ABSTRACT
IgA enters the gastrointestinal tract both from exogenous and endogenous sources. During the neonatal period, the ingestion of breast milk introduces large amounts of secretory IgA (slgA) into the lumen of the suckling infant's intestine. This chapter describes the various pathways for entry of IgA into intestinal fluids. IgG in breast milk crosses the intestinal epithelium of the suckling newborn to enter the circulation. The cellular mechanisms for vectorial transport of endogenously synthesized IgA into the intestinal lumen were controversial. The origin of the IgA plasma cells found in the intestinal mucosa appears to be principally IgA-bearing lymphocytes in the gut-associated lymphoid tissues, although similar cells might migrate to the gut from other mucosa-associated lymphoid tissues. The secretion of slgA into human hepatic bile has far reaching pathophysiologic implications for the liver, biliary tract, an intestine, and perhaps even for systemic immunologic reactions, but the actual importance of biliary slgA remains to be defined.
