In humans, IgA is produced in quantities larger than those of all other immunoglobulin isotypes combined. In comparison with immunoglobulin of other isotypes, human IgA displays a unique heterogeneity of its molecular forms with a characteristic distribution in various body fluids. Serum IgA is represented by predominantly monomeric molecules of the IgAl subclass and is produced mainly in the bone marrow, while in external secretions most of the IgA, produced locally in mucosal tissues, occurs in the polymeric configuration with a relatively increased proportion of IgA2 molecules. Cells that produce monomeric or polymeric IgA and IgAl or IgA2 molecules are characteristically distributed in various lymphoid and nonlymphoid tissues. The differential interactions of monomeric and polymeric IgA molecules with various cells lead to their selective distribution in body fluids and to differences in the effector functions. Contrary to immunoglobulins of other isotypes, IgA participates in the disposal of antigens by mechanisms mostly devoid of inflammatory consequences. Various modes of immunization can induce an immune response in the systemic and/or secretory IgA compartments. Although most of the total serum IgA is momomeric, recent studies indicate that specific serum IgA antibodies induced by either mucosal or systemic immunizations with microbial antigens appear first in the polymeric form. Thus, cautious immunochemical analyses of IgA antibodies in systemic and secretory compartments with respect to their kinetics of appearance, monomeric and polymeric forms, and subclass may elucidate their tissue origin and character of the antigen involved in their induction. © 1988, National Kidney Foundation, Inc.. All rights reserved.