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The five major immunoglobulin classes are designated IgA IgD IgE IgG and IgM on the basis of the C regions in their heavy chains The antibodies of each class have different biological roles and locations in the body IgA - IgA monomer exists in limited amounts in plasma The IgA dimer also called secretory IgA is found in body secretions such as saliva sweat intestinal juice and milk and helps prevent attachment of pathogens to epithelial surfaces including mucous membranes of the epidermis It plays a major role in preventing pathogens from gaining entry into the body IgD - Always bound to the external surface of a B cell functioning as the antigen receptor of the B cell and is important in B cell activation IgE - IgE antibodies are almost never found in blood They are the antibodies involved in some allergies IgE is slightly larger than the IgG antibody It is secreted by plasma cells in skin mucosae of the gastrointestinal and respiratory tracts and tonsils Its stem region becomes bound to mast cells and basophils and when its receptor ends are triggered by an antigen it causes the cells to release histamine and other chemicals that mediate inflammation and an allergic reaction Typically only traces of IgE are found in plasma but levels rise during severe allergic attacks or chronic parasitic infections of the gastrointestinal tract IgG - The most abundant and diverse antibody in plasma accounting for 75-85 of circulating antibodies It protects against bacteria viruses and toxins circulating in blood and lymph readily fixes complement and is the main antibody of both primary and secondary responses It is also the only Ig class that crosses the placental barrier Therefore the passive immunity that a mother transfers to her fetus is from her IgG antibodies IgM - Exists in monomer and pentamer forms The monomer which is attached to the B cell surface serves as an
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