Cellular and molecular mechanisms for induction of mucosal immunity.

The epithelial glycoprotein called secretory component (SC) is quantitatively the most important receptor of the immune system because it is responsible for external transport of locally produced polymeric IgA (pIgA) to generate remarkably large amounts of secretory IgA. Antibodies of this type constitute the major mediators of specific humoral immunity. Transmembrane SC belongs to the Ig supergene family and functions as a common pIg receptor, also translocating pentameric IgM externally to form secretory IgM. The B cells responsible for mucosal production of Ig polymers are initially stimulated in organized mucosa-associated lympho-epithelial structures, particularly the Peyer's patches in the distal small intestine; from these inductive sites they migrate ("home") as memory cells to exocrine tissues all over the body. Mucous membranes are thus furnished with secretory antibodies in an integrated way, ensuring a variety of specificities at every secretory effector site. There is currently great interest in exploiting this integrated or "common" mucosal immune system for oral vaccination against pathogenic infectious agents and also to induce tolerance in T cell-mediated auto-immune diseases. However, much remains to be learned about mechanisms for antigen uptake and processing necessary to elicit stimulatory or suppressive mucosal immune responses in humans. Moreover, evidence is emerging for the existence of considerable regionalization with regard to functional links between inductive sites and effector sites of mucosal immunity.