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.
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