local inflammatory reactions, and others) of vaccination. With so many recommended vaccines, there has been an effort to combine
vaccines in order to minimize the number of required injections. One of the first licensed vaccine mixtures was DPT, which now usually contains diphtheria (D) and tetanus (T) toxoids and acellular pertussis vaccine (aP). More recent mixtures include DTaP with inactivated poliovirus, either with Hib conjugate to tetanus toxoid or with hepatitis B antigen. Common problems confronting vaccination programs include access to the
programs in less developed countries or lack of compliance of the susceptible population even when vaccination programs are available. A certain percentage of the population will be genetically unresponsive or less responsive to a particular vaccine and therefore will not produce a protective immune response. As many as 10% of the population may not respond adequately to the recommended series of injections. With most vaccines, the percentage of unresponsive individuals is low, and they will benefit from successful immunization of the rest of the population. Depending on the microorganism, a certain percentage of the population (usually about 85%) should be immunized in order to achieve protection of the total population. This is referred to as herd immunity. If this level of immunization is not
achieved, outbreaks of infection can occur. More recently resistance to immunization with measles has increased, and in early 2008 the number of measles cases in the United States increased by about fourfold. In several European countries antivaccine groups have disrupted immunization programs. As a result the incidence of pertussis (whooping cough) increased by 10 to 100 times in those countries compared with neighboring countries that maintained a high incidence of immunization. Immunizations should be complete before children start school. The reluctance to vaccinate has generally been based on potential vaccine
dangers.23 As with any medicine, complications can arise. In the case of vaccines, these include pain and redness at the injection site, fever, allergic reactions to vaccine ingredients, infection associated with attenuated viruses in immune-deficient individuals, and others. More severe dangers do exist, although they are extremely rare. More commonly the reluctance to vaccination is based on inadequate information.24 A common fear related to the presence of the preservative thimerosal in vaccines. Thimerosal is a mercury-containing compound that had been used as a preservative since the 1930s. Although no cases of mercury toxicity have been reported secondary to vaccination, thimerosal was removed from all vaccines in 2001, with the exception of some inactivated influenza vaccines. In 2003 groups in northern Nigeria claimed that the oral vaccine was unsafe and were tainted with antifertility drugs (estradiol), HIV, and cancer-causing agents.25 The reasoning appeared to be secondary to mounting distrust of Western nations because of conflicts in the Middle East. The effect was suspension of polio immunization for almost 1 year in two Nigerian states and reduction of immunization in three other states. The incidence of polio rose dramatically, and more than 27,000 cases of paralysis resulted. The goal of the WHO is to eradicate polio worldwide by 2022. As of November 2014, the total global number of wild polio (naturally occurring) cases was 291; the highest number of cases were in Pakistan (246).26
Passive Immunotherapy Passive immunotherapy is a form of countermeasure against pathogens in which preformed antibodies are given to the individual. Passive immunotherapy with human immunoglobulin has been approved for several infections, including hepatitis A and hepatitis B. Treatment of potential rabies infection after a bite combines passive and active immunization. The rabies virus proliferates very slowly.27 Individuals who have been bitten receive a onetime injection with human rabies immunoglobulin, or, more recently, with monoclonal antibody to slow further viral proliferation, followed by multiple injections with a killed viral vaccine to induce greater protective immunity. More specific therapy with
monoclonal antibodies is being evaluated for other infectious diseases. A monoclonal antibody against respiratory syncytial virus has been approved for therapy, and recently an experimental monoclonal antibody preparation seems to have neutralized the Ebola virus. In the past, vaccines and therapeutic antibodies were developed only for the most
deadly pathogens. With the increase in antibiotic-resistant microorganisms, the development and widespread use of new vaccines and antibodies against these microorganisms must be considered.28
Quick Check 8-1
1. How do antigenic changes in viral pathogens promote disease?
2. What are three mechanisms pathogens use to block the immune system?
3. What is the difference between an endotoxin and an exotoxin?
4. How do bacteria develop antibiotic resistance?
Deficiencies in Immunity An immune deficiency is the failure of the immune or inflammatory response to function normally, resulting in increased susceptibility to infections. Primary (congenital) immune deficiency is caused by a genetic defect, whereas secondary (acquired) immune deficiency is caused by another condition, such as cancer, infection, or normal physiologic changes, such as aging. Acquired forms of immune deficiency are far more common than the congenital forms.
Initial Clinical Presentation The clinical hallmark of immune deficiency is a tendency to develop unusual or recurrent, severe infections. The most severe primary immune deficiencies develop in young children, 2 years old and younger. Preschool and school-age children normally may have 6 to 12 infections per year, and adults may have 2 to 4 infections per year. Most of these are not severe and are limited to viral infections of the upper respiratory tract, recurrent streptococcal pharyngitis, or mild otitis media (middle ear infections). Potential immune deficiencies should be considered if the individual has
experienced severe, documented bouts of pneumonia, otitis media, sinusitis (sinus infection), bronchitis, septicemia (blood infection), or meningitis or infections with rare opportunistic microorganisms (e.g., Pneumocystis carinii).29 Infections are generally recurrent with only short intervals of relative health, and multiple simultaneous infections are common. Individuals with immune deficiencies often have eight or more purulent ear infections, two or more serious sinus infections, and two or more pneumonias, recurrent abscesses, or persistent fungal infections (particularly thrush) within a year. Invasive fungal infections are rare in healthy individuals and strongly indicate a defective immune system. Recurrent internal infections, such as meningitis, osteomyelitis, or sepsis, are common. Prolonged antibiotic use is commonly ineffective by oral or injected routes and may necessitate intravenous administration. Children frequently present with failure to thrive because of chronic diarrhea and other chronic symptoms. A familial history of immune deficiency may be found in some types of primary deficiency. Routine care of individuals with immune deficiencies must be tempered with the
knowledge that the immune system may be totally ineffective. It is unsafe to administer conventional immunizing agents or blood products to many of these individuals because of the risk of causing an uncontrolled infection. Infection is a particular problem when attenuated vaccines that contain live but weakened microorganisms are used (e.g., live polio vaccine; vaccines against measles,
mumps, and rubella). The type of recurrent infections may indicate the type of immune defect.
Deficiencies in T-cell immune responses are associated with recurrent infections caused by certain viruses (e.g., varicella herpes, cytomegalovirus), fungi, and yeasts (e.g., Candida, Histoplasma), or atypical microorganisms (e.g., P. carinii). B-cell deficiencies and phagocyte deficiencies, however, are suggested if the individual has documented, recurrent infections with microorganisms that require opsonization (e.g., encapsulated bacteria) or with viruses against which humoral immunity is normally effective (e.g., rubella). Some complement deficiencies resemble defects in antibody or phagocyte function, but others are associated with disseminated infections with bacteria of the genus Neisseria (Neisseria meningitides and Neisseria gonorrhoeae).
Primary (Congenital) Immune Deficiencies Most primary immune deficiencies are the result of single gene defects (Table 8-10). Generally, the mutations are sporadic and not inherited: a family history exists in only about 25% of individuals. The sporadic mutations occur before birth, but the onset of symptoms may be early or later, depending on the particular syndrome. In some instances, symptoms of immune deficiency appear within the first 2 years of life. Other immune deficiencies are progressive, with the onset of symptoms appearing in the second or third decade of life.
TABLE 8-10 Examples of Primary Immune Deficiencies
Classification Example Immune Deficiency Outcome Combined Immune Deficiencies: Without Nonimmune Defects Defective development of both B and T cells
Severe combined immunodeficiencies (SCIDs) X-linked SCID
Lack of both T and B cells, little or no antibody production or cellular immunity Defective interleukin receptors needed for lymphocyte maturation
Recurrent, life-threatening infections with variety of microorganisms Recurrent, life-threatening infections with variety of microorganisms
Defects in cooperation among B cells, T cells, and antigen- presenting cells
Bare lymphocyte syndrome
No antigen presentation because of lack of MHC class I or MHC class II molecules on cell surface
Recurrent, life-threatening infections with variety of microorganisms
Combined Immune Deficiencies: With Nonimmune Defects Defect in actin cytoskeleton Wiskott-Aldrich
syndrome (WAS) Decreased IgM antibody Recurrent infections with encapsulated bacteria;
thrombocytopenia; eczema Defective development of T cells in central lymphoid organ (thymus)
DiGeorge syndrome Lack of T cells Recurrent, life-threatening fungal and viral infections; defective parathyroid gland; abnormal facial development
Predominantly Antibody Deficiencies Defect in class-switch to IgA Selective IgA
deficiency Diminished or absent IgA Asymptomatic or recurrent mild sinus, pulmonary,
and gastrointestinal infections Defect in development of B cells in the bone marrow
Bruton agammaglobulinemia
Few B cells Recurrent bacterial infections
Phagocytic Defects Defects in production of neutrophils
Severe congenital neutropenia
Lack of neutrophils Recurrent, life-threatening bacterial infections
Defects in bacterial killing Chronic granulomatous disease
Lack of production of oxygen products (e.g., hydrogen peroxide)
Recurrent infections with bacteria that are sensitive to killing by oxygen-dependent mechanisms
Defects in Innate Immunity Defect in development of cellular immunity against specific antigen
Chronic mucocutaneous candidiasis
Lack of T-cell response to Candida Recurrent and disseminated infections with fungus Candida albicans
Complement Deficiencies Defective production of C3 C3 deficiency Little or no C3 produced Recurrent, life-threatening bacterial infections Defective production of component of membrane attack complex
C6, C7, C8, or C9 deficiency
Little or no C6, C7, C8, or C9 produced Recurrent disseminated infections with Neisseria gonorrhoeae or N. meningitides
Defective production of component of lectin pathway
Mannose-binding lectin (MBL) deficiency
Little or no activation of lectin pathway Recurrent infections with bacteria and yeast with mannose-containing capsules
Individually, primary immune deficiencies are rare. For instance, only 30 to 50 new cases of severe combined immunodeficiency (SCID) are diagnosed in the United States yearly. However, more than 250 different deficiencies have been identified, and the number is growing rapidly.30 Together, primary immune deficiencies are more common than cystic fibrosis, hemophilia, childhood leukemia, or many other well-known diseases. Many are subtle with minor deficiencies, but several result from major defects and lead to recurrent life- threatening infections. The distribution between genders is about even, although some specific diseases have a male or female predominance. The three most commonly diagnosed deficiencies are common variable immune deficiency (34% of individuals with primary immune deficiencies), selective immunoglobulin A (IgA) deficiency (24%), and IgG subclass deficiency (17%). Primary immune deficiencies have recently been reclassified into nine groups,
based on the principal component of the immune or inflammatory systems that is defective.31 The major groups include combined with or without nonimmune defects (both B and T lymphocytes are deficient, although this group contains some diseases previously classified as T-cell defects), predominantly antibody deficiencies, immune dysregulation (defects in control of lymphocyte proliferation, T-regulatory cells defects), phagocytic defects (inadequate numbers or function), defects in innate immunity, and complement defects. To provide a better understanding of the diversity and severity of primary immune deficiencies, a few select examples will be discussed.
Combined Deficiencies Combined deficiencies include the most life-threatening disorders and result from defects that directly affect the development of both T and B lymphocytes. However, the severity depends upon the degree to which B and T cells are affected.32 The most severe disorders are called severe combined immunodeficiencies (SCIDs). Most individuals with SCIDs have few detectable lymphocytes in the circulation and secondary lymphoid organs (spleen, lymph nodes). The thymus usually is underdeveloped because of the absence of T cells. Immunoglobulin levels, especially IgM and IgA, are absent or greatly reduced. Several forms of SCID are caused by autosomal recessive enzymatic defects that result in the accumulation of toxic metabolites, and rapidly dividing cells, such as lymphocytes, are especially sensitive. For instance, deficiency of adenosine deaminase (ADA deficiency) results in the accumulation of toxic purines. X-linked SCID results from a common defect in most of the important interleukin (IL) receptors needed for lymphocyte maturation (e.g., IL-2, IL-4, IL-7, and others). Even if nearly adequate numbers of B and T cells are produced, their cooperation
may be defective. The bare lymphocyte syndrome is an immune deficiency characterized by an inability of lymphocytes and macrophages to produce major histocompatibility complex (MHC) class I or class II molecules. Without MHC molecules, antigen presentation and intercellular cooperation cannot occur effectively. Children with this deficiency develop serious, life-threatening infections and usually die before the age of 5 years. Some combined immune deficiencies result in depressed development of a small
portion of the immune system. For instance, an individual can be unable to produce a certain class of antibody, as in Wiskott-Aldrich syndrome (WAS, an X-linked recessive disorder), where IgM antibody production is greatly depressed. Antibody responses against antigens that elicit primarily an IgM response, such as polysaccharide antigens from bacterial cell walls (e.g., P. aeruginosa, S.
pneumoniae, Haemophilus influenzae, and other microorganisms with polysaccharide outer capsules), are deficient. Many combined immune deficiencies also are associated with other characteristic
defects, some of which appear to be unrelated to the immune system yet may be life- threatening by themselves. These associated symptoms can be useful diagnostically and can clarify the pathophysiology of the disease. WAS results from a mutation in the WAS gene that affects the actin cytoskeleton, which is important for platelet function. Thus, WAS has an associated major defect in platelet function and is classified as a combined deficiency with nonimmune defects. Clinical manifestations include bleeding secondary to thrombocytopenia (low platelet counts), eczema, and recurrent infections (e.g., otitis media, pneumonia, herpes simplex, cytomegalovirus). DiGeorge syndrome (congenital thymic aplasia or hypoplasia and diminished
parathyroid gland development) is caused by the lack or partial lack of the thymus, resulting in greatly decreased T-cell numbers and function. Defective development of the third and fourth pharyngeal pouches during embryonic development results in the thymic defects and the lack of the parathyroid gland (causing an inability to regulate calcium concentration). Low blood calcium levels cause the development of tetany or involuntary rigid muscular contraction. DiGeorge syndrome is frequently associated with abnormal development of facial features that are controlled by the same embryonic pouches; these include low-set ears, fish-shaped mouth, and other altered features (Figure 8-6). Other examples of combined immune deficiencies include defects in CD3 resulting in the loss of T-cell receptor intracellular signaling, defective somatic gene rearrangement of variable region genes or constant region genes, IL-2 receptor defects, and defects in DNA repair.
FIGURE 8-6 Facial Anomalies Associated with DiGeorge Syndrome. Note the wide-set eyes, low-set ears, and shortened structure of the upper lip. (From Male D et al: Immunology, ed 8, Philadelphia, 2013,
Mosby.)
Predominantly Antibody Deficiencies Predominantly antibody deficiencies result from defects in B-cell maturation or function and are the most common of immune deficiencies.33 T-cell immune responses are not affected in pure B-lymphocyte deficiencies. The results are lower levels of circulating immunoglobulins (hypogammaglobulinemia) or occasionally totally or nearly absent immunoglobulins (agammaglobulinemia). Some defects may involve a particular class of antibody, such as selective IgA
deficiency, in which only IgA is suppressed. This occurs in 1 in 700 to 1 in 400 individuals and may result from a failure to class-switch to IgA and mature into IgA-producing plasma cells. Many individuals are asymptomatic, although others have a history of recurring sinus, pulmonary, and gastrointestinal infections. Individuals with IgA deficiency often have chronic intestinal candidiasis (infection with C. albicans). Complications of IgA deficiency include severe allergic disease and autoimmune diseases. Secretory IgA normally may prevent the uptake of allergens from the environment; therefore IgA deficiency may lead to a more intense challenge to the immune system by environmental antigens. Bruton agammaglobulinemia is caused by blocked development of mature B
cells in the bone marrow. There are few or no circulating B cells, although T-cell number and function are normal, resulting in repeated bacterial infections, such as otitis media, streptococcal sore throat, and conjunctivitis, and more serious conditions, such as septicemia.
Other predominantly antibody deficiencies include severe reduction in particular classes or subclasses of antibody; defects in B-cell surface receptors, such as CD21 and CD40; and defects in class-switch, which may result in a hyper-IgM syndrome.
Phagocyte Defects Phagocyte defects range from inadequate numbers of phagocytes (e.g., severe congenital neutropenia) to defects in phagocyte function that can result in recurrent infections with the same group of microorganisms (encapsulated bacteria) associated with antibody, and complement deficiencies. Chronic granulomatous disease (CGD) is a severe defect in the myeloperoxidase–hydrogen peroxide system—a major means of bacterial destruction using the enzyme myeloperoxidase, halides (e.g., chloride ion), and hydrogen peroxide (H2O2).
34 As a result of phagocytosis, neutrophils and other phagocytes switch much of their glucose metabolism to the hexose-monophosphate shunt. A byproduct of this pathway is the conversion of molecular oxygen by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase into highly reactive oxygen derivatives, including hydrogen peroxide. Mutations in NADPH oxidase result in deficient production of hydrogen peroxide and other oxygen products needed for phagocytic killing. Thus, affected individuals have adequate myeloperoxidase and halide but lack the necessary hydrogen peroxide. This results in recurrent severe pneumonias; tumor-like granulomata in lungs, skin, and bones; and other infections with some opportunistic microorganisms, such as Staphylococcus aureus, Serratia marcescens, and Aspergillus species. Other phagocytic deficiencies include defects in various leukocyte adhesion molecules, defects in the phagocytosis process or bacterial killing, and defects in cytokine receptors.
Defects in Innate Immunity Some immune deficiencies are characterized by a defect in the capacity to produce an immune response against a particular antigen. In chronic mucocutaneous candidiasis, interaction between the Th17 lymphocytes and macrophages is ineffective related to a specific infectious agent, C. albicans. Thus the macrophage cannot be activated and these individuals usually have mild to extremely severe recurrent Candida infections involving the mucous membranes and skin. Other defects in innate immunity include defects in Toll-like receptors and natural killer cells.
Complement Deficiencies Many complement deficiencies have been described. C3 deficiency is the most
severe defect because of its central role in the complement cascade. Loss of C3b and C3a production and the inability to activate C5 result in recurrent life-threatening infections with encapsulated bacteria (e.g., Haemophilus influenzae and Streptococcus pneumoniae) at an early age. Deficiencies of any of the terminal components of the complement cascade (C5, C6, C7, C8, or C9 deficiencies) are associated with increased infections with only one group of bacteria—those of the genus Neisseria (Neisseria meningitides or N. gonorrhoeae). Neisseria bacteria usually cause localized infections (meningitis or gonorrhea), but terminal pathway defects result in an 8000-fold increased risk for systemic infections with atypical strains of these microorganisms. Mannose-binding lectin (MBL) deficiency is the primary defect of the lectin
pathway of complement activation. This defect, as well as defects in the alternative pathway, results in increased risk of infection with microorganisms that have polysaccharide capsules rich in mannose, particularly the yeast Saccharomyces cerevisiae and encapsulated bacteria such as N. meningitidis and S. pneumoniae. Other complement deficiencies include defects in components C1, C4, C2, C5, C1 inhibitor, factor B, factor D, properdin, complement control factors, MASP, or complement receptors.
Secondary (Acquired) Immune Deficiencies Secondary, or acquired, immune deficiencies are far more common than primary deficiencies. These deficiencies are complications of other physiologic or pathophysiologic conditions. Some conditions that are known to be associated with acquired deficiencies are summarized in Box 8-1.
Box 8-1 Some Conditions Known to Be Associated with Acquired Immunodeficiencies Normal Physiologic Conditions
Pregnancy
Infancy
Aging
Psychologic Stress
Emotional trauma
Eating disorders
Dietary Insufficiencies
Malnutrition caused by insufficient intake of large categories of nutrients, such as protein or calories
Insufficient intake of specific nutrients, such as vitamins, iron, or zinc
Infections
Congenital infections, such as rubella, cytomegalovirus, hepatitis B
Acquired infections, such as AIDS
Malignancies
Malignancies of lymphoid tissues, such as Hodgkin disease, acute or chronic leukemia, or myeloma
Malignancies of nonlymphoid tissues, such as sarcomas and carcinomas
Physical Trauma
Burns
Medical Treatments
Stress caused by surgery
Anesthesia
Immunosuppressive treatment with corticosteroids or antilymphocyte antibodies
Splenectomy
Cancer treatment with cytotoxic drugs or ionizing radiation
Other Diseases or Genetic Syndromes
Diabetes
Alcoholic cirrhosis
Sickle cell disease
Systemic lupus erythematosus (SLE)
Chromosome abnormalities, such as trisomy 21
Although secondary deficiencies are common, many are not clinically relevant. In many cases, the degree of the immune deficiency is relatively minor and without any apparent increased susceptibility to infection. Alternatively, the immune system may be substantially suppressed, but only for a short duration, thus minimizing the incidence of clinically relevant infections. Some secondary immune deficiencies (e.g., AIDS or immunosuppression by cancer), however, are extremely severe and may result in recurrent life-threatening infections.
Evaluation and Care of Those with Immune Deficiency A review of clinical characteristics can help select the appropriate tests. A basic screening test is a complete blood count (CBC) with a differential. The CBC provides information on the numbers of red blood cells, white blood cells, and platelets, and the differential indicates the quantities of lymphocytes, granulocytes, and monocytes in the blood. Quantitative determination of immunoglobulins (IgG, IgM, IgA) is a screening test for antibody production, and an assay for total complement (total hemolytic complement, CH50) is useful if a complement defect is suspected. Further testing is described in Table 8-11.
TABLE 8-11 Laboratory Evaluation of Immune Deficiencies
Function Tested
Laboratory Test Significance of Test
Tests of Humoral Immune Function Antibody production
Total immunoglobulin levels, including IgG, IgM, and IgA
Decrease or absence of total antibody production or of specific classes of antibody, which is associated with many B-cell and combined deficiencies
Levels of isohemagglutinins Production of specific IgM antibodies, which is decreased in some combined deficiencies; not useful with persons who are blood type AB and do not have naturally occurring isohemagglutinins
Levels of antibodies against vaccines—especially diphtheria and tetanus toxoids
Production of specific IgG antibodies, which is decreased when B cells are deficient or class-switch is blocked
B-cell numbers Numbers of lymphocytes with surface immunoglobulin
Production of circulating B cells, which is decreased in many severe B-cell or combined deficiencies
Antibody subclasses
Level-specific subclasses, particularly IgG1, IgG2, and IgG3
Decrease or absence of a particular subclass, which is characteristic of several immune deficiencies
Tests of Cellular Immune Function Delayed hypersensitivity skin test
Skin test reaction against previously encountered antigens, especially Candida albicans or tetanus toxoid
Defects in antigen-responsive T cells and skin test cellular interactions (e.g., lymphokine activity and macrophage function)
T-cell numbers Numbers of T cells expressing characteristic membrane antigens (CD3 or CD11)
Defects in production of circulating T cells
T-cell proliferation in vitro
Proliferative response to nonspecific mitogens (e.g., phytohemagglutinin)
General T-cell defects in response to nonspecific stimulation (mitogens)
Proliferative response to antigens (e.g., tetanus toxoid)
Defects in response of T cells to specific antigens
T-cell subpopulations
Quantify percentage of T cells with specific markers for total T cells (CD3), Th cells (CD4), Tc cells (CD8)
Decrease in numbers of CD4 cells, which is related to AIDS progression
Replacement Therapies for Immune Deficiencies Many immune deficiencies can be successfully treated by replacing the missing component of the immune system. Individuals with B-cell deficiencies that cause hypogammaglobulinemia or agammaglobulinemia usually are treated by administration of intravenous immune globulin (IVIg), antibody-rich fractions prepared from plasma pooled from large numbers of donors.35 Administration of IVIg replaces the individual's antibodies temporarily; these antibodies have a half- life of 3 to 4 weeks. Thus individuals must be treated repeatedly to maintain a protective level of antibodies in the blood. Defects in lymphoid cell development in the primary lymphoid organs (e.g.,
SCID, Wiskott-Aldrich syndrome) can sometimes be treated by replacement of stem cells through transplantation of bone marrow, umbilical cord cells, or other cell populations that are rich in stem cells. Thymic defects (e.g., DiGeorge syndrome, chronic mucocutaneous candidiasis) may be treated by transplantation of fetal thymus tissue or thymic epithelial cells (the cells that produce thymic hormones). However, in most cases improvement is only temporary. Enzymatic defects that cause SCID (e.g., adenosine deaminase deficiency) have
been treated successfully with transfusions of glycerol frozen-packed erythrocytes.
The donor erythrocytes contain the needed enzyme and can, at least temporarily, provide sufficient enzyme for normal lymphocyte function. Bone marrow transplants containing hematopoietic stem cells are routinely used
to treat SCID. However, as discussed later in this chapter, the donor and recipient should be matched as closely as possible for HLA antigens. Individuals with SCID are at risk for graft-versus-host disease (GVHD). This occurs if T cells in a transplanted graft (e.g., transfused blood, bone marrow transplants) are mature and therefore capable of cell-mediated immunity against the recipient's HLA. The primary targets for GVHD are the skin (e.g., rash, loss or increase of pigment, thickening of skin), liver (e.g., damage to bile duct, hepatomegaly), mouth (e.g., dry mouth, ulcers, infections), eyes (e.g., burning, irritation, dryness), and gastrointestinal tract (e.g., severe diarrhea), and the disease may lead to death from infections. The risk of GVHD can be diminished by removing mature T cells from tissue used to treat individuals with immune deficiencies.36 Injection of mesenchymal stem cells (MSCs) may be useful in these individuals.
Stem cells are relatively undifferentiated cells and can be obtained from a variety of sources (e.g., embryos, bone marrow, adult tissues). MSCs are present in all adult tissues. These particular stem cells undergo differentiation into other cell types and, more importantly, have potent immunosuppressive properties.37 Several clinical trials have demonstrated complete suppression of GVHD in a large number of recipients of MSCs.38 The first successful therapeutic replacement of defective genes was performed in
two girls with SCID caused by an ADA deficiency.39 The normal gene for ADA was cloned and inserted into a retroviral vector.40 The gene for ADA replaced some retroviral genes, resulting in a virus that carried the normal human gene but did not cause disease. The virus was used to infect bone marrow stem cells from these children. The retrovirus inserted the normal ADA gene into the individuals' genetic material. The genetically altered stem cells were infused into the children, resulting in reconstitution of their immune systems. Gene therapy trials have verified immune reconstitution in individuals with ADA deficiency, X-linked SCID, CGD, and WAS.41 However, the treatment trials have not been without some major complications, such as leukemia, that raise questions concerning the use of retroviral vectors for the insertion of new genes.
Acquired Immunodeficiency Syndrome (AIDS) Acquired immunodeficiency syndrome is a secondary immune deficiency that develops in response to viral infection. The human immunodeficiency virus (HIV) infects and destroys the CD4-positive (CD4+) Th cells, which are necessary for the
development of both plasma cells and cytotoxic T cells. Therefore HIV suppresses the immune response against itself and secondarily creates a generalized immune deficiency by suppressing the development of immune responses against other pathogens and opportunistic microorganisms, leading to the development of acquired immunodeficiency syndrome (AIDS). Despite major efforts by healthcare agencies around the world, the number of
cases and deaths from HIV infection and AIDS (HIV/AIDS) remains a major health concern. The WHO estimated that at the end of 2013, 35.3 million people were living with HIV/AIDS worldwide and more than 2.5 million were newly infected.42 Approximately 3 million deaths occur each year from AIDS. Since 1980 it is estimated that more than 36 million individuals have died from AIDS worldwide. The majority of cases are in sub–Saharan Africa where about 1 in 20 adults is living with HIV, but the epidemic is worldwide and the number of new cases is increasing rapidly, particularly in Asia. In the United States the spread of HIV/AIDS remains somewhat stable. The CDC
estimated in 2013 (the most recent data) that approximately 47,352 people were newly infected with HIV.43 Although new infections remain at about 50,000 per year, both encouraging and discouraging trends were apparent. Although new HIV infections in black women decreased by 12% between 2008 and 2010, new infections in young gay and bisexual men increased by 21%. Men who had sex with men accounted for 78% of new HIV infections in 2010. Heterosexual transmission accounted for about 25% of new HIV infections. with two thirds of those cases occurring in women, with the highest number among black women. Deaths related to HIV/AIDS were 13,712 in 2012, and appear to be decreasing. The cumulative number of HIV/AIDS-related deaths in the United States is in excess of 658,507, and more than 1,201,100 persons age 13 years and older are currently living with HIV/AIDS. Before the implementation of massive public health campaigns and the use of
antiviral drugs in the United States, the progression from HIV infection to AIDS and death was unrelenting. In 1995 AIDS became the number one killer of individuals between the ages of 25 and 44 years and remains the eighth most common cause of death in that age group. With the advent of effective therapy to stabilize progression of the disease in the mid-1990s, HIV infection has become a chronic disease in the United States, with many fewer deaths.
Epidemiology of AIDS HIV is a blood-borne pathogen with the typical routes of transmission: blood or blood products, intravenous drug abuse, both heterosexual and homosexual activity,
and maternal-child transmission before or during birth. Although the disease first gained attention in the United States related to sexual transmission between males, the most common route worldwide is through heterosexual activity (see Health Alert: Risk of HIV Transmission Associated with Sexual Practices). Worldwide, women constitute more than half of those living with HIV/AIDS. In the United States, as in the rest of the world, the predominant means of transmission to women is through heterosexual contact. Hundreds of thousands of cases of HIV/AIDS have been reported in children who contracted the virus from their mothers across the placenta, through contact with infected blood during delivery, or through the milk during breast-feeding.
Health Alert Risk of HIV Transmission Associated with Sexual Practices
High Risk (in descending order of risk)
Receptive anal intercourse with ejaculation (no condom)
Receptive vaginal intercourse with ejaculation (no condom)
Insertive anal intercourse (no condom)
Insertive vaginal intercourse (no condom)
Receptive anal intercourse with withdrawal before ejaculation
Insertive anal intercourse with withdrawal before ejaculation
Receptive vaginal intercourse (with spermicidal foam but no condom)
Insertive vaginal intercourse (with spermicidal foam but no
condom)
Receptive anal or vaginal intercourse (with a condom)
Insertive anal or vaginal intercourse (with a condom)
Some Risk (in descending order of risk)
Oral sex with men with ejaculation
Oral sex with women
Oral sex with men with preejaculation fluid (precum)
Oral sex with men, no ejaculation or precum
Oral sex with men (with a condom)
Some Risk (depending on situation, intactness of mucous membranes, etc.)
Mutual masturbation with external or internal touching
Sharing sex toys
Anal or vaginal fisting
No Risk
Masturbating with another person without touching one another
Hugging/massage/dry kissing
Frottage (rubbing genitals while remaining clothed)
Masturbating alone
Abstinence
Unresolved Issues
The role of precum in transmission
The protection offered by covering female genitals with a dental dam during oral sex on the women
The risk of transmission from wet kissing
Pathogenesis of AIDS HIV is a member of a family of viruses called retroviruses, which carry genetic information in the form of RNA rather than DNA (Figure 8-7). Retroviruses use a viral enzyme, reverse transcriptase, to convert RNA into double-stranded DNA. Using a second viral enzyme, HIV integrase, the new DNA is inserted into the infected cell's genetic material, where it may remain dormant. If the cell is activated, translation of the viral information may be initiated, resulting in the formation of new virions, lysis and death of the infected cell, and shedding of infectious HIV particles. During that process, HIV protease is essential in processing proteins needed from the viral internal structure (capsid). If, however, the cell remains relatively dormant, the viral genetic material may remain latent for years and is probably present for the life of the individual.
FIGURE 8-7 The Structure and Genetic Map of HIV-1. The HIV-1 virion consists of a core of two identical strands of viral RNA molecules of viral enzymes (reverse transcriptase [RT], protease [PR], integrase [IN]) encoated in a core capsid structure consisting primarily of the structural viral protein p24. The capsid is further encased in a matrix consisting primarily of viral protein p17. The outer surface is an envelope consisting of the plasma membrane of the cell from
which the virus budded (lipid bilayer) and two viral glycoproteins: a transmembrane glycoprotein, gp41, and a noncovalently attached surface protein, gp120. The HIV-1 genome contains regions that encode the structural proteins (gag), the viral enzymes (pol), and the envelope proteins (env). The genome of complex retroviruses, like HIV-1, often contains a
variety of small regions that regulate expression of the virus. (Modified from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
The primary surface receptor on HIV is the envelope protein gp120, which binds to the molecule CD4 on the surface of Th cells. Several other necessary co- receptors, particularly the chemokine receptor CCR5, have been identified on target
cells. Thus the major immunologic finding in AIDS is the striking decrease in the number of CD4+ Th cells (Figure 8-8).
FIGURE 8-8 Life Cycle and Possible Sites of Therapeutic Intervention of Human Immunodeficiency Virus (HIV). The HIV virion consists of a core of two identical strands of viral
RNA encoated in a protein structure with viral proteins gp41 and gp120 on its surface (envelope). HIV infection begins when a virion binds to CD4 and chemokine co-receptors on a susceptible cell and follows the process described here. The provirus may remain latent in the cell's DNA until it is activated (e.g., by cytokines). The HIV life cycle is susceptible to blockage at
several sites (see the text for further information), including entrance inhibitors, reverse transcriptase inhibitors, integrase inhibitors, and protease inhibitors. (Modified from Kumar V et al, editors:
Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
Clinical Manifestations of AIDS Depletion of CD4+ cells has a profound effect on the immune system, causing a severely diminished response to a wide array of infectious pathogens and cancers (Box 8-2). At the time of diagnosis, the individual may present with one of several different conditions: serologically negative (no detectable antibody), serologically positive (positive for antibody against HIV proteins) but asymptomatic, early stages of HIV disease, or AIDS (Figure 8-9).
Box 8-2 AIDS-Defining Opportunistic Infections and
Neoplasms Found in Individuals with HIV Infection Infections
Protozoal and Helminthic Infections
Cryptosporidiosis or isosporiasis (enteritis)
Pneumocystosis (pneumonia or disseminated infection)
Toxoplasmosis (pneumonia or CNS infection)
Fungal Infections
Candidiasis (esophageal, tracheal, or pulmonary)
Coccidioidomycosis (disseminated)
Cryptococcosis (CNS infection)
Histoplasmosis (disseminated)
Bacterial Infections
Mycobacteriosis (“atypical,” e.g., Mycobacterium avium-intracellulare, disseminated or extrapulmonary
M. tuberculosis, disseminated or extrapulmonary)
Nocardiosis (pneumonia, meningitis, disseminated)
Salmonella infections (septicemia, recurrent)
Viral Infections
Cytomegalovirus (pulmonary, intestinal, retinitis, or CNS)
Herpes simplex virus (localized or disseminated)
Progressive multifocal leukoencephalopathy
Varicella-zoster virus (localized or disseminated)
Neoplasms
Invasive cancer of the uterine cervix
Kaposi sarcoma
Non-Hodgkin lymphomas (Burkitt, immunoblastic)
Primary lymphoma of brain
CNS, Central nervous system.
From Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.
FIGURE 8-9 Typical Progression from HIV Infection to AIDS in Untreated Persons. A, Clinical progression begins within weeks after infection; the person may experience symptoms of
acute HIV syndrome. During this early period, the virus progressively infects T cells and other cells and spreads to the lymphoid organs, with a sharp decrease in the number of circulating CD4+ T cells. During a period of clinical latency, the virus replicates and T-cell destruction continues, although the person is generally asymptomatic. The individual may develop HIV-
related disease (constitutional symptoms)—a variety of symptoms of acute viral infection that do not involve opportunistic infections or malignancies. When the number of CD4+ cells is
critically suppressed, the individual becomes susceptible to a variety of opportunistic infections and cancers with a diagnosis of AIDS. The length of time for progression from HIV infection to
AIDS may vary considerably from person to person. B, Laboratory tests are changing throughout infection. Antibody and Tc cell (cytotoxic T lymphocytes [CTLs]) levels change during the progression to AIDS. During the initial phase antibodies against HIV-1 are not yet detectable (window period), but viral products, including proteins and RNA, and infectious virus may be detectable in the blood a few weeks after infection. Most antibodies against HIV are not
detectable in the early phase. During the latent phase of infection antibody levels against p24 and other viral proteins, as well as HIV-specific CTLs, increase, and then remain constant until the development of AIDS. (A redrawn from Fauci AS, Lane HC: Human immunodeficiency virus disease: AIDS and related conditions. In Fauci AS et al, editors: Harrison's principles of internal medicine, ed 14, New York, 1997, McGraw-Hill; B from Kumar V et al: Robbins and Cotran
pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
The presence of circulating antibody against the HIV protein p24 followed by
more complex tests for antibodies against additional HIV proteins (e.g., Western blot analysis) or for HIV DNA (e.g., polymerase chain reaction) indicates infection by the virus, although many of these individuals are asymptomatic. Antibody appears rather rapidly after infection through blood products, usually within 4 to 7 weeks, although some individuals have been seronegative for longer periods. The period between infection and the appearance of antibody is referred to as the window period. Although a person does not have antibody against HIV, he or she may have virus growing, have virus in the blood and body fluids, and be infectious to others. Those with the early stages of HIV disease (early-stage disease) usually initially
present with relatively mild and nonspecific symptoms resembling influenza, such as headaches, fever, or fatigue. These symptoms disappear after 1 to 6 weeks, and although individuals appear to be in clinical latency the virus is actively proliferating in lymph nodes. The currently accepted definition of AIDS relies on both laboratory tests and
clinical symptoms. If the individual is positive for antibodies against HIV, the diagnosis of AIDS is made in association with various clinical symptoms (Figure 8- 10; also see Box 8-2). The symptoms include atypical or opportunistic infections and cancers, as well as indications of debilitating chronic disease (e.g., wasting syndrome, recurrent fevers). Most commonly, new cases of AIDS are diagnosed initially by decreased CD4+ T cell numbers. Individuals who are not HIV infected typically have 800 to 1000 CD4+ cells per cubic millimeter of blood, with a range from 600/mm3 to 1200/mm3. A diagnosis of AIDS can be made if the CD4+ T cell numbers decrease to less than 200/mm3. Without treatment, the average time from infection to development of AIDS is just over 10 years. Some estimates are that approximately 99% of untreated HIV-infected individuals would eventually progress to AIDS.
FIGURE 8-10 Clinical Symptoms of AIDS. A, Severe weight loss and anorexia. B, Kaposi sarcoma lesions. C, Perianal lesions of herpes simplex infection. D, Deterioration of vision from cytomegalovirus retinitis leading to areas of infection, which can lead to blindness. (A and D from
Taylor PK: Diagnostic picture tests in sexually transmitted diseases, London, 1995, Mosby; B and C from Morse SA et al, editors: Atlas of sexually transmitted diseases and AIDS, ed 4, London, 2011, Saunders.)
Treatment and Prevention of AIDS Approved AIDS medications are classified by mechanism of action: nucleoside and non-nucleoside inhibitors of reverse transcriptase (reverse transcriptase inhibitors), inhibitors of the viral protease (HIV protease inhibitors), inhibitors of the viral integrase (HIV integrase inhibitors), inhibitors of viral entrance into the target cell (HIV fusion inhibitors), and a CCR5 antagonist (inhibitor of viral
attachment) (see Figure 8-8). The current regimen for treatment of HIV infection is a combination of drugs, termed antiretroviral therapy (ART). ART protocols require a combination of synergist drugs from different classes and specific regimens (e.g., timing of drug administration, doses, drug combinations) are adapted based on age of the individual, secondary clinical symptoms (renal or hepatic insufficiency), CD4+ T cell levels, viral load, specific coinfections, pre- existing cardiac risk factors, past history of treatment failure, suspected drug resistance, and other parameters.44,45 The clinical benefits of ART are profound. Death from AIDS-related diseases has been reduced significantly since the introduction of ART. However, resistant variants to these drugs have been identified. Drug therapy for AIDS is not curative because HIV incorporates into the genetic material of the host, particularly CD4+ T memory cells, and may never be removed by antimicrobial therapy.46 Therefore drug administration to control the virus may have to continue for the lifetime of the individual. Additionally, HIV may persist in regions where the antiviral drugs are not as effective, such as the CNS. The chronic nature of HIV/AIDS resulting from successful ART has led to
additional concerns. Long-term toxicity of ART drugs has resulted in increased risk for cardiovascular disease, metabolic disorders, and organ failure. Treated individuals frequently fail to reconstitute their immune system and develop chronic immune activation characterized by activation of monocytes and T cells, production of pro-inflammatory cytokines (e.g., interferon- γ [IFN-γ], interleukin-6 [IL-6]), and depletion of Th17 cells.47 Chronic immune activation tends to exacerbate clinical disease in adults and neonates.48 Vaccine development should be the most effective means of preventing HIV
infection and may be useful in treating preexisting infection. Most of the common viral vaccines (e.g., rubella, mumps, influenza) induce protective antibodies that block the initial infection. Only one vaccine (rabies) is used after the infection has occurred. The rabies vaccine is successful because the rabies virus proliferates and spreads very slowly. However, the ability of an HIV vaccine to either successfully prevent or treat HIV infection is questionable for several reasons.49 First, the AIDS virus is genetically and antigenically variable, like the influenza virus, so that a vaccine created against one variant may not provide protection against another variant. Second, although individuals with HIV/AIDS have high levels of circulating antibodies against the virus, these antibodies do not appear to be protective. Therefore even if a circulating antibody response can be induced by vaccination, that response might not be effective. A vaccine may have to induce both circulating and secretory (to prevent initial infection of the mucosal T cell) antibody and Tc cells.
Pediatric AIDS and Central Nervous System Involvement HIV can be transmitted from mother to child during pregnancy, at the time of delivery, or through breast-feeding, although the risk of mother-to-child transmission has dropped precipitously since the use of anti-retroviral drugs in pregnant women. The clinical diagnosis of HIV infection in young children born of HIV-infected mothers is very often a difficult task because the presence of maternal antibodies may result in a misleading false-positive test for antibodies against HIV for as long as 18 months after birth. Testing for antibody against HIV can be performed recurrently from birth until 18 months; if the test results become negative and remain so after 12 months, the child can be considered uninfected. The 2008 revised surveillance case definition for HIV infection in children
younger than 18 months, which remains in effect today, recommends testing for HIV or viral components in two separate specimens, not including cord blood.50 These include detection of HIV nucleic acid or p24 antigen, or direct isolation of HIV in viral cultures. HIV infection of babies is generally more aggressive than in adults; on average
an untreated child will die by his or her second birthday. Neurologic involvement occurs more commonly in children than in adults and results from CNS involvement, rather than effects on peripheral portions of the nervous system. HIV encephalopathy occurs with varying degrees of severity and is a clinical component in the diagnosis of AIDS in children. Most HIV-infected newborns appear normal, but may progressively develop signs of CNS involvement. These usually appear as failure to attain, or loss of, developmental milestones or loss of intellectual ability, verified by standard developmental scale or neuropsychologic tests; acquired symmetric motor deficits, seen in children older than age 1 month; impaired brain growth or acquired microcephaly, demonstrated by head circumference measurements; or brain atrophy, demonstrated by computed tomography (CT) or magnetic resonance imaging (MRI) with serial imaging and required in children younger than 2 years of age. It may be difficult to completely differentiate the effect of HIV infection on the
CNS from other risk factors, including prenatal drug exposure, prematurity, chronic illness, and a chaotic social atmosphere. The pathogenesis of HIV encephalopathy in children is poorly understood, but the presence of inflammatory mediators may be a contributing factor. Because HIV infection in infants progresses very rapidly, treatment must begin at
the diagnosis of infection. In older children the criteria for treatment are similar to those used in adults. A growing number of investigational protocols are available for treatment of children with HIV. In general, treatment is focused on the
preservation and maintenance of the immune system, aggressive response to opportunistic infections, support and relief of symptomatic occurrences, and administration of ART.
Quick Check 8-2
1. Why is the development of recurrent or unusual infections the clinical hallmark of immunodeficiency?
2. Compare and contrast the most common infections in individuals with defects in cell-mediated immune response and those with defects in humoral immune response.
3. What are the new treatments for HIV?
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity Allergy, autoimmunity, and alloimmunity are classified as hypersensitivity reactions. Hypersensitivity is an altered immunologic response to an antigen that results in disease or damage to the individual. Allergy, autoimmunity, and alloimmunity (also termed isoimmunity) can be most easily understood in relationship to the source of the antigen against which the hypersensitivity response is directed (Table 8-12). Allergy refers to a hypersensitivity to environmental antigens. These can include medicines, natural products (e.g., pollens, bee stings), infectious agents, and any other antigen that is not naturally found in the individual.
TABLE 8-12 Relative Incidence and Examples of Hypersensitivity Diseases*
Target Antigen
MECHANISM Type I (IgE Mediated)
Type II (Tissue Specific)
Type III (Immune Complex Mediated) Type IV (Cell Mediated)
Allergy ++++ + + ++ Environmental antigens
Hay fever Hemolysis in drug allergies
Gluten (wheat) allergy Poison ivy allergy
Autoimmunity + ++ +++ ++ Self-antigens May contribute to some
type III reactions Autoimmune thrombocytopenia
Systemic lupus erythematosus Hashimoto thyroiditis
Alloimmunity + ++ + ++ Another person's antigens
May contribute to some type III reactions
Hemolytic disease of the newborn
Individuals who do not make their own IgA may have an anaphylactic response against IgA in human immune globulin
Graft rejection
*The frequency of each reaction is indicated in a range from rare (+) to very common (++++). An example of each reaction is given.
Autoimmunity is a disturbance in the immunologic tolerance of self-antigens. The immune system normally does not strongly recognize the individual's own antigens. Healthy individuals of all ages, but particularly the elderly, may produce low quantities of antibodies against their own antigens (autoantibodies) without developing overt autoimmune disease. Therefore the presence of low quantities of autoantibodies does not necessarily indicate a disease state. Autoimmune diseases occur when the immune system reacts against self-antigens to such a degree that autoantibodies or autoreactive T cells damage the individual's tissues. Many clinical disorders are associated with autoimmunity and are generally referred to as autoimmune diseases (Table 8-13). Autoimmune diseases are more prevalent in women and the overall prevalence is rising.51
TABLE 8-13 Examples of Autoimmune Disorders
System Disease Organ or Tissue Probable Self-Antigen Endocrine System Hyperthyroidism (Graves disease) Thyroid gland Receptors for thyroid-stimulating hormone on plasma membrane of thyroid cells Hashimoto hypothyroidism Thyroid gland Thyroid cell surface antigens, thyroglobulin Insulin-dependent diabetes Pancreas Islet cells, insulin, insulin receptors on pancreatic cells Addison disease Adrenal gland Surface antigens on steroid-producing cells; microsomal antigens Male infertility Testis Surface antigens on spermatozoa Skin Pemphigus vulgaris Skin Intercellular substances in stratified squamous epithelium Bullous pemphigoid Skin Basement membrane Vitiligo Skin Surface antigens on melanocytes (melanin-producing cells) Neuromuscular Tissue Multiple sclerosis Neural tissue Surface antigens of nerve cells Myasthenia gravis Neuromuscular junction Acetylcholine receptors; striations of skeletal and cardiac muscle Rheumatic fever Heart Cardiac tissue antigens that cross-react with group A streptococcal antigen Cardiomyopathy Heart Cardiac muscle Gastrointestinal System Ulcerative colitis Colon Mucosal cells Pernicious anemia Stomach Surface antigens of parietal cells; intrinsic factor Primary biliary cirrhosis Liver Cells of bile duct Chronic active hepatitis Liver Surface antigens of hepatocytes, nuclei, microsomes, smooth muscle Eye Sjögren syndrome Lacrimal gland Antigens of lacrimal gland, salivary gland, thyroid, and nuclei of cells Connective Tissue Ankylosing spondylitis Joints Sacroiliac and spinal apophyseal joint Rheumatoid arthritis Joints Collagen, IgG Systemic lupus erythematosus Multiple sites Numerous antigens in nuclei, organelles, and extracellular matrix Renal System Immune complex glomerulonephritis Kidney Numerous immune complexes Goodpasture syndrome Kidney Glomerular basement membrane Hematologic System Idiopathic neutropenia Neutrophil Surface antigens on polymorphonuclear neutrophils Idiopathic lymphopenia Lymphocytes Surface antigens on lymphocytes Autoimmune hemolytic anemia Erythrocytes Surface antigens on erythrocytes Autoimmune thrombocytopenic purpura Platelets Surface antigens on platelets Respiratory System Goodpasture syndrome Lung Septal membrane of alveolus
Alloimmune diseases occur when the immune system of one individual produces an immunologic reaction against tissues of another individual. Alloimmunity can be observed during immunologic reactions against transfusions, transplanted tissue, or the fetus during pregnancy. The mechanism that initiates the onset of hypersensitivity, whether allergy,
autoimmunity, or alloimmunity, is not completely understood. It is generally accepted that genetic, infectious, and possibly environmental factors contribute to the development of hypersensitivity reactions.
Mechanisms of Hypersensitivity Hypersensitivity reactions can be characterized also by the particular immune mechanism that results in the disease (Table 8-14). These mechanisms are apparent
in most hypersensitivity reactions and have been divided into four distinct types: type I (IgE-mediated reactions), type II (tissue-specific reactions), type III (immune complex–mediated reactions), and type IV (cell-mediated reactions). This classification is artificial and seldom is a particular disease associated with only a single mechanism. The four mechanisms are interrelated, and in most hypersensitivity reactions several mechanisms can be functioning simultaneously or sequentially.
TABLE 8-14 Immunologic Mechanisms of Tissue Destruction
Type Name Rate of Development
Class of Antibody Involved
Principal Effector Cells Involved
Participation of Complement Examples of Disorders
I IgE-mediated reaction
Immediate IgE Mast cells No Seasonal allergic rhinitis Asthma
II Tissue-specific reaction
Immediate IgG IgM
Macrophages in tissues
Frequently Autoimmune thrombocytopenic purpura, Graves disease, autoimmune hemolytic anemia
III Immune complex– mediated reaction
Immediate IgG IgM
Neutrophils Yes Systemic lupus erythematosus
IV Cell-mediated reaction
Delayed None Lymphocytes Macrophages
No Contact sensitivity to poison ivy, metals (jewelry), and latex
As with all immune responses, hypersensitivity reactions require sensitization against a particular antigen that results in a primary immune response. Disease symptoms appear after an adequate secondary immune response occurs. Hypersensitivity reactions are immediate or delayed, depending on the time required to elicit clinical symptoms after reexposure to the antigen. Reactions that occur within minutes to a few hours after exposure to antigen are termed immediate hypersensitivity reactions. Delayed hypersensitivity reactions may take several hours to appear and are at maximal severity days after reexposure to the antigen. Generally, immediate reactions are caused by antibody, whereas delayed reactions are caused by cells (e.g., T cells, NK cells, macrophages). The most rapid and severe immediate hypersensitivity reaction is anaphylaxis.
Anaphylaxis occurs within minutes of reexposure to the antigen and can be either systemic (generalized) or cutaneous (localized). Symptoms of systemic anaphylaxis include pruritus, erythema, vomiting, abdominal cramps, diarrhea, and breathing difficulties, and the most severe reactions may include contraction of bronchial smooth muscle, edema of the throat, and decreased blood pressure that can lead to shock and death.52 Examples of systemic anaphylaxis are allergic reactions to bee stings (see p. 206), peanuts, shellfish, or eggs. Cutaneous anaphylaxis results in local symptoms, such as pain, swelling, and redness, which occur at the site of exposure to an antigen (e.g., a painful local reaction to an injected vaccine or drug).
Type I: IgE-Mediated Hypersensitivity Reactions Type I hypersensitivity reactions are mediated by antigen-specific IgE and the products of tissue mast cells (Figure 8-11). Most common allergic reactions are type I reactions. In addition, most type I reactions occur against environmental antigens and are therefore allergic. Because of this strong association, many healthcare professionals use the term allergy to indicate only IgE-mediated reactions. However, IgE can contribute to some autoimmune and alloimmune diseases, and many common allergies (e.g., poison ivy) are not mediated by IgE.
FIGURE 8-11 Mechanism of Type I, IgE-Mediated Reactions. First exposure to an allergen leads to antigen processing and presentation of antigen by an antigen-presenting cell (APC) to B
lymphocytes, which is under the direction of T-helper 2 (Th2) cells. Th2 cells produce specific cytokines (e.g., IL-4, IL-13, and others) that favor maturation of the B lymphocytes into plasma cells that secrete IgE. The IgE is adsorbed to the surface of the mast cell by binding with IgE- specific Fc receptors. When an adequate amount of IgE is bound the mast cell is sensitized.
During a reexposure, the allergen cross-links the surface-bound IgE and causes degranulation of the mast cell. Contents of the mast cell granules, primarily histamine, induce local edema,
smooth muscle contraction, mucous secretion, and other characteristics of an acute inflammatory reaction. (See Chapter 6 for more details on the role of mast cells in
inflammation.)
IgE has a relatively short life span in the blood because it rapidly binds to Fc receptors on mast cells.53 Unlike Fc receptors on phagocytes, which bind IgG that has previously reacted with antigen, the Fc receptors on mast cells specifically bind IgE that has not previously interacted with antigen. After a large amount of IgE has bound to the mast cells, an individual is considered sensitized. Further exposure of a sensitized individual to the allergen results in degranulation of the mast cell and the release of mast cell products (see Chapter 6).
Mechanisms of IgE-mediated hypersensitivity. The most potent mediator of IgE-mediated hypersensitivity is histamine, which affects several key target cells. Acting through H1 receptors, histamine contracts bronchial smooth muscles (bronchial constriction), increases vascular permeability (edema), and causes vasodilation (increased blood flow) (see Chapter 6). The interaction of histamine with H2 receptors results in increased gastric acid secretion. Blocking histamine receptors with antihistamines can control some type I responses.
Clinical manifestations of IgE-mediated hypersensitivity. The clinical manifestations of type I reactions are attributable mostly to the biologic effects of histamine. The tissues most commonly affected by type I responses contain large numbers of mast cells and are sensitive to the effects of histamine released from them. These tissues are found in the gastrointestinal tract, the skin, and the respiratory tract (Figure 8-12 and Table 8-15).
FIGURE 8-12 Type I Hypersensitivity Reactions. Manifestations of allergic reactions as a result of type I hypersensitivity include pruritus, angioedema (swelling caused by exudation), edema of the larynx, urticaria (hives), bronchospasm (constriction of airways in the lungs), hypotension (low blood pressure), and dysrhythmias (irregular heartbeat) because of anaphylactic shock,
and gastrointestinal cramping caused by inflammation of the gastrointestinal mucosa. Photographic inserts show a diffuse allergic-like eye and skin reaction on an individual. The skin lesions have raised edges and develop within minutes or hours, with resolution occurring after
about 12 hours. (Inserts from Male D et al: Immunology, ed 8, St Louis, 2013, Mosby.)
TABLE 8-15 Causes of Clinical Allergic Reactions
Typical Allergen Mechanism of Hypersensitivity Clinical Manifestation Ingestants Foods Type I Gastrointestinal allergy Drugs Types I, II, III Urticaria, immediate drug reaction, hemolytic anemia, serum sickness Inhalants Pollens, dust, molds Type I Allergic rhinitis, bronchial asthma Aspergillus fumigatus Types I, III Allergic bronchopulmonary aspergillosis Thermophilic actinomycetes* Types III, IV Extrinsic allergic alveolitis Injectants Drugs Types I, II, III Immediate drug reaction, hemolytic anemia, serum sickness Bee venom Type I Anaphylaxis Vaccines Type III Localized Arthus reaction Serum Types I, III Anaphylaxis, serum sickness Contactants Poison ivy, metals Type IV Contact dermatitis Latex Type I, IV Contact dermatitis, anaphylaxis
*An order of fungi that grows best at high temperatures (between 45° and 80° C [113° and 176° F]).
Modified from Bellanti JA: Immunology III, Philadelphia, 1985, Saunders.
Gastrointestinal allergy is caused by allergens that enter through the mouth— usually foods or medicines. Symptoms include vomiting, diarrhea, or abdominal pain. Foods most often implicated in gastrointestinal allergies are milk, chocolate, citrus fruits, eggs, wheat, nuts, peanut butter, and fish.54 The most common food allergy in adults is a reaction to shellfish, which may initiate an anaphylactic response and death.55 When food is the source of an allergen, the active immunogen may be an unidentifiable product of how the food is processed during manufacture or broken down by digestive enzymes.56 Sometimes the allergen is a drug, an additive, or a preservative in the food. For example, cows treated for mastitis with penicillin yield milk containing trace amounts of this antibiotic. Thus hypersensitivity apparently caused by milk proteins may instead be the result of an allergy to penicillin. Urticaria, or hives, is a dermal (skin) manifestation of allergic reactions (see
Figure 8-12). The underlying mechanism is the localized release of histamine and increased vascular permeability, resulting in limited areas of edema. Urticaria is characterized by white fluid-filled blisters (wheals) surrounded by areas of redness (flares). This wheal and flare reaction is usually accompanied by pruritus. Not all urticarial symptoms are caused by immunologic reactions. Some, termed nonimmunologic urticaria, result from exposure to cold temperatures, emotional stress, medications, systemic diseases, or malignancies (e.g., lymphomas). Effects of allergens on the mucosa of the eyes, nose, and respiratory tract include
conjunctivitis (inflammation of the membranes lining the eyelids) (see Figure 8-12), rhinitis (inflammation of the mucous membranes of the nose), and asthma
(constriction of the bronchi). Symptoms are caused by vasodilation, hypersecretion of mucus, edema, and swelling of the respiratory mucosa. Because the mucous membranes lining the respiratory tract are continuous, they are all adversely affected. The degree to which each is affected determines the symptoms of the disease; most anaphylactic reactions are type I hypersensitivities. The central problem in allergic diseases of the lung is obstruction of the large
and small airways (bronchi) of the lower respiratory tract by bronchospasm (constriction of smooth muscle in airway walls), edema, and thick secretions. This leads to ventilatory insufficiency, wheezing, and difficult or labored breathing (see Chapter 27). Certain individuals are genetically predisposed to develop allergies and are
called atopic. In families in which one parent has an allergy, allergies develop in about 40% of the offspring. If both parents have allergies, the incidence may be as high as 80%. Atopic individuals tend to produce higher quantities of IgE and have more Fc receptors for IgE on their mast cells. The airways and the skin of atopic individuals have increased responsiveness to a wide variety of both specific and nonspecific stimuli.
Evaluation and treatment of IgE hypersensitivity. Allergic reactions can be life-threatening; therefore it is essential that severely allergic individuals be informed of the specific allergen against which they are sensitized and instructed to avoid contact with that material. Several tests are available to evaluate allergic individuals. These include food challenges, skin tests with allergens, and laboratory tests for total IgE and allergen-specific IgE.
Type II: Tissue-Specific Hypersensitivity Reactions Type II hypersensitivities are generally reactions against a specific cell or tissue. Cells express a variety of antigens on their surfaces, some of which are called tissue-specific antigens because they are expressed on the plasma membranes of only certain cells. Platelets, for example, have groups of antigens that are found on no other cells of the body. The symptoms of many type II diseases are determined by which tissue or organ expresses the particular antigen. Environmental antigens (e.g., drugs or their metabolites) may bind to the plasma membranes of specific cells (especially erythrocytes and platelets) and function as targets of type II reactions. The five general mechanisms by which type II hypersensitivity reactions can affect cells are shown in Figure 8-13. Each mechanism begins with antibody binding to tissue-specific antigens or antigens that have attached to particular tissues.
FIGURE 8-13 Mechanisms of Type II, Tissue-Specific Reactions. Antigens on the target cell bind with antibody and are destroyed or prevented from functioning by one of the following
mechanisms: (A) complement-mediated lysis (an erythrocyte target is illustrated here); (B) clearance (phagocytosis) by macrophages in the tissue; (C) neutrophil-mediated immune
destruction; (D) antibody-dependent cell-mediated cytotoxicity (ADCC) (apoptosis of target cells is induced by natural killer [NK] cells by two mechanisms: by the release of granzymes and perforin, which is a molecule that creates pores in the plasma membrane, and enzymes
[granzymes] that enter the target through the perforin pores; by the interactions of Fas ligand [FasL; a molecule similar to TNF-α] on the surface of NK cells with Fas [the receptor for FasL] on the surface of target cells); or (E) modulation or blocking of the normal function of receptors by
antireceptor antibody. This example of mechanism (E) depicts myasthenia gravis in which acetylcholine receptor antibodies block acetylcholine from attaching to its receptors on the motor end plates of skeletal muscle, thereby impairing neuromuscular transmission and causing muscle weakness. C1, Complement component C1; C3b, complement fragment
produced from C3, which acts as an opsonin; C5a, complement fragment produced from C5, which acts as a chemotactic factor for neutrophils; Fcγ receptor, cellular receptor for the Fc
portion of IgG; FcR, Fc receptor.
The cell may be destroyed by antibody and complement (see Figure 8-13, A). The antibody (IgM or IgG) reacts with an antigen on the surface of the cell, causing activation of the complement cascade through the classical pathway. Formation of the membrane attack complex (C5-9) damages the membrane and may result in lysis of the cell. For example, erythrocytes are destroyed by complement-mediated lysis in individuals with autoimmune hemolytic anemia (see Chapters 21 and 22) or as a result of an alloimmune reaction to mismatched transfused blood cells. Antibody may cause cell destruction through phagocytosis by macrophages (see
Figure 8-13, B). The antibody may additionally activate complement, resulting in the deposition of C3b on the cell surface. Receptors on the macrophage recognize and bind opsonins (e.g., antibody or C3b) and increase phagocytosis of the target cell. For example, antibodies against platelet-specific antigens or against red blood cell antigens of the Rh system cause their removal by phagocytosis in the spleen. Tissue damage may be caused by toxic products produced by neutrophils (see
Figure 8-13, C). Soluble antigens such as medications, molecules released from infectious agents, or molecules released from an individual's own cells may enter the circulation. In some instances, the antigens are deposited on the surface of tissues, where they bind antibody. The antibody may activate complement, resulting in the release of C3a and C5a, which are chemotactic for neutrophils, and the deposition of complement component C3b. Neutrophils are attracted, bind to the tissues through receptors for the Fc portion of antibody (Fc receptor) or for C3b, and release their granules onto the healthy tissue. The components of neutrophil granules, as well as the toxic oxygen products produced by these cells, will damage the tissue. Antibody-dependent cell-mediated cytotoxicity (ADCC) involves natural killer
(NK) cells (see Figure 8-13, D). Antibody on the target cell is recognized by Fc receptors on the NK cells, which release toxic substances that destroy the target cell. The last mechanism does not destroy the target cell but rather causes the cell to
malfunction (see Figure 8-13, E). The antibody is usually directed against antigenic determinants associated with specific cell surface receptors. The antibody changes the function of the receptor by preventing interactions with their normal ligands, replacing the ligand and inappropriately stimulating the receptor, or destroying the receptor. For example, in the hyperthyroidism (excessive thyroid activity) of Graves disease, autoantibody binds to and activates receptors for thyroid-stimulating
hormone (TSH) (a pituitary hormone that controls the production of the hormone thyroxine by the thyroid). In this way, the antibody stimulates the thyroid cells to produce thyroxine. Under normal conditions, the increasing levels of thyroxine in the blood would signal the pituitary to decrease TSH production, which would result in less stimulation of the TSH receptor in the thyroid and a concomitant decrease in thyroxine production. Increasing amounts of thyroxine in the blood have no effect on anti-TSH receptor antibodies, which continue to stimulate despite decreasing amounts of TSH (see Chapter 19).
Type III: Immune Complex–Mediated Hypersensitivity Reactions
Mechanisms of type III hypersensitivity. Most type III hypersensitivity diseases reactions are caused by antigen-antibody (immune) complexes that are formed in the circulation and deposited later in vessel walls or other tissues (Figure 8-14). The primary difference between type II and type III mechanisms is that in type II hypersensitivity antibody binds to antigen on the cell surface, whereas in type III antibody binds to soluble antigen that was released into the blood or body fluids, and the complex is then deposited in the tissues. Type III reactions are not organ specific, and symptoms are mostly unrelated to the particular antigenic target of the antibody. The harmful effects of immune complex deposition are caused by complement activation, particularly through the generation of chemotactic factors for neutrophils. The neutrophils bind to antibody and C3b contained in the complexes and attempt to ingest the immune complexes. They are often unsuccessful because the complexes are bound to large areas of tissue. During the attempted phagocytosis, large quantities of lysosomal enzymes are released into the inflammatory site instead of into phagolysosomes. The attraction of neutrophils and the subsequent release of lysosomal enzymes cause most of the resulting tissue damage.
FIGURE 8-14 Mechanism of Type III, Immune Complex–Mediated Reactions. Immune complexes form in the blood from circulating antigen and antibody. Both small and large
immune complexes are removed successfully from the circulation and do not cause tissue damage. Intermediate-sized complexes are deposited in certain target tissues in which the circulation is slow or filtration of the blood occurs. The complexes activate the complement cascade through C1 and generate fragments including C5a and C3b. C5a is chemotactic for neutrophils, which migrate into the inflamed area and attach to the IgG and C3b in the immune
complexes. The neutrophils attempt unsuccessfully to phagocytose the tissue and in the process release a variety of degradative enzymes that destroy the healthy tissues. Fcγ receptor
is the cellular receptor for the Fc portion of IgG.
Immune complex disease. Two prototypic models of type III hypersensitivity help explain the variety of diseases in this category. Serum sickness is a model of systemic type III hypersensitivities, and the Arthus reaction is a model of localized or cutaneous reactions. Serum sickness–type reactions are caused by the formation of immune
complexes in the blood and their subsequent generalized deposition in target tissues. Typically affected tissues are the blood vessels, joints, and kidneys. Symptoms include fever, enlarged lymph nodes, rash, and pain at sites of inflammation. Serum sickness was initially described as a complication of therapeutic administration of horse serum that contained antibody against tetanus toxin. Foreign serum is not administered to individuals today, although serum sickness reactions can be caused by the repeated intravenous administration of other antigens, such as drugs, and the characteristics of serum sickness are observed in systemic type III autoimmune diseases. A form of serum sickness is Raynaud phenomenon, a condition caused by the
temperature-dependent deposition of immune complexes in the capillary beds of the peripheral circulation. Certain immune complexes precipitate at temperatures below normal body temperature, particularly in the tips of the fingers, toes, and nose, and are called cryoglobulins. The precipitates block the circulation and cause localized pallor and numbness, followed by cyanosis (a bluish tinge resulting from oxygen deprivation) and eventually gangrene if the circulation is not restored. An Arthus reaction is caused by repeated local exposure to an antigen that reacts
with preformed antibody and forms immune complexes in the walls of the local blood vessels. Symptoms of an Arthus reaction begin within 1 hour of exposure and peak 6 to 12 hours later. The lesions are characterized by a typical inflammatory reaction, with increased vascular permeability, an accumulation of neutrophils, edema, hemorrhage, clotting, and tissue damage. Arthus reactions may be observed after injection, ingestion, or inhalation of
allergens. Skin reactions can follow subcutaneous or intradermal inoculation with drugs, fungal extracts, or antigens used in skin tests. Gastrointestinal reactions, such as gluten-sensitive enteropathy (celiac disease), follow ingestion of antigen, usually gluten from wheat products (see Chapter 37). Allergic alveolitis (farmer lung, pigeon breeder disease) is an Arthus-like acute hemorrhagic inflammation of the air sacs (alveoli) of the lungs resulting from inhalation of fungal antigens, usually particles from moldy hay or pigeon feces (see Chapter 27).
Type IV: Cell-Mediated Hypersensitivity Reactions Whereas types I, II, and III hypersensitivity reactions are mediated by antibody, type IV hypersensitivity reactions are mediated by T lymphocytes and do not involve antibody (Figure 8-15). Type IV mechanisms occur through either cytotoxic T lymphocytes (Tc cells) or lymphokine-producing Th1 and Th17 cells. Tc cells attack and destroy cellular targets directly. Th1 and Th17 cells produce cytokines that recruit and activate phagocytic cells, especially macrophages. Destruction of the tissue is usually caused by direct killing by Tc cells or the release of soluble factors, such as lysosomal enzymes and toxic reactive oxygen species, from activated macrophages.
FIGURE 8-15 Mechanism of Type IV, Cell-Mediated Reactions. Antigens from target cells stimulate T cells to differentiate into T-cytotoxic cells (Tc cells), which have direct cytotoxic
activity, and T-helper cells (Th1 cells) involved in delayed hypersensitivity. The Th1 cells produce lymphokines (especially interferon-γ [IFN-γ]) that activate the macrophage through specific receptors (e.g., IFN-γ receptor [IFNγR]). The macrophages can attach to targets and release enzymes and reactive oxygen species that are responsible for most of the tissue destruction.
Clinical examples of type IV hypersensitivity reactions include graft rejection, the skin test for tuberculosis, and allergic reactions resulting from contact with such substances as poison ivy and metals. A type IV component also may be present in many autoimmune diseases. For example, T cells against type II collagen (a protein present in joint tissues) contribute to the destruction of joints in rheumatoid arthritis; T cells against a thyroid cell–surface antigen contribute to the destruction of the thyroid in autoimmune thyroiditis (Hashimoto disease); and T cells against an antigen on the surface of pancreatic beta cells (the cell that normally produces insulin) are responsible for beta-cell destruction in insulin-dependent (type 1) diabetes mellitus. In 1891 Ehrlich was the first to thoroughly describe a type IV hypersensitivity
reaction in the skin, leading to the development of a diagnostic skin test for tuberculosis. The reaction follows an intradermal injection of tuberculin antigen into a suitably sensitized individual and is called a delayed hypersensitivity skin test because of its slow onset—24 to 72 hours to reach maximal intensity. The reaction site is infiltrated with T lymphocytes and macrophages, resulting in a clear
hard center (induration) and a reddish surrounding area (erythema). Allergic type IV reactions are elicited by some environmental antigens that are
haptens (Chapter 7) and become immunogenic after binding to larger (carrier) proteins in the individual. In allergic contact dermatitis, the carrier protein is in the skin. The best-known example is poison ivy (Figure 8-16). The antigen is a plant catechol, urushiol, which reacts with normal skin proteins and evokes a cell- mediated immune response. Skin reactions to industrial chemicals, cosmetics, detergents, clothing, food, metals, and topical medicines (such as penicillin) are elicited by the same mechanism. Contact dermatitis consists of lesions only at the site of contact with the allergen, such as a metal allergy to jewelry.
Quick Check 8-3
1. Distinguish among the four types of hypersensitivity mechanisms.
2. What is the mechanism of anaphylaxis?
3. What are some clinical examples of type IV hypersensitivity?
FIGURE 8-16 Development of Allergic Contact Dermatitis. A, The development of type IV hypersensitivity to poison ivy. The first (primary) contact with allergen sensitizes (produces reactive T cells) the individual but does not produce a rash (dermatitis). Secondary contact
activates a type IV cell-mediated reaction that causes dermatitis. B, Contact dermatitis caused by a delayed hypersensitivity reaction leading to vesicles and scaling at the sites of
contact. (From Damjanov I, Linder J: Anderson's pathology, ed 10, St Louis, 1996, Mosby.)
Antigenic Targets of Hypersensitivity Reactions Allergy
Allergens. Allergies are the most common hypersensitivity reactions. The majority of allergies
are type I reactions that lead to annoying symptoms, including rhinitis, sneezing, and other relatively mild reactions. In some individuals, however, these reactions can be excessive and life-threatening (anaphylaxis). Antigens that cause allergic responses are called allergens. It is not known why some antigens are allergens and others are not. Typical allergens include pollens (e.g., ragweed), molds and fungi (e.g., Penicillium chrysogenum), foods (e.g., milk, eggs, fish), animals (e.g., cat dander, dog dander), cigarette smoke, and components of house dust (e.g., fecal pellets of house mites). Often the allergen is contained within a particle that is too large to be phagocytosed or is surrounded by a protective nonallergenic coat. The actual allergen is released after enzymatic breakdown (e.g., by lysozyme in secretions) of the larger particle.
Allergic disease: bee sting allergy. Bee venoms contain a mixture of enzymes and other proteins that may serve as allergens and cause a type I hypersensitivity reaction. About 1% of children may have an anaphylactic reaction to bee venom.57 Within minutes they may develop excessive swelling (edema) at the bee sting site, followed by generalized hives, pruritus, and swelling in areas distal from the sting (e.g., eyes, lips), and other systemic symptoms including flushing, sweating, dizziness, and headache. The most severe symptoms may include gastrointestinal (e.g., stomach cramps, vomiting), respiratory (e.g., tightness in the throat, wheezing, difficulty breathing), and vascular (e.g., low blood pressure, shock) reactions. Severe respiratory tract and vascular reactions may lead to death. For an individual with known bee sting hypersensitivity, lifestyle changes include
avoidance of stinging or biting insects. If a child has experienced a previous anaphylactic reaction, the chance of having another is about 60%. The primary life- threatening symptoms result from contraction of respiratory tract smooth muscle. Autonomic nervous system mediators, such as epinephrine, bind to specific receptors on smooth muscle and reverse the effects of histamine, resulting in muscle relaxation. Thus most individuals with bee sting allergies carry self- injectable epinephrine. The administration of antihistamines has little effect because histamine has already bound H1 receptors and initiated severe bronchial smooth muscle contraction. Clinical desensitization to allergens can be achieved in some individuals. Minute
quantities of the allergen are injected in increasing doses over a prolonged period. The procedure may reduce the severity of the allergic reaction in the treated individual. However, this form of therapy may trigger systemic anaphylaxis, which can be severe and life-threatening. This approach works best for routine respiratory
tract allergens and biting insect allergies (80% to 90% rate of desensitization over 5 years of treatment).58 Food allergies have been very difficult to suppress, but some promising trials are underway to evaluate desensitization by oral or sublingual administration of increasing amounts of allergen.
Autoimmunity Autoimmune diseases originate from an initiating event in a genetic predisposed individual. Current models of factors related to autoimmune diseases include genetic factors, environmental factors, and random or stochastic changes.59 Some autoimmune diseases can be familial and attributed to the presence of a very small number of susceptibility genes; affected family members may not all develop the same disease, but have different disorders characterized by a variety of hypersensitivity reactions, including autoimmune and allergic. For instance, the HLA antigen B27 (HLA is discussed further under transplant rejection, p. 209) is a risk factor for developing ankylosing spondylitis (AS), an autoimmune inflammatory disease of the spine; 95% of individuals diagnosed with AS express HLA-B27 whereas only 4% to 8% of the general population expresses this antigen.60 Although most autoimmune diseases appear as isolated events without a positive family history, susceptibility for developing such diseases appears to be linked to a combination of multiple genes.
Breakdown of tolerance. An individual is usually tolerant to his or her own antigens. Tolerance is a state of immunologic control so that the individual does not make a detrimental immune response against his or her own cells and tissues. Autoimmune disease results from a breakdown of this tolerance. The initiating event that breaks tolerance is unclear for most autoimmune
diseases. It is also unclear as to the bodily site initially involved to cause autoimmunity.59 Potential infectious initiators of autoimmune disease are being investigated,61 but only one example is known: acute rheumatic fever. In a small number of individuals with group A streptococcal sore throats, the M proteins in the bacterial capsule mimic (antigenic mimicry) normal heart antigens and induce antibodies that also react with proteins in the heart valve, damaging the valve.62 Thus acute rheumatic fever is a type II autoimmune hypersensitivity. Additionally, some streptococcal skin or throat infections release bacterial antigens into the blood that form circulating immune complexes. The complexes may deposit in the kidneys and initiate an immune complex–mediated glomerulonephritis (inflammation of the kidney).63 Thus streptococcal antigens (an environmental antigen) may also cause a
type III allergic hypersensitivity (poststreptococcal glomerulonephritis).
Autoimmune disease: systemic lupus erythematosus. Systemic lupus erythematosus (SLE) is the most common, complex, and serious of the autoimmune disorders. SLE is characterized by the production of a large variety of antibodies (autoantibodies) against self-antigens, including nucleic acids, erythrocytes, coagulation proteins, phospholipids, lymphocytes, platelets, and many other self-components. The most characteristic autoantibodies are against nucleic acids (e.g., single-stranded DNA, double-stranded DNA), histones, ribonucleoproteins, and other nuclear materials. Approximately 98% of persons with SLE have detectable antibodies against nuclear antigens. The blood normally contains many of these products of cellular turnover and breakdown so that autoantibodies react with the circulating antigen and form circulating immune complexes. The deposition of circulating DNA/anti-DNA complexes in the kidneys can cause severe kidney inflammation. Similar reactions can occur in the brain, heart, spleen, lung, gastrointestinal tract, peritoneum, and skin. Thus some of the symptoms of SLE result from a type III hypersensitivity reaction. Other symptoms, such as destruction of red blood cells (anemia), lymphocytes (lymphopenia), and other cells, may be type II hypersensitivity reactions. SLE, like most autoimmune diseases, occurs more often in women
(approximately a 9 : 1 predominance of females), especially in the 20- to 40-year- old age group.64 Blacks are affected more often than whites (about an eightfold increased risk). A genetic predisposition for the disease has been implicated on the basis of increased incidence in twins and the existence of autoimmune disease in the families of individuals with SLE. As with many autoimmune diseases, clinical manifestations of SLE may wax and
wane; the individual may go through periods of remission and be relatively disease free until the onset of a flare (exacerbated disease activity). Symptoms include arthralgias or arthritis (90% of individuals), vasculitis and rash (70% to 80% of individuals), renal disease (40% to 50% of individuals), hematologic abnormalities (50% of individuals, with anemia being the most common complication), and cardiovascular diseases (30% to 50% of individuals) (see discoid lupus erythematosus in Chapter 41). Because the signs and symptoms affect almost every body system and tend to vacillate, SLE is extremely difficult to diagnose. This has led to the development of a list of 11 common clinical findings,65 which has been modified slightly to increase sensitivity of the diagnosis.66 The serial or simultaneous presence of at least four of these findings indicates that the individual has SLE. The findings are as follows:
1. Facial rash confined to the cheeks (malar rash)
2. Discoid rash (raised patches, scaling)
3. Photosensitivity (development of skin rash as a result of exposure to sunlight)
4. Oral or nasopharyngeal ulcers
5. Nonerosive arthritis of at least two peripheral joints
6. Serositis (inflammation of membranes of lung [pleurisy] or heart [pericarditis])
7. Renal disorder (persistent proteinuria of >0.5 g/day or >3 on dipstick, or cellular casts)
8. Neurologic disorders (seizures or psychosis in the absence of known causes)
9. Hematologic disorders (hemolytic anemia, leukopenia, lymphopenia, or thrombocytopenia)
10. Immunologic disorders (anti–double-stranded DNA (dsDNA), anti–Smith [Sm] antigen, false-positive serologic test for syphilis, or antiphospholipid antibodies [anticardiolipin antibody or lupus anticoagulant])
11. Presence of antinuclear antibody (ANA)
Laboratory diagnosis is usually based on a positive ANA screening test; about 98% of individuals with SLE are positive, but a substantial number of false-positives occur in healthy individuals and those with other diseases. Because SLE is a progressive and slowly developing disease, some laboratory tests, including the ANA, may be positive years before the onset of clinical symptoms.64 Detection of a positive ANA is usually followed by one or more specific tests (e.g., antibodies against Sm, dsDNA) that are complicated by low sensitivity (only a portion of individuals with SLE will be positive, although the number of false-positives is low). There is no cure for SLE or most other autoimmune diseases. Fatalities resulting
from SLE are usually related to infection, organ failure, or cardiovascular disease. The goals of treatment are to control symptoms and prevent further damage by suppressing the autoimmune response. Nonsteroidal anti-inflammatory drugs, such as aspirin, ibuprofen, or naproxen, reduce inflammation and relieve pain. Corticosteroids are often prescribed for more serious active disease.
Immunosuppressive drugs (e.g., methotrexate, azathioprine, or cyclophosphamide) are used to treat severe symptoms involving internal organs. Antimalarial medications (e.g., hydroxychloroquine) are preferred treatments for individuals with stable disease.64 Ultraviolet light may initiate flares and protection from sun exposure is helpful. Prolonged use of certain drugs can cause transient SLE-like symptoms, and the medication history is important for differential diagnosis.
Alloimmunity
Alloantigens. Genetic diversity is the norm in humans. Diversity also is observed among self- antigens, so that two individuals may have different antigens on their tissues and, therefore, make an immune response against each other's tissues. Some self- antigens, such as the ABO blood group, have limited diversity with very few different antigens being expressed in the population, whereas others, such as the HLA system, have tremendous diversity.
Alloimmune disease: transfusion reactions. Red blood cells (erythrocytes) express several important surface antigens, which are known collectively as the blood group antigens and can be targets of alloimmune reactions. More than 80 different red cell antigens are grouped into several dozen blood group systems. The most important of these, because they provoke the strongest humoral alloimmune response, are the ABO and Rh systems. The ABO blood group consists of two major carbohydrate antigens, labeled A
and B (Figure 8-17), that are expressed on virtually all cells. These are codominant so that both A and B can be simultaneously expressed, resulting in an individual having any one of four different blood types. The erythrocytes of blood type A express the type A carbohydrate antigen, those with blood type B express the B antigen, those with blood type AB express both A and B antigens on the same cell, and those of blood type O express neither the A nor the B antigen. A person with type A blood also has circulating antibodies to the B carbohydrate antigen. If this person receives blood from a type AB or B individual, a severe transfusion reaction occurs, and the transfused erythrocytes are destroyed by agglutination or complement-mediated lysis. Similarly, a type B individual (whose blood contains anti-A antibodies) cannot receive blood from a type A or AB donor. Type O individuals, who have neither antigen but have both anti-A and anti-B antibodies, cannot accept blood from any of the other three types. These naturally occurring antibodies, called isohemagglutinins, are IgM immunoglobulins and are induced
early in life against similar antigens expressed on naturally occurring bacteria in the intestinal tract.
FIGURE 8-17 ABO Blood Types. This figure shows the relationship of antigens and antibodies associated with the ABO blood groups. The surfaces of erythrocytes of individuals with blood group O have a core carbohydrate that is present on cells of all ABO blood groups (H antigen).
The sera of blood group O individuals contain IgM antibodies against both A and B carbohydrates. In individuals of the blood group A, some of the H antigens have been modified into A antigens. The sera of these individuals have IgM antibodies against the B antigen. In individuals with blood group B, some of the H antigens have been modified into B antigens. These individuals have IgM antibodies against the A antigen in their sera. In individuals of the blood group AB, some of the H antigens have been modified into both the A and B antigens.
These individuals do not have antibody to either A or B antigens.
Because individuals with type O blood lack both types of antigens, they are considered universal donors, meaning that anyone can accept their red blood cells. Similarly, type AB individuals are considered universal recipients because they lack both anti-A and anti-B antibodies and can be transfused with any ABO blood type. Harmful transfusion reactions can be prevented only by complete and careful ABO matching between donor and recipient. The Rh blood group is a group of antigens expressed only on red blood cells.
This is the most diverse group of red cell antigens, consisting of at least 45 separate antigens, although only 1 is considered of major importance: the D antigen. Individuals who express the D antigen on their red cells are Rh-positive, whereas
individuals who do not express the D antigen are Rh-negative. When discussing the gene for the Rh antigen, the letter d is used to indicate lack of D. Rh-positive individuals can have either a DD or Dd genotype, whereas Rh-negative individuals have the dd genotype. About 85% of North Americans are Rh-positive. Rh-negative individuals can make an IgG antibody to the D antigen (anti-D) if exposed to Rh- positive erythrocytes. A disease called hemolytic disease of the newborn was most commonly caused by
IgG anti-D alloantibody produced by Rh-negative mothers against erythrocytes of their Rh-positive fetuses (see Chapter 22). The mother's antibody crossed the placenta and destroyed the red blood cells of the fetus. The occurrence of this particular form of the disease has decreased dramatically because of the use of prophylactic anti-D immunoglobulin (i.e., RhoGAM). By mechanisms that are still not completely understood, administration of anti-D antibody within a few days of exposure to RhD-positive erythrocytes completely prevents sensitization against the D antigen. Because hemolytic disease of the newborn related to the D antigen has been controlled, alloantibodies against the other Rh antigens have become more important. In general, these alloantibodies are associated with a less severe hemolytic disease.
Alloimmune disease: transplant rejection. Molecules of the major histocompatibility complex (MHC) were discussed in Chapter 7 as antigen-presenting molecules. MHC molecules are also a major target of transplant rejection. As a result of studies of transplantation, the human MHC molecules are also referred to as human leukocyte antigens (HLAs) and the different MHC genetic loci are commonly called HLA-A, HLA-B, HLA-C, HLA- DR, HLA-DQ, and HLA-DP (Figure 8-18). Additional genes for complement components (e.g., C4, factor B) are also contained in the MHC region and are referred to as class III loci. The class I (HLA-A, -B, and -C) and class II MHC loci (HLA-DR, -DQ, and -DP) are the most genetically diverse (polymorphic) of any human genetic loci. Within the human population, the number of possible different alleles (i.e., forms of the gene) expressed by each locus is astounding. For example, more than 300 different HLA-A molecules are expressed in the population. These numbers are based on the polymorphism of observed DNA sequences and may not reflect differences in function.
FIGURE 8-18 Human Leukocyte Antigens (HLAs). The major histocompatibility complex (MHC) is located on the short arm of chromosome 6 and contains genes that code for class I antigens, class II antigens, and class III proteins (i.e., complement proteins and cytokines). (From Peakman M,
Vergani D: Basic and clinical immunology, ed 2, London, 2009, Churchill Livingstone.)
Clearly, not every allele is expressed in the same individual. Humans have two copies of each MHC locus (one inherited from each parent) that are codominant so that molecules encoded by each parent's genes are expressed on the surface of every cell, except erythrocytes. Within an individual, each locus will express only one allele. For instance, each person will have at most two different HLA-A proteins (one from each parent). However, with the tremendous number of possible alleles that can be expressed throughout the population, it is likely that any two unrelated individuals will have different MHC antigens. The diversity of MHC molecules becomes clinically relevant during organ
transplantation. The recipient of a transplant can mount an immune response against the foreign HLA antigens on the donor tissue, resulting in rejection. To minimize the chance of tissue rejection, the donor and recipient are often tissue-typed beforehand to identify differences in HLA antigens. Because of the large number of different alleles, it is highly unlikely that a perfect match can be found between someone who needs a transplant and a potential donor from the general population. The more similar two individuals are in their HLA tissue type, the more likely a transplant from one to the other will be successful. Clearly, the most successful transplants would be between identical twins because they are identical genetically. The specific combination of alleles at the six major HLA loci on one
chromosome (A, B, C, DR, DQ, and DP) is termed a haplotype. Each individual has two HLA haplotypes, one from the paternal chromosome 6 and another from the maternal chromosome (Figure 8-19). Each parent passes on one set of HLA antigens to each of his or her offspring, meaning that children usually share half their HLA antigens with each parent. Odds dictate that children will share one haplotype with half their siblings and either no haplotypes or both haplotypes with a quarter of their siblings. Thus the chance of finding a match among siblings is much higher (25%) than the general population.
FIGURE 8-19 Inheritance of HLA. HLA alleles are inherited in a codominant fashion; both maternal and paternal antigens are expressed. Specific HLA alleles are commonly given
numbers to indicate different antigens. In this example, the mother has linked genes for HLA-A3 and HLA-B12 on one chromosome 6 and genes for HLA-A10 and HLA-B5 on the second
chromosome 6. The father has HLA-A28 and HLA-B7 on one chromosome and HLA-A1 and HLA- B35 on the second chromosome. The children from this pairing may have one of four possible
combinations of maternal and paternal HLA.
Transplant rejection may be classified as hyperacute, acute, or chronic, depending on the amount of time that elapses between transplantation and rejection. Hyperacute rejection is immediate and rare. When the circulation is reestablished to the grafted area, the graft may immediately turn white (the so-called white graft) instead of a normal pink color. Hyperacute rejection usually occurs because of preexisting antibody (type II reaction) to HLA antigens on the vascular endothelial cells in the grafted tissue. Acute rejection is a cell-mediated immune response that occurs within days to
months after transplantation. This type of rejection occurs when the recipient develops an immune response against unmatched HLA antigens after transplantation. A biopsy of the rejected organ usually shows an infiltration of lymphocytes and macrophages characteristic of a type IV reaction. Chronic rejection may occur after a period of months or years of normal
function. It is characterized by slow, progressive organ failure. Chronic rejection usually results from a weak cell-mediated (type IV) reaction against minor histocompatibility antigens on the grafted tissue. However, antibodies against HLA and other antigens also may cause chronic rejection through activation of complement or antibody-dependent cellular cytotoxicity (ADCC) with NK cells.
Quick Check 8-4
1. Why do certain drugs become immunogenic to the host?
2. Why is SLE considered an autoimmune disease?
3. Define the different types of graft rejection.
Did You Understand? Infection 1. Infectious disease is a significant cause of morbidity and mortality in the United States and worldwide.
2. Pathogens have unique characteristics that influence their ability to cause disease.
3. Bacteria injure cells by producing exotoxins or endotoxins. Exotoxins are enzymes that can damage the plasma membranes of host cells or can inactivate enzymes critical to protein synthesis, and endotoxins activate the inflammatory response and produce fever.
4. Septicemia is the proliferation of bacteria in the blood. Endotoxins released by blood-borne bacteria cause the release of vasoactive enzymes that increase the permeability of blood vessels. Leakage from vessels causes hypotension that can result in septic shock.
5. Viruses enter host cells and use the metabolic processes of host cells to proliferate and cause disease.
6. Viruses that have invaded host cells may decrease protein synthesis, disrupt lysosomal membranes, form inclusion bodies where synthesis of viral nucleic acids is occurring, fuse with host cells to produce giant cells, alter antigenic properties of the host cell, transform host cells into cancerous cells, and promote bacterial infection.
7. Diseases caused by fungi are called mycoses, and they occur in two forms: yeasts (spheres) and molds (filaments or hyphae).
8. Dermatophytes are fungi that infect skin, hair, and nails with diseases such as ringworm and athlete's foot.
9. Fungi release toxins and enzymes that are damaging to tissue. Candida albicans is the most common cause of fungal infections in humans.
10. Parasitic microorganisms range from unicellular protozoa to large worms. Although less common in the United States, parasites and protozoa are common causes of infection worldwide.
11. Parasitic and protozoal infections are rarely transmitted from human to human. Infection mainly spreads through vectors (e.g., by mosquito bites) or through contaminated water or food (i.e., malaria, Chagas disease, sleeping sickness, and leishmaniasis).
12. Infection control measures include implementation of clean food and water, management of sewage and waste, control of insects that transmit disease, vaccination, appropriate use of antimicrobials, and passive immunotherapy.
Deficiencies in Immunity 1. Immunodeficiency is the failure of mechanisms of self-defense to function in their normal capacity.
2. Immunodeficiencies are either congenital (primary) or acquired (secondary). Congenital immunodeficiencies are caused by genetic defects that disrupt lymphocyte development, whereas acquired immunodeficiencies are secondary to disease or other physiologic alterations.
3. The clinical hallmark of immunodeficiency is a propensity to unusual or recurrent severe infections. The type of infection usually reflects the immune system defect.
4. The most common infections in individuals with defects of cell-mediated immune response are fungal and viral, whereas infections in individuals with defects of the humoral immune response or complement function are primarily bacterial.
5. Severe combined immunodeficiency (SCID) is a total lack of T-cell function and a severe (either partial or total) lack of B-cell function.
6. Wiskott-Aldrich syndrome is caused by decreased production of IgM antibody.
7. DiGeorge syndrome (congenital thymic aplasia or hypoplasia) is characterized by complete or partial lack of the thymus (resulting in depressed T-cell immunity), frequently associated with diminished or absent parathyroid gland activity (resulting in hypocalcemia) and cardiac anomalies.
8. Antibody deficiencies result from defects in B-cell maturation or function and range from a complete lack of the human bursal equivalent, the lymphoid organs required for B-cell maturation (as in Bruton agammaglobulinemia), to deficiencies
in a single class of immunoglobulins (e.g., selective IgA deficiency).
9. Phagocyte defects include inadequate numbers or alteration in function, such as inadequate adhesion to bacteria or ineffective killing.
10. Complement and mannose-binding lectin deficiencies also are rare causes of increased risk for infection.
11. Acquired immunodeficiencies are caused by superimposed conditions, such as malnutrition, medical therapies, physical or psychologic trauma, or infections.
12. Immunodeficiency syndromes usually are treated by replacement therapy. Deficient antibody production is treated by replacement of missing immunoglobulins with commercial gamma-globulin preparations. Lymphocyte deficiencies are treated by the replacement of host lymphocytes with transplants of bone marrow, fetal liver, or fetal thymus from a donor. There are ongoing trials for gene therapy.
13. AIDS is an acquired dysfunction of the immune system caused by a retrovirus (HIV) that infects and destroys CD4+ lymphocytes (T-helper cells).
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity 1. Hypersensitivity is an immune response misdirected against the host's own tissues (autoimmunity) or directed against beneficial foreign tissues, such as transfusions or transplants (alloimmunity); or it can be exaggerated responses against environmental antigens (allergy).
2. Mechanisms of hypersensitivity are classified as type I (IgE-mediated) reactions, type II (tissue-specific) reactions, type III (immune complex–mediated) reactions, and type IV (cell-mediated) reactions.
3. Hypersensitivity reactions can be immediate (developing within seconds or hours) or delayed (developing within hours or days).
4. Anaphylaxis, the most rapid immediate hypersensitivity reaction, is an explosive reaction that occurs within minutes of reexposure to the antigen and can lead to cardiovascular shock.
5. Type I (IgE-mediated) reactions occur after antigen reacts with IgE on mast cells, leading to mast cell degranulation and the release of histamine and other inflammatory substances.
6. Type II (tissue-specific) reactions are caused by four possible mechanisms: complement-mediated lysis, opsonization and phagocytosis, antibody-dependent cell-mediated cytotoxicity, and modulation of cellular function.
7. Type III (immune complex–mediated) reactions are caused by the formation of immune complexes that are deposited in target tissues, where they activate the complement cascade, generating chemotactic fragments that attract neutrophils into the inflammatory site.
8. Immune complex disease can be a systemic reaction, such as serum sickness (e.g., Raynaud phenomenon), or localized, such as the Arthus reaction.
9. Type IV (cell-mediated) reactions are caused by specifically sensitized T cells, which either kill target cells directly or release lymphokines that activate other cells, such as macrophages.
10. Allergens are antigens that cause allergic responses, usually a type I hypersensitivity response.
11. Autoimmune disease is loss of tolerance to self-antigens. There can be a genetic predisposition and the diseases can be a type II or type III hypersensitivity reaction.
12. Alloimmunity is the immune system's reaction against antigens on the tissues of other members of the same species.
13. Alloimmune disorders include transient neonatal disease, in which the maternal immune system becomes sensitized against antigens expressed by the fetus; and transplant rejection and transfusion reactions, in which the immune system of the recipient of an organ transplant or blood transfusion reacts against foreign antigens on the donor's cells.
Key Terms ABO blood group, 209
Acquired immunodeficiency syndrome (AIDS), 194
Acute rejection, 210
Adenosine deaminase (ADA) deficiency, 191
Agammaglobulinemia, 191
Allergen, 206
Allergy, 199
Alloimmune disease, 199
Alloimmunity, 199
Anaphylaxis, 201
Ankylosing spondylitis, 207
Antibiotic resistance, 181, 187
Antibody-dependent cell-mediated cytotoxicity (ADCC), 203
Antigenic drift, 183
Antigenic shift, 183
Antiretroviral therapy (ART), 197
Antitoxin, 180
Arthus reaction, 205
Atopic, 203
Attenuated virus, 188
Autoimmune disease, 199, 208
Autoimmunity, 199
β-lactamase, 181
Bacteremia, 182
Bare lymphocyte syndrome, 191
Biofilms, 180
Blood group antigen, 208
Bruton agammaglobulinemia, 192
C3 deficiency, 192
CCR5 antagonist, 197
Chronic granulomatous disease (CGD), 192
Chronic mucocutaneous candidiasis, 192
Chronic rejection, 211
Communicability, 177
Complement deficiency, 192
Contact dermatitis, 206
Cryoglobulins, 205
Defects in innate immunity, 191
Delayed hypersensitivity reaction, 201
Delayed hypersensitivity skin test, 206
Dermatophyte, 184
Desensitization, 207
DiGeorge syndrome, 191
Dimorphic fungus (pl., fungi), 184
Endotoxic shock, 182
Endotoxin (lipopolysaccharide [LPS]), 181
Erythema, 206
Exotoxin, 180
Graft-versus-host disease (GVHD), 193
Herd immunity, 189
HIV integrase, 194
HIV integrase inhibitor, 197
HIV fusion inhibitor, 197
HIV protease, 195
HIV protease inhibitor, 197
Human immunodeficiency virus (HIV), 194
Human leukocyte antigen (HLA), 210
Hyperacute rejection, 210
Hypersensitivity, 199
Hypogammaglobulinemia, 191
Immediate hypersensitivity reaction, 201
Immune deficiency, 189
Immunogenicity, 188
Induration, 206
Infectivity, 177
Isohemagglutinin, 209
Major histocompatibility complex (MHC), 209
Mannose-binding lectin deficiency, 192
Mesenchymal stem cell (MSC), 193
Methicillin-resistant Staphylococcus aureus (MRSA), 188
Multiple-antibiotic resistance, 187
Mycosis (pl., mycoses), 184
Parasitic microorganisms, 185
Passive immunotherapy, 189
Pathogenicity, 177
Phagocytic defects, 192
Portal of entry, 177
Predominantly antibody deficiency, 191
Primary (congenital) immune deficiency, 189
Raynaud phenomenon, 205
Reverse transcriptase, 194
Reverse transcriptase inhibitor, 197
Rh blood group, 209
Secondary (acquired) immune deficiency, 189
Selective IgA deficiency, 191
Septicemia, 182
Serum sickness, 205
Severe combined immunodeficiency (SCID), 191
Severe congenital neutropenia, 192
Systemic lupus erythematosus (SLE), 208
Tissue-specific antigen, 203
Tolerance, 208
Toxigenicity, 177
Toxoid, 188
Type I hypersensitivity, 201
Type II hypersensitivity, 203
Type III hypersensitivity, 205
Type IV hypersensitivity, 206
Universal donor, 209
Universal recipient, 209
Urticaria (hives), 202
Vaccination, 188
Vaccine, 188
Virulence, 177
Wheal and flare reaction, 202
Wiskott-Aldrich syndrome, 191
X-linked SCID, 191
References 1. Khabbaz RF, et al. Challenges of infectious diseases in the USA. Lancet. 2014;384(9937):53–63.
2. Hoyert DL, Xu J. no. 6. National Center for Health Statistics: Hyattsville, Md; 2012. Deaths: preliminary data for 2011, national vital statistics reports. vol 61.
3. Murthy S, et al. Infections of the developing world. Crit Care Clin. 2013;29(3):485–507.
4. Brown AF, et al. Staphylococcus aureus colonization: modulation of host immune response and impact on human vaccine design. Front Immunol. 2014;4:507.
5. Scherr TD, et al. Hiding in plain sight: interplay between staphylococcal biofilms and host immunity. Front Immunol. 2014;5:37.
6. Foster TJ, et al. Adhesion, invasion and evasion: the many functions of the surface proteins of Staphylococcus aureus. Nat Rev Microbiol. 2014;12(1):49–62.
7. Zecconi A, Scali F. Staphylococcus aureus virulence factors in evasion from innate immune defenses in human and animal diseases. Immunol Lett. 2013;150:12–22.
8. Monack DM, Hultgren SJ. The complex interactions of bacterial pathogens and host defenses. Curr Opin Microbiol. 2013;16(1):1–3.
9. Baxt LA, et al. Bacterial subversion of host innate immune pathways. Science. 2013;340(6133):697–701.
10. Alonzo F 3rd, Torres VJ. The bicomponent pore-forming leucocidins of Staphylococcus aureus. Microbiol Mol Biol Rev. 2014;78(2):199–230.
11. Scully IL, et al. Covering all the bases: preclinical development of an effective Staphylococcus aureus vaccine. Front Immunol. 2014;5:109.
12. Huet O, Chin-Dusting JPF. Septic shock: desperately seeking treatment. Clin Sci. 2014;126(1):31–39.
13. Hotchkiss RS, et al. Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy. Nat Rev Immunol. 2013;13(12):862–874.
14. Bean AGD, et al. Studying immunity to zoonotic diseases in the natural host —keeping it real. Nat Rev Immunol. 2013;13(12):851–861.
15. Ramsey CD, Kumar A. Influenza and endemic viral pneumonia. Crit Care Clin. 2013;29:1069–1086.
16. Sun X, Whittaker GR. Entry of influenza virus. Adv Exp Med Biol. 2013;790:72–82.
17. Schrauwen EJA, et al. Determinants of virulence of influenza A virus. Eur J
Clin Microbiol Infect Dis. 2014;33(4):479–490. 18. World Health Organization (WHO). Malaria Fact Sheet No 94 Reviewed
April 2015. Author: Geneva Switzerland; 2015 [Available at] http://www.who.int/mediacentre/factsheets/fs094/en/.
19. Centers for Disease Control and Prevention (CDC). Antibiotic resistance threats in the United States, 2013. [Available at] www.cdc.gov/drugresistance/threat-report-2013/pdf/ar-threats-2013- 508.pdf; 2013 [Accessed to May 12, 2015].
20. Rubinstein E, Keynan Y. Vancomycin-resistant enterococci. Crit Care Clin. 2013;29:841–852.
21. Kennedy D. Time to deal with antibiotics. Science. 2013;342(6160):776–777. 22. Rappuoli R, et al. Vaccines, new opportunities for a new society. Proc Natl
Acad Sci U S A. 2014;111(34):12288–12293. 23. Larson HJ, et al. Measuring vaccine confidence: analysis of data obtained by
a media surveillance system used to analyse public concerns about vaccines. Lancet Infect Dis. 2013;13(7):606–613.
24. Smith JC, et al. Building confidence in vaccines. Adv Exp Med Biol. 2013;764:81–98.
25. Centers for Disease Control and Prevention (CDC). Progress toward poliomyelitis eradication—Nigeria, January 2009–June 2010. MMWR Morb Mortal Wkly Rep. 2010;59(26):802–807.
26. Global Polio Eradication Initiative. Data and monitoring, November 19, 2014. [Available at] http://polioeradication.org/Dataandmonitoring/Poliothisweek.aspx.
27. Hemachudha T, et al. Human rabies: neuropathogenesis, diagnosis, and management. Lancet Neurol. 2013;12(5):498–513.
28. Graham BS. Advances in antiviral vaccine development. Immunol Rev. 2013;255(1):230–242.
29. Cant A, Battersby A. When to think of immunodeficiency. Adv Exp Med Biol. 2013;764:167–177.
30. Milner JD, Holland SM. The cup runneth over: lessons from the ever- expanding pool of primary immunodeficiency diseases. Nat Rev Immunol. 2013;13(9):635–648.
31. Al-Herz W, et al. Primary immunodeficiency diseases: an update on the classification from the International Union of Immunological Societies Expert Committee for Primary Immunodeficiency. Front Immunol. 2014;5:162.
32. van der Burg M, van Zelm MC. Clinical spectrum of SCID: the key is the thymus? Front Immunol. 2014;5:111.
33. Durandy A, et al. Primary antibody deficiencies. Nat Rev Immunol. 2013;13(7):510–533.
34. Holland SM. Chronic granulomatous disease. Hematol Oncol Clin N Am. 2013;27:89–99.
35. Schwab I, Nimmerjahn F. Intravenous immunoglobulin therapy: how does IgG modulate the immune system. Nat Rev Immunol. 2013;13(3):176–189.
36. Horn B, Cowan MJ. Unresolved issues in hematopoietic stem cell transplantation for severe combined immunodeficiency: need for safer conditioning and reduced late effects. J Allergy Clin Immunol. 2013;131(5):1306–1311.
37. Eggenhofer E, et al. The life and fate of mesenchymal stem cells. Front Immunol. 2014;55:148.
38. Toubai T, et al. Mesenchymal stem cells for treatment and prevention of graft-versus-host disease after allogeneic hematopoietic cell transplantation. Curr Stem Cell Res Therapy. 2009;4(4):252–259.
39. Blaese RM. Development of gene therapy for immunodeficiency: adenosine deaminase deficiency. Pediatr Res. 1993;33(1 Suppl):S49–S53.
40. Onodera M, et al. Gene therapy for severe combined immunodeficiency caused by adenosine deaminase deficiency: improved retroviral vectors for clinical trials. Acta Haematol. 1999;101(2):89–96.
41. Mukherjee S, Thrasher AJ. Gene therapy for PIDs: progress, pitfalls, and prospects. Gene. 2013;525:174–181.
42. World Health Organization (WHO). Towards universal access: scaling up priority HIV/AIDS interventions in the health sector: progress report 2009. Author: Geneva Switzerland; 2009.
43. Centers for Disease Control and Prevention (CDC). Diagnosis of HIV infection in the United States and dependent areas, 2013. [HIV Surveillance Report, Volume 25; Available at] http://www.cdc.gov/hiv/library/reports/surveillance/2013/surveillance_Report_vol_25.html [Last updated: February 23, 2015; Accessed March 27, 2015].
44. World Health Organization (WHO). Patient evaluation and antiretroviral treatment for adults and adolescents clinical protocol for the WHO European Region 2012, revision. Author: Geneva, Switzerland; 2012.
45. National Institutes of Health (NIH). Guidelines for the use of antiretroviral agents in HIV-1-infected adults and adolescents. [Available at] http://aids info.nih.gov/guidelines [Accessed August 11, 2014].
46. Archin NM, Margolis DM. Emerging strategies to deplete the HIV reservoir. Curr Opin Infect Dis. 2014;27(1):29&ndash
