The structure and function of human IgA.

IgA is present in normal human serum at about one-fifth of the concentration of IgG. However, it is catabolized around five times faster than IgG and therefore the rates of synthesis of the two immunoglobulins must be similar [1]. IgA is themost abundant immunoglobulin in secretions. Secretory IgA (sIgh) is the product of local synthesis at the mucosal surfaces which are the main source of antigenic material for the body. In mucosal tissue, IgA synthesis far exceeds that of other classes. As a result, in man, more IgA is produced than all other immunoglobulin classes combined. In contrast to other immunoglobulins, human IgA displays a unique heterogeneity in its molecular forms, each with a characteristic distribution in various body fluids [2]. Human IgA occurs in two isotypic forms, IgAl and IgA2, with IgA2 existing as two allotypic variants IgA2m(l) and IgA2m(2). Each of these forms is found in various degrees of aggregation. Although the importance of IgA in mucosal secretions is well established, it is now clear that in humans much of the IgA is secreted directly into the blood and never reaches the mucosal surfaces [3]. Serum IgA is predominantly monomeric IgAl which is produced in the bone marrow, while in external secretions most of the locally produced IgA is polymeric with a relative increase in the proportion of IgA2 [4, 5]. The lymphocytes which produce monomeric or polymeric IgA, IgAl or IgA2 are characteristically distributed in various lymphoid and nonlymphoid tissues. The differential interaction of monomeric and polymeric IgA molecules with various cells leads to their selective distribution in body fluids and possibly to differences in their effector functions. Secretory and serum IgA are therefore molecules with different biochemical and immunochemical properties produced by cells with different organ distributions.