The Human Body

Humoral Immune Response

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The humoral immune response involves a complex series of events after antigens enter the body. First, macrophages take up some of the antigen and attach it to class II MHC molecules, which then present the antigen to T helper cells. The T helper cells bind the presented antigen, which stimulates the T helper cells to divide and secrete stimulatory molecules called interleukins. The interleukins in turn activate any B lymphocytes that have also bound the antigen. The activated B cells then divide and secrete antibodies. Finally, the secreted antibodies bind the antigen and help destroy it.

Antibodies

Antibodies are Y-shaped proteins called immunoglobulins (Ig) and are made only by B cells. The antibody binds to the antigen at the ends of the arms of the Y. The area at the base of the Y determines how the antibody will destroy the antigen. This area is used to categorize antibodies into five main classes: IgM, IgG, IgA, IgD, and IgE. During the humoral immune response, IgM is the first class of antibody made. After several days, other classes appear. Exactly which other Ig classes a B cell makes depends on the kind of interleukins it receives from the T helper cells.

Antibodies can sometimes stop an antigen’s disease-causing activities simply by neutralization—that is, by binding the antigen and preventing it from interfering with the cell’s normal activities. For example, the toxin made by tetanus bacteria binds to nerve cells and interferes with their control of muscles. Antibodies against tetanus toxin stick to the toxin and cover the part of it that binds to nerve cells, thereby preventing serious disease. All classes of antibodies can neutralize antigens.

Antibodies also help destroy antigens by preparing them for ingestion by macrophages in a process called opsonization. In opsonization, antibodies coat the surface of antigens. Since macrophages have receptors that stick to the base of the antibody’s Y structure, antigens coated with antibodies are more likely to stick to the macrophages and be ingested. Opsonization is especially important in helping the body resist bacterial diseases.

Finally, IgM and IgG antibodies can trigger the complement system, a group of proteins that cause cells to disintegrate by cutting holes in the cell membrane. Complement is important in resisting bacteria that are hard to destroy in other ways. For example, some of the bacteria that cause pneumonia have a slimy coating, making it hard for macrophages to ingest and eliminate them. However, if IgM and IgG antibodies bind to the pneumonia bacteria and activate the complement system, it is able to cut holes in the bacteria to destroy them.

Although the IgM and IgG classes of antibodies work best in the circulatory system, IgA can exit the bloodstream and appear in other body fluids. IgA is thus important in preventing infection at mucosal surfaces, such as the intestine and the lung. Since these are the sites where most infectious agents enter, IgA is particularly important in resistance to many diseases. IgA is also found in mother’s milk and may help nursing newborns resist disease.