Selective transport of IgA. Cellular and molecular aspects.

The principal characteristic immunoglobulin of mucosal surfaces, secretory immunoglobulin A (S-IgA), is the product of two different types of cell present in mucosal and glandular tissues. Submucosal plasma cells, which are generated largely within the common mucosal immune system, synthesize predominantly polymeric, J chain-containing IgA, which is selectively bound by polymeric immunoglobulin receptor or secretory component (SC) on the basolateral surfaces of mucosal and glandular epithelial cells. The molecular and cellular events involved in SC expression, its intravesicular transport together with its polymeric IgA ligand to the apical surface of the epithelial cell, during which IgA becomes covalently linked to SC, and the proteolytic cleavage of SC from the apical membrane to release S-IgA into the lumen have been elucidated. Additional receptors and mechanisms for the uptake, catabolism, and transport of IgA exist, especially in the liver. The biologic significance of IgA transport lies in the secretion of large quantities of S-IgA antibodies for the protection of huge areas of mucosal surfaces and for the provision of passive immunity to suckling infants, and in the immune elimination of antigenic materials by hepatobiliary transport.