Lymphatic and immune systems

In this section, we will examine the physiology, or how the immune and lymphatic systems work.

As the afferent lymph vessels collect excess fluid from the capillaries, they take the fluid to the lymph nodes, which filter the lymph fluid and activate the immune response if pathogens are found. The filtered lymph returns to the circulation via the efferent lymph vessels.

Immune Response

There are three major kinds of immune response, classified by their timing in the body defense process: barriers, innate immune responses, and adaptive immune responses.

Table: Comparison of innate and adaptive immunity

Barriers: Skin and mucous membranes act immediately to protect the body from pathogens. This includes the digestive system’s salvia, mucous lining, low pH/acidic environment, and intestinal flora (“good bacteria”). The barriers are sometimes considered part of the innate immune response.

Innate immune response: This is a rapid, non-specific response to pathogens. The innate immune response is quick to kick in when any kind of pathogen is present. The innate immune response relies on phagocytosis, where cells engulf and destroy invading cells and particles. Macrophages, neutrophils, and dendritic cells have phagocytic properties. Macrophages (which are in the agranulocyte white blood cell category) serve as a first line of defense, as they reside through many body tissues. Neutrophils, a type of agranulocyte, act as reinforcements; the neutrophils are called to action via chemotaxis in the bloodstream surrounding infected tissues. Monocytes, which are also agranulocytes, can differentiate into either macrophages or dendritic cells to contribute to the defense process; the monocytes are signaled into action when inflammation is present. Inflammation and/or fever are common in the innate immune response. The inflammatory process includes 4 steps:

1. Histamine release to respond to the pathogen

2. Redness, heat, and pain as the blood vessels dilate

3. Chemotaxis and swelling as the blood vessels increase permeability

4. Clean up and repair via phagocytosis

Adaptive immune response: This slower response is specific, as lymphocytes are called into play to fight specific known pathogens that the body has been exposed to before. Adaptive responses come in two major forms:

1. Cellular immunity is mediated by T-cells. The T-cells respond with cytotoxic chemicals to destroy specific antigens.

2. Humoral immunity is mediated by B-cells. The B-cells respond with antibodies to specific antigens. Humans have 5 different classes of antibodies: IgM, IgD, IgG, IgA, and IgE.

· IgM is the main antibody in a primary response.

· IgG is the main antibody of a secondary response.

· IgA is found in mucus, tears, and saliva. It is also important for newborns, as they acquire IgA immunity from their mother’s breast milk.

· IgE responds to allergies and parasites

· IgD is a type of B cell receptor

Humoral Immunity

Humoral immunity can be acquired first-hand (actively) or second-hand (passively). It can also be acquired naturally via exposure to the antigen, or artificially, such as via a vaccination.

Table: Ways to acquire humoral immunity. Humoral immunity is part of the B lymphocyte mediated adaptive response to specific antigens.

Immune Responses

Summary of immune responses (Betts et al., 2013)

Alterations can occur in immune responses when parts of the immune system are damaged. Three of the most common alterations are immunodeficiency, autoimmunity, and hypersensitivity.

Immunodeficiency: In immunodeficiency conditions, such as HIV, hepatitis, and side effects of corticosteroid therapy, chemotherapy, and leukemia, the immune system becomes less responsive to pathogens.

Autoimmunity: In autoimmune conditions, such as rheumatoid arthritis, lupus, multiple sclerosis, and type I diabetes, the immune system begins to attack its own healthy cells.

Hypersensitivity: In hypersensitivity conditions, the immune system overresponds to an antigen, and this can lead to allergic responses.