This chapter provides an overview of immunoglobulin A (IgA). IgA is found in adult human serum in concentrations that range from 66 to 344 mg/dl. In secretions IgA represents, under normal conditions, the predominant immunoglobulin isotype. Serum IgA occurs mostly in the form of monomeric (6.5-7 S) molecules composed of two H (MW = 53,000) and two L (MW = 22,500) chains. A minor portion of the IgA present in sera of normal individuals is found in polymeric form with monomers connected by disulfide bonds and linked to an additional polypeptide called J chain. In external secretions monomeric IgA constitutes only a minor component; most of the IgA occurs in a polymeric form. The chapter further discusses subclasses and allotypes of IgA. Two subclasses (or isotypes) of IgA, IgAl and IgA2, are identified in human serum and secretions. IgA1 accounts for 80-90% of serum IgA, whereas in external secretions IgA1 constitutes only 50 to 74% of the total IgA. Determination of subclasses and allotypes are also described in the chapter. © 1985, Elsevier Inc. All rights reserved.