Antibodies are proteins that bind to specific antigens. B cells, located in lymphoid tissue, release the antibodies, which then circulate in the blood plasma, lymph, or extracellular fluids. Some antibodies migrate to other areas of the body, such as the respiratory tract or the placenta, or enter various body secretions, such as saliva, sweat, and milk. Additional properties of antibodies include the following:
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There are five classes of antibodies (or immunoglobulins): IgA, IgD, IgE, IgG, and IgM. Antibodies circulating in the blood are primarily IgG, IgA, and IgM; IgD and a second form of IgM antibodies are found on the plasma membranes of B cells, where they act as antigen receptors. IgE antibodies attach to basophils and mast cells (both white blood cells found in connective tissue) and induce them to secrete histamine.
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The basic structure of an antibody is Y‐shaped protein that consists of constant and variable regions. The variable regions are sequences of amino acids that differ among antibodies and give them specificity to antigens.
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Antibodies bind to antigens, thus forming an antigen‐antibody complex. This complex attracts macrophages, which will phagocytize any foreign substance that has that specific antigen‐antibody complex. The formation of these complexes may also cause agglutination (clumping) of antigens or foreign cells.