In mammals, there are five classes of antibody, called IgG, IgM, IgA, IgD, and IgE, each with a different function (Fig. 43.10). IgG is the Y-shaped antibody depicted in Figs. 43.9 and 43.10. IgG circulates in the blood and is particularly effective against bacteria and viruses.

IgM is a pentamer in mammals and a tetramer in fish. (A pentamer is a molecule made of five monomers; a tetramer is a molecule made of four monomers.) The individual units—the monomers—are linked by a joining chain. IgM also exists as a monomer on the surface of B cells. IgM is important in the early response to infection.

IgA is usually a dimer consisting of two antibody molecules linked by a joining chain. It is the major antibody on mucosal surfaces, such as those of the respiratory, gastrointestinal, and genitourinary tracts. It is also present in secretions, including tears, saliva, and breast milk.

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IgD and IgE are monomers. IgD is typically found on the surface of B cells. This immunoglobulin helps initiate inflammation. IgE plays a central role in allergies, asthma, and other immediate hypersensitivity reactions, which are characterized by a heightened or inappropriate immune response to common antigens. Immediate hypersensitivity reactions result from the binding of IgE to receptors on the surface of mast cells and basophils. Binding leads to the release of histamine and cytokines, resulting in inflammation that can be life threatening.