Immune System Physiology: Mechanisms of Immune Response and Defense Against
Pathogens
Introduction
The immune system is a complex network of specialized organs, cells and defense molecules
that provides protection against infectious disease-causing microbes and other foreign
antigens. Functioning as the body's defense police force, it utilizes adaptive, innate and
mechanical barriers working in coordinated manner to identify and eradicate pathogens or
transformed tumor cells. Its ability to mount specific memory responses prevents reinfection
by the same microbe providing lifelong immunity. This overview aims to describe the key
cellular and molecular components involved in immune surveillance and mediate the intricate
defensive strategies mounted upon challenge.
Innate Immune Barriers
The first line of defense utilizes non-specific physical and chemical methods to curb entry
and spread of invading microbes:
- Intact Skin - Acts as a protective covering to keep most pathogens from entering body.
- Stomach Acid - Low pH in stomach deactivates most ingested pathogens and protects
intestine.
- Saliva - Contains lysozyme that disrupts bacterial cell walls and sweat maintains skin
acidity.
- Cilia and Mucus - Ciliated cells in respiratory tract flush out trapped microbes and debris in
mucus.
- Normal Body Flora - Commensal microbes occupy mucosal niches competitively inhibiting
pathogens.
However, microbes occasionally breach these barriers gaining entry into tissues and
bloodstream triggering inflammation response.
Inflammatory Response
The early detection of infected and damaged cells is handled by the innate immune
mechanisms which provides non-specific but rapid response against pathogens:
- Phagocytes - Neutrophils and macrophages recognize pathogens via pattern recognition
receptors and engulf microbes using phagocytosis.
- Phagosome Maturation - Formation of phagolysosome fuses ingested pathogen containing
vesicle with lysosomes exposing it to enzymes and reactive oxygen/nitrogen species to kill
microbes.
- Complement System - Series of plasma proteins activated by pathogens trigger
inflammatory cascade and opsonization facilitating phagocytosis.
- Cytokines and Chemokines - Proteins released by macrophages recruit leukocytes to
infection site and mediate acute inflammatory functions like fever to stunt pathogen growth.
- Mast Cells - Release histamine and other compounds promoting edema, vomiting and
diarrhea to expel or restrict microbes at entry sites like skin.
While swift, the inflammatory reaction must be tightly regulated to prevent tissue damage
from excessive activity. It also bridges the gap between innate and adaptive defenses when
prolonged.
Antigen Presentation and T cell Activation
The specific cellular adaptive immunity depends on T and B lymphocytes that undergo clonal
selection and proliferation upon encountering "non-self" antigen. Their activation involves
antigen presenting cells - macrophages and dendritic cells that uptake, process and present
pathogen peptides via MHC class I/II molecules:
- Exogenous antigen from phagocytosed pathogens are degraded and loaded on MHC class II
in dendritic cells which then migrate to lymph nodes.
- Endogenous antigens from viruses replicating within host cells are presented via MHC class
I molecules ubiquitously expressed on nucleated cells.
- T cells bearing cognate TCR recogize peptide-MHC complex and receive co-stimulatory
signals causing them to differentiate and divide clonally as effector cells.
- Helper CD4+ T cells activate and regulate functions of other immune cells like
macrophages and B cells producing antibodies.
- Cytotoxic CD8+ T cells directly kill infected cells displaying antigens through secretion of
granzymes and perforin to eliminate intracellular pathogens.
This highly specific interaction between antigen presenting cells, T cell receptors and co-
stimulatory signals initiates the central events of adaptive immunity.
Humoral Immune Response
On receiving help from activated CD4+ T cells, naive B cells specific to pathogen epitopes
proliferate into memory B cells and plasma cells that produce large amounts of protective
antibodies:
- B cell receptor recognizes intact pathogen and binds its surface antigens.
- Engulfed pathogen is processed into peptides that combine with MHC class II and stimulate
specific TH cell subset.
- TH cells recognize and provide survival signals in the form of CD40 ligand and cytokines
like IL-4.
- B cells become plasma blasts and migrate to secondary lymphoid follicles undergoing
somatic hypermutation that strengthens antibody affinity.
- Plasma cells continue to secrete high affinity antibodies in circulation and tissues to directly
neutralize extracellular pathogens or coat them facilitating phagocytosis.
- Memory B cells persist long term to mount rapid anamnestic responses upon reinfection by
the same pathogen.
Antibodies are highly specific and versatile molecules capable of sterilizing immunity against
humoral pathogens and toxins.
Cell-Mediated Immunity
The cell-mediated immunity relies on CD8+ CTLs and macrophage activity to eliminate
intracellular infectious agents that evade humoral defenses:
- Macrophages sample antigens released from dying infected cells and present peptides on
MHC class I to activate virus-specific CD8+ T cells.
- Upon recognizing cognate antigen, CTLs proliferate clonally within lymph nodes and
migrate as effector CTLs to infected tissues guided by chemokines.
- Recognition of specific peptide-MHC complex on infected cell surface prompts CTLs to
release cytotoxic granules containing perforins and granzymes through immunological
synapse.
- Perforins form pores in target cell membrane facilitating entry of granzymes like tryptases
that trigger apoptosis through caspase activation pathways.
- CTLs also secrete IFN-γ recruiting more macrophages and maintaining inflammation at
infected sites to eradicate persistent intracellular pathogens.
- Memory CD8+ T cells provide long term protection from viruses able to evade antibodies.
CTLs are critical mediators of defenses against viruses, some bacteria and tumors through
direct cell-killing functions.
Immunological Memory and Adaptation
The hallmark feature of the adaptive immunity is its ability to generate immunological
memory against specific pathogens through two mechanisms:
- Memory B & T cells - Upon resolution of infection, some stimulated lymphocytes survive
for years as quiescent memory cells primed to respond more vigorously and rapidly via clonal
expansion on secondary antigen exposure.
- Somatic Hypermutation - Lymphocytes undergoing multiple rounds of division in germinal
centers undergo point mutations in antibody variable region genes and TCRs increasing
affinity for epitopes on re-exposure, ensuring durable protection.
- Immunological Tolerance - Self-reactive naive lymphocytes are eliminated in thymus/bone
marrow or become regulatory T cells to prevent autoimmunity when self-antigens are
encountered.
- Vaccination - Mimicking pathogenesis using weakened live or dead viruses/toxins allows
memory induction without disease risks conferring long lasting protection.
Thus persistence of highly specific memory cell populations confers host protective
immunity through lifetime while limiting collateral autoimmune pathology.
Immune Regulation and Resolution
Failure to switch-off immune effectors post infection can lead to chronic inflammation that
damages tissues:
- Regulatory T cells (Tregs) secrete immunosuppressive cytokines like IL-10 and TGF-β
inhibiting dendritic, neutrophil and macrophage functions and proliferation of lymphocytes.
- Complement and acute phase regulatory proteins tightly control complement cascade
activation spatially and temporally to prevent bystander tissue injury.
- Programmed cell death by apoptosis allows efficient clearance of no longer needed
activated lymphocytes preventing excessive inflammation.
- Resolution phase involves clearance of apoptotic cells and production of pro-resolving
mediators like lipoxins by phagocytes that reduces vascular permeability and brings
inflammation to an end.
- Immune privilege sites like eyes, testes, placenta employ additional tolerance mechanisms
like absence of traditional lymphatic drainage to circumvent damage from adaptive
immunity.
Thus inflammation and immunity must transition timely under regulatory control to eradicate
acute threats but avoid autoimmunity when no longer required.
Immune Disorders
Dysregulation or defect in any aspect of immune mechanisms can compromise host defenses
or else damage self-tissues:
- Immunodeficiencies -SCID, CVID involving impairment of B/T/phagocyte function raise
infection risk from opportunistic microbes.
- Hypersensitivities - Type I (IgE mediated), Type IV (T cell) delayed reactions trigger
allergy, asthma and autoimmunity through inappropriate inflammatory signaling.
- Autoimmunity -Failure of central/peripheral tolerance allows self-reactive clones to mediate
organ specific diseases like diabetes, lupus through molecular mimicry.
- Transplant rejection - Allograft recognition as foreign triggers acute or chronic cytotoxic
rejection through indirect and direct allorecognition pathways.
- Cancer immunology - Tumors escape intact immunity through evasion strategies or induce
suppressive networks to grow and metastasize systematically.
- Immunotherapy -Engineered immunological techniques modify host immune mechanisms
to treat malignancies, chronic infections and allergies.
Understanding disease mechanisms and maintaining immune balance aids preventive and
therapeutic advancements.
Conclusion
The human immune system relies upon layers of protective mechanisms working seamlessly
to shield the host from diverse classes of pathogens. The intricate interplay between innate
sensors, adaptive recognition and subsequent effector functions achieves sterilizing immunity
through lifelong training and memory. Its ability to rapidly adapt also safeguards against
dysregulated responses through regulatory checkpoints. Further insights into underlying
genetic, cellular and molecular circuitries offer promising opportunities to combat immune
disorders, cancers and infections that continue to challenge global health.
The immune system is a complex network of specialized organs, cells and defense molecules
that provides protection against infectious disease-causing microbes and other foreign
antigens. Functioning as the body's defense police force, it utilizes adaptive, innate and
mechanical barriers working in coordinated manner to identify and eradicate pathogens or
transformed tumor cells. Its ability to mount specific memory responses prevents reinfection
by the same microbe providing lifelong immunity. This overview aims to describe the key
cellular and molecular components involved in immune surveillance and mediate the intricate
defensive strategies mounted upon challenge.
Innate Immune Barriers
The first line of defense utilizes non-specific physical and chemical methods to curb entry
and spread of invading microbes:
- Intact Skin - Acts as a protective covering to keep most pathogens from entering body.
- Stomach Acid - Low pH in stomach deactivates most ingested pathogens and protects
intestine.
- Saliva - Contains lysozyme that disrupts bacterial cell walls and sweat maintains skin
acidity.
- Cilia and Mucus - Ciliated cells in respiratory tract flush out trapped microbes and debris in
mucus.
- Normal Body Flora - Commensal microbes occupy mucosal niches competitively inhibiting
pathogens.
However, microbes occasionally breach these barriers gaining entry into tissues and
bloodstream triggering inflammation response.
Inflammatory Response
The early detection of infected and damaged cells is handled by the innate immune
mechanisms which provides non-specific but rapid response against pathogens:
- Phagocytes - Neutrophils and macrophages recognize pathogens via pattern recognition
receptors and engulf microbes using phagocytosis.
- Phagosome Maturation - Formation of phagolysosome fuses ingested pathogen containing
vesicle with lysosomes exposing it to enzymes and reactive oxygen/nitrogen species to kill
microbes.
- Complement System - Series of plasma proteins activated by pathogens trigger
inflammatory cascade and opsonization facilitating phagocytosis.
- Cytokines and Chemokines - Proteins released by macrophages recruit leukocytes to
infection site and mediate acute inflammatory functions like fever to stunt pathogen growth.
- Mast Cells - Release histamine and other compounds promoting edema, vomiting and
diarrhea to expel or restrict microbes at entry sites like skin.
While swift, the inflammatory reaction must be tightly regulated to prevent tissue damage
from excessive activity. It also bridges the gap between innate and adaptive defenses when
prolonged.
Antigen Presentation and T cell Activation
The specific cellular adaptive immunity depends on T and B lymphocytes that undergo clonal
selection and proliferation upon encountering "non-self" antigen. Their activation involves
antigen presenting cells - macrophages and dendritic cells that uptake, process and present
pathogen peptides via MHC class I/II molecules:
- Exogenous antigen from phagocytosed pathogens are degraded and loaded on MHC class II
in dendritic cells which then migrate to lymph nodes.
- Endogenous antigens from viruses replicating within host cells are presented via MHC class
I molecules ubiquitously expressed on nucleated cells.
- T cells bearing cognate TCR recogize peptide-MHC complex and receive co-stimulatory
signals causing them to differentiate and divide clonally as effector cells.
- Helper CD4+ T cells activate and regulate functions of other immune cells like
macrophages and B cells producing antibodies.
- Cytotoxic CD8+ T cells directly kill infected cells displaying antigens through secretion of
granzymes and perforin to eliminate intracellular pathogens.
This highly specific interaction between antigen presenting cells, T cell receptors and co-
stimulatory signals initiates the central events of adaptive immunity.
Humoral Immune Response
On receiving help from activated CD4+ T cells, naive B cells specific to pathogen epitopes
proliferate into memory B cells and plasma cells that produce large amounts of protective
antibodies:
- B cell receptor recognizes intact pathogen and binds its surface antigens.
- Engulfed pathogen is processed into peptides that combine with MHC class II and stimulate
specific TH cell subset.
- TH cells recognize and provide survival signals in the form of CD40 ligand and cytokines
like IL-4.
- B cells become plasma blasts and migrate to secondary lymphoid follicles undergoing
somatic hypermutation that strengthens antibody affinity.
- Plasma cells continue to secrete high affinity antibodies in circulation and tissues to directly
neutralize extracellular pathogens or coat them facilitating phagocytosis.
- Memory B cells persist long term to mount rapid anamnestic responses upon reinfection by
the same pathogen.
Antibodies are highly specific and versatile molecules capable of sterilizing immunity against
humoral pathogens and toxins.
Cell-Mediated Immunity
The cell-mediated immunity relies on CD8+ CTLs and macrophage activity to eliminate
intracellular infectious agents that evade humoral defenses:
- Macrophages sample antigens released from dying infected cells and present peptides on
MHC class I to activate virus-specific CD8+ T cells.
- Upon recognizing cognate antigen, CTLs proliferate clonally within lymph nodes and
migrate as effector CTLs to infected tissues guided by chemokines.
- Recognition of specific peptide-MHC complex on infected cell surface prompts CTLs to
release cytotoxic granules containing perforins and granzymes through immunological
synapse.
- Perforins form pores in target cell membrane facilitating entry of granzymes like tryptases
that trigger apoptosis through caspase activation pathways.
- CTLs also secrete IFN-γ recruiting more macrophages and maintaining inflammation at
infected sites to eradicate persistent intracellular pathogens.
- Memory CD8+ T cells provide long term protection from viruses able to evade antibodies.
CTLs are critical mediators of defenses against viruses, some bacteria and tumors through
direct cell-killing functions.
Immunological Memory and Adaptation
The hallmark feature of the adaptive immunity is its ability to generate immunological
memory against specific pathogens through two mechanisms:
- Memory B & T cells - Upon resolution of infection, some stimulated lymphocytes survive
for years as quiescent memory cells primed to respond more vigorously and rapidly via clonal
expansion on secondary antigen exposure.
- Somatic Hypermutation - Lymphocytes undergoing multiple rounds of division in germinal
centers undergo point mutations in antibody variable region genes and TCRs increasing
affinity for epitopes on re-exposure, ensuring durable protection.
- Immunological Tolerance - Self-reactive naive lymphocytes are eliminated in thymus/bone
marrow or become regulatory T cells to prevent autoimmunity when self-antigens are
encountered.
- Vaccination - Mimicking pathogenesis using weakened live or dead viruses/toxins allows
memory induction without disease risks conferring long lasting protection.
Thus persistence of highly specific memory cell populations confers host protective
immunity through lifetime while limiting collateral autoimmune pathology.
Immune Regulation and Resolution
Failure to switch-off immune effectors post infection can lead to chronic inflammation that
damages tissues:
- Regulatory T cells (Tregs) secrete immunosuppressive cytokines like IL-10 and TGF-β
inhibiting dendritic, neutrophil and macrophage functions and proliferation of lymphocytes.
- Complement and acute phase regulatory proteins tightly control complement cascade
activation spatially and temporally to prevent bystander tissue injury.
- Programmed cell death by apoptosis allows efficient clearance of no longer needed
activated lymphocytes preventing excessive inflammation.
- Resolution phase involves clearance of apoptotic cells and production of pro-resolving
mediators like lipoxins by phagocytes that reduces vascular permeability and brings
inflammation to an end.
- Immune privilege sites like eyes, testes, placenta employ additional tolerance mechanisms
like absence of traditional lymphatic drainage to circumvent damage from adaptive
immunity.
Thus inflammation and immunity must transition timely under regulatory control to eradicate
acute threats but avoid autoimmunity when no longer required.
Immune Disorders
Dysregulation or defect in any aspect of immune mechanisms can compromise host defenses
or else damage self-tissues:
- Immunodeficiencies -SCID, CVID involving impairment of B/T/phagocyte function raise
infection risk from opportunistic microbes.
- Hypersensitivities - Type I (IgE mediated), Type IV (T cell) delayed reactions trigger
allergy, asthma and autoimmunity through inappropriate inflammatory signaling.
- Autoimmunity -Failure of central/peripheral tolerance allows self-reactive clones to mediate
organ specific diseases like diabetes, lupus through molecular mimicry.
- Transplant rejection - Allograft recognition as foreign triggers acute or chronic cytotoxic
rejection through indirect and direct allorecognition pathways.
- Cancer immunology - Tumors escape intact immunity through evasion strategies or induce
suppressive networks to grow and metastasize systematically.
- Immunotherapy -Engineered immunological techniques modify host immune mechanisms
to treat malignancies, chronic infections and allergies.
Understanding disease mechanisms and maintaining immune balance aids preventive and
therapeutic advancements.
Conclusion
The human immune system relies upon layers of protective mechanisms working seamlessly
to shield the host from diverse classes of pathogens. The intricate interplay between innate
sensors, adaptive recognition and subsequent effector functions achieves sterilizing immunity
through lifelong training and memory. Its ability to rapidly adapt also safeguards against
dysregulated responses through regulatory checkpoints. Further insights into underlying
genetic, cellular and molecular circuitries offer promising opportunities to combat immune
disorders, cancers and infections that continue to challenge global health.
The immune system is a complex network of specialized organs, cells and defense molecules
that provides protection against infectious disease-causing microbes and other foreign
antigens. Functioning as the body's defense police force, it utilizes adaptive, innate and
mechanical barriers working in coordinated manner to identify and eradicate pathogens or
transformed tumor cells. Its ability to mount specific memory responses prevents reinfection
by the same microbe providing lifelong immunity. This overview aims to describe the key
cellular and molecular components involved in immune surveillance and mediate the intricate
defensive strategies mounted upon challenge.
Innate Immune Barriers
The first line of defense utilizes non-specific physical and chemical methods to curb entry
and spread of invading microbes:
- Intact Skin - Acts as a protective covering to keep most pathogens from entering body.
- Stomach Acid - Low pH in stomach deactivates most ingested pathogens and protects
intestine.
- Saliva - Contains lysozyme that disrupts bacterial cell walls and sweat maintains skin
acidity.
- Cilia and Mucus - Ciliated cells in respiratory tract flush out trapped microbes and debris in
mucus.
- Normal Body Flora - Commensal microbes occupy mucosal niches competitively inhibiting
pathogens.
However, microbes occasionally breach these barriers gaining entry into tissues and
bloodstream triggering inflammation response.
Inflammatory Response
The early detection of infected and damaged cells is handled by the innate immune
mechanisms which provides non-specific but rapid response against pathogens:
- Phagocytes - Neutrophils and macrophages recognize pathogens via pattern recognition
receptors and engulf microbes using phagocytosis.
- Phagosome Maturation - Formation of phagolysosome fuses ingested pathogen containing
vesicle with lysosomes exposing it to enzymes and reactive oxygen/nitrogen species to kill
microbes.
- Complement System - Series of plasma proteins activated by pathogens trigger
inflammatory cascade and opsonization facilitating phagocytosis.
- Cytokines and Chemokines - Proteins released by macrophages recruit leukocytes to
infection site and mediate acute inflammatory functions like fever to stunt pathogen growth.
- Mast Cells - Release histamine and other compounds promoting edema, vomiting and
diarrhea to expel or restrict microbes at entry sites like skin.
While swift, the inflammatory reaction must be tightly regulated to prevent tissue damage
from excessive activity. It also bridges the gap between innate and adaptive defenses when
prolonged.
Antigen Presentation and T cell Activation
The specific cellular adaptive immunity depends on T and B lymphocytes that undergo clonal
selection and proliferation upon encountering "non-self" antigen. Their activation involves
antigen presenting cells - macrophages and dendritic cells that uptake, process and present
pathogen peptides via MHC class I/II molecules:
- Exogenous antigen from phagocytosed pathogens are degraded and loaded on MHC class II
in dendritic cells which then migrate to lymph nodes.
- Endogenous antigens from viruses replicating within host cells are presented via MHC class
I molecules ubiquitously expressed on nucleated cells.
- T cells bearing cognate TCR recogize peptide-MHC complex and receive co-stimulatory
signals causing them to differentiate and divide clonally as effector cells.
- Helper CD4+ T cells activate and regulate functions of other immune cells like
macrophages and B cells producing antibodies.
- Cytotoxic CD8+ T cells directly kill infected cells displaying antigens through secretion of
granzymes and perforin to eliminate intracellular pathogens.
This highly specific interaction between antigen presenting cells, T cell receptors and co-
stimulatory signals initiates the central events of adaptive immunity.
Humoral Immune Response
On receiving help from activated CD4+ T cells, naive B cells specific to pathogen epitopes
proliferate into memory B cells and plasma cells that produce large amounts of protective
antibodies:
- B cell receptor recognizes intact pathogen and binds its surface antigens.
- Engulfed pathogen is processed into peptides that combine with MHC class II and stimulate
specific TH cell subset.
- TH cells recognize and provide survival signals in the form of CD40 ligand and cytokines
like IL-4.
- B cells become plasma blasts and migrate to secondary lymphoid follicles undergoing
somatic hypermutation that strengthens antibody affinity.
- Plasma cells continue to secrete high affinity antibodies in circulation and tissues to directly
neutralize extracellular pathogens or coat them facilitating phagocytosis.
- Memory B cells persist long term to mount rapid anamnestic responses upon reinfection by
the same pathogen.
Antibodies are highly specific and versatile molecules capable of sterilizing immunity against
humoral pathogens and toxins.
Cell-Mediated Immunity
The cell-mediated immunity relies on CD8+ CTLs and macrophage activity to eliminate
intracellular infectious agents that evade humoral defenses:
- Macrophages sample antigens released from dying infected cells and present peptides on
MHC class I to activate virus-specific CD8+ T cells.
- Upon recognizing cognate antigen, CTLs proliferate clonally within lymph nodes and
migrate as effector CTLs to infected tissues guided by chemokines.
- Recognition of specific peptide-MHC complex on infected cell surface prompts CTLs to
release cytotoxic granules containing perforins and granzymes through immunological
synapse.
- Perforins form pores in target cell membrane facilitating entry of granzymes like tryptases
that trigger apoptosis through caspase activation pathways.
- CTLs also secrete IFN-γ recruiting more macrophages and maintaining inflammation at
infected sites to eradicate persistent intracellular pathogens.
- Memory CD8+ T cells provide long term protection from viruses able to evade antibodies.
CTLs are critical mediators of defenses against viruses, some bacteria and tumors through
direct cell-killing functions.
Immunological Memory and Adaptation
The hallmark feature of the adaptive immunity is its ability to generate immunological
memory against specific pathogens through two mechanisms:
- Memory B & T cells - Upon resolution of infection, some stimulated lymphocytes survive
for years as quiescent memory cells primed to respond more vigorously and rapidly via clonal
expansion on secondary antigen exposure.
- Somatic Hypermutation - Lymphocytes undergoing multiple rounds of division in germinal
centers undergo point mutations in antibody variable region genes and TCRs increasing
affinity for epitopes on re-exposure, ensuring durable protection.
- Immunological Tolerance - Self-reactive naive lymphocytes are eliminated in thymus/bone
marrow or become regulatory T cells to prevent autoimmunity when self-antigens are
encountered.
- Vaccination - Mimicking pathogenesis using weakened live or dead viruses/toxins allows
memory induction without disease risks conferring long lasting protection.
Thus persistence of highly specific memory cell populations confers host protective
immunity through lifetime while limiting collateral autoimmune pathology.
Immune Regulation and Resolution
Failure to switch-off immune effectors post infection can lead to chronic inflammation that
damages tissues:
- Regulatory T cells (Tregs) secrete immunosuppressive cytokines like IL-10 and TGF-β
inhibiting dendritic, neutrophil and macrophage functions and proliferation of lymphocytes.
- Complement and acute phase regulatory proteins tightly control complement cascade
activation spatially and temporally to prevent bystander tissue injury.
- Programmed cell death by apoptosis allows efficient clearance of no longer needed
activated lymphocytes preventing excessive inflammation.
- Resolution phase involves clearance of apoptotic cells and production of pro-resolving
mediators like lipoxins by phagocytes that reduces vascular permeability and brings
inflammation to an end.
- Immune privilege sites like eyes, testes, placenta employ additional tolerance mechanisms
like absence of traditional lymphatic drainage to circumvent damage from adaptive
immunity.
Thus inflammation and immunity must transition timely under regulatory control to eradicate
acute threats but avoid autoimmunity when no longer required.
Immune Disorders
Dysregulation or defect in any aspect of immune mechanisms can compromise host defenses
or else damage self-tissues:
- Immunodeficiencies -SCID, CVID involving impairment of B/T/phagocyte function raise
infection risk from opportunistic microbes.
- Hypersensitivities - Type I (IgE mediated), Type IV (T cell) delayed reactions trigger
allergy, asthma and autoimmunity through inappropriate inflammatory signaling.
- Autoimmunity -Failure of central/peripheral tolerance allows self-reactive clones to mediate
organ specific diseases like diabetes, lupus through molecular mimicry.
- Transplant rejection - Allograft recognition as foreign triggers acute or chronic cytotoxic
rejection through indirect and direct allorecognition pathways.
- Cancer immunology - Tumors escape intact immunity through evasion strategies or induce
suppressive networks to grow and metastasize systematically.
- Immunotherapy -Engineered immunological techniques modify host immune mechanisms
to treat malignancies, chronic infections and allergies.
Understanding disease mechanisms and maintaining immune balance aids preventive and
therapeutic advancements.
Conclusion
The human immune system relies upon layers of protective mechanisms working seamlessly
to shield the host from diverse classes of pathogens. The intricate interplay between innate
sensors, adaptive recognition and subsequent effector functions achieves sterilizing immunity
through lifelong training and memory. Its ability to rapidly adapt also safeguards against
dysregulated responses through regulatory checkpoints. Further insights into underlying
genetic, cellular and molecular circuitries offer promising opportunities to combat immune
disorders, cancers and infections that continue to challenge global health.
The immune system is a complex network of specialized organs, cells and defense molecules
that provides protection against infectious disease-causing microbes and other foreign
antigens. Functioning as the body's defense police force, it utilizes adaptive, innate and
mechanical barriers working in coordinated manner to identify and eradicate pathogens or
transformed tumor cells. Its ability to mount specific memory responses prevents reinfection
by the same microbe providing lifelong immunity. This overview aims to describe the key
cellular and molecular components involved in immune surveillance and mediate the intricate
defensive strategies mounted upon challenge.
Innate Immune Barriers
The first line of defense utilizes non-specific physical and chemical methods to curb entry
and spread of invading microbes:
- Intact Skin - Acts as a protective covering to keep most pathogens from entering body.
- Stomach Acid - Low pH in stomach deactivates most ingested pathogens and protects
intestine.
- Saliva - Contains lysozyme that disrupts bacterial cell walls and sweat maintains skin
acidity.
- Cilia and Mucus - Ciliated cells in respiratory tract flush out trapped microbes and debris in
mucus.
- Normal Body Flora - Commensal microbes occupy mucosal niches competitively inhibiting
pathogens.
However, microbes occasionally breach these barriers gaining entry into tissues and
bloodstream triggering inflammation response.
Inflammatory Response
The early detection of infected and damaged cells is handled by the innate immune
mechanisms which provides non-specific but rapid response against pathogens:
- Phagocytes - Neutrophils and macrophages recognize pathogens via pattern recognition
receptors and engulf microbes using phagocytosis.
- Phagosome Maturation - Formation of phagolysosome fuses ingested pathogen containing
vesicle with lysosomes exposing it to enzymes and reactive oxygen/nitrogen species to kill
microbes.
- Complement System - Series of plasma proteins activated by pathogens trigger
inflammatory cascade and opsonization facilitating phagocytosis.
- Cytokines and Chemokines - Proteins released by macrophages recruit leukocytes to
infection site and mediate acute inflammatory functions like fever to stunt pathogen growth.
- Mast Cells - Release histamine and other compounds promoting edema, vomiting and
diarrhea to expel or restrict microbes at entry sites like skin.
While swift, the inflammatory reaction must be tightly regulated to prevent tissue damage
from excessive activity. It also bridges the gap between innate and adaptive defenses when
prolonged.
Antigen Presentation and T cell Activation
The specific cellular adaptive immunity depends on T and B lymphocytes that undergo clonal
selection and proliferation upon encountering "non-self" antigen. Their activation involves
antigen presenting cells - macrophages and dendritic cells that uptake, process and present
pathogen peptides via MHC class I/II molecules:
- Exogenous antigen from phagocytosed pathogens are degraded and loaded on MHC class II
in dendritic cells which then migrate to lymph nodes.
- Endogenous antigens from viruses replicating within host cells are presented via MHC class
I molecules ubiquitously expressed on nucleated cells.
- T cells bearing cognate TCR recogize peptide-MHC complex and receive co-stimulatory
signals causing them to differentiate and divide clonally as effector cells.
- Helper CD4+ T cells activate and regulate functions of other immune cells like
macrophages and B cells producing antibodies.
- Cytotoxic CD8+ T cells directly kill infected cells displaying antigens through secretion of
granzymes and perforin to eliminate intracellular pathogens.
This highly specific interaction between antigen presenting cells, T cell receptors and co-
stimulatory signals initiates the central events of adaptive immunity.
Humoral Immune Response
On receiving help from activated CD4+ T cells, naive B cells specific to pathogen epitopes
proliferate into memory B cells and plasma cells that produce large amounts of protective
antibodies:
- B cell receptor recognizes intact pathogen and binds its surface antigens.
- Engulfed pathogen is processed into peptides that combine with MHC class II and stimulate
specific TH cell subset.
- TH cells recognize and provide survival signals in the form of CD40 ligand and cytokines
like IL-4.
- B cells become plasma blasts and migrate to secondary lymphoid follicles undergoing
somatic hypermutation that strengthens antibody affinity.
- Plasma cells continue to secrete high affinity antibodies in circulation and tissues to directly
neutralize extracellular pathogens or coat them facilitating phagocytosis.
- Memory B cells persist long term to mount rapid anamnestic responses upon reinfection by
the same pathogen.
Antibodies are highly specific and versatile molecules capable of sterilizing immunity against
humoral pathogens and toxins.
Cell-Mediated Immunity
The cell-mediated immunity relies on CD8+ CTLs and macrophage activity to eliminate
intracellular infectious agents that evade humoral defenses:
- Macrophages sample antigens released from dying infected cells and present peptides on
MHC class I to activate virus-specific CD8+ T cells.
- Upon recognizing cognate antigen, CTLs proliferate clonally within lymph nodes and
migrate as effector CTLs to infected tissues guided by chemokines.
- Recognition of specific peptide-MHC complex on infected cell surface prompts CTLs to
release cytotoxic granules containing perforins and granzymes through immunological
synapse.
- Perforins form pores in target cell membrane facilitating entry of granzymes like tryptases
that trigger apoptosis through caspase activation pathways.
- CTLs also secrete IFN-γ recruiting more macrophages and maintaining inflammation at
infected sites to eradicate persistent intracellular pathogens.
- Memory CD8+ T cells provide long term protection from viruses able to evade antibodies.
CTLs are critical mediators of defenses against viruses, some bacteria and tumors through
direct cell-killing functions.
Immunological Memory and Adaptation
The hallmark feature of the adaptive immunity is its ability to generate immunological
memory against specific pathogens through two mechanisms:
- Memory B & T cells - Upon resolution of infection, some stimulated lymphocytes survive
for years as quiescent memory cells primed to respond more vigorously and rapidly via clonal
expansion on secondary antigen exposure.
- Somatic Hypermutation - Lymphocytes undergoing multiple rounds of division in germinal
centers undergo point mutations in antibody variable region genes and TCRs increasing
affinity for epitopes on re-exposure, ensuring durable protection.
- Immunological Tolerance - Self-reactive naive lymphocytes are eliminated in thymus/bone
marrow or become regulatory T cells to prevent autoimmunity when self-antigens are
encountered.
- Vaccination - Mimicking pathogenesis using weakened live or dead viruses/toxins allows
memory induction without disease risks conferring long lasting protection.
Thus persistence of highly specific memory cell populations confers host protective
immunity through lifetime while limiting collateral autoimmune pathology.
Immune Regulation and Resolution
Failure to switch-off immune effectors post infection can lead to chronic inflammation that
damages tissues:
- Regulatory T cells (Tregs) secrete immunosuppressive cytokines like IL-10 and TGF-β
inhibiting dendritic, neutrophil and macrophage functions and proliferation of lymphocytes.
- Complement and acute phase regulatory proteins tightly control complement cascade
activation spatially and temporally to prevent bystander tissue injury.
- Programmed cell death by apoptosis allows efficient clearance of no longer needed
activated lymphocytes preventing excessive inflammation.
- Resolution phase involves clearance of apoptotic cells and production of pro-resolving
mediators like lipoxins by phagocytes that reduces vascular permeability and brings
inflammation to an end.
- Immune privilege sites like eyes, testes, placenta employ additional tolerance mechanisms
like absence of traditional lymphatic drainage to circumvent damage from adaptive
immunity.
Thus inflammation and immunity must transition timely under regulatory control to eradicate
acute threats but avoid autoimmunity when no longer required.
Immune Disorders
Dysregulation or defect in any aspect of immune mechanisms can compromise host defenses
or else damage self-tissues:
- Immunodeficiencies -SCID, CVID involving impairment of B/T/phagocyte function raise
infection risk from opportunistic microbes.
- Hypersensitivities - Type I (IgE mediated), Type IV (T cell) delayed reactions trigger
allergy, asthma and autoimmunity through inappropriate inflammatory signaling.
- Autoimmunity -Failure of central/peripheral tolerance allows self-reactive clones to mediate
organ specific diseases like diabetes, lupus through molecular mimicry.
- Transplant rejection - Allograft recognition as foreign triggers acute or chronic cytotoxic
rejection through indirect and direct allorecognition pathways.
- Cancer immunology - Tumors escape intact immunity through evasion strategies or induce
suppressive networks to grow and metastasize systematically.
- Immunotherapy -Engineered immunological techniques modify host immune mechanisms
to treat malignancies, chronic infections and allergies.
Understanding disease mechanisms and maintaining immune balance aids preventive and
therapeutic advancements.
Conclusion
The human immune system relies upon layers of protective mechanisms working seamlessly
to shield the host from diverse classes of pathogens. The intricate interplay between innate
sensors, adaptive recognition and subsequent effector functions achieves sterilizing immunity
through lifelong training and memory. Its ability to rapidly adapt also safeguards against
dysregulated responses through regulatory checkpoints. Further insights into underlying
genetic, cellular and molecular circuitries offer promising opportunities to combat immune
disorders, cancers and infections that continue to challenge global health.
