1 - Secretory Component: The Receptor That Mediates External Secretion of Polymeric Immunoglobulins

Abstract

Secretory component (SC) is the receptor on glandular epithelial cells and hepatocytes that mediates the transport of immunoglobulin A and M (IgA and IgM) polymers to external body fluids. The SC-polymeric IgA system represents a unique type of receptor-ligand interaction; the receptor is neither recycled nor degraded subsequent to ligand binding, but is secreted by the cell in complex with its ligand. This chapter presents the evidence that led to the concept that SC is the receptor for polymeric immunoglobulins. It reviews the importance of SC-polymeric IgA interactions in the mucosal immune system. The polymeric IgA binds to SC on the abluminal surface of the epithelial cells by noncovalent interactions that are saturable, reversible, and specific for polymeric IgA and IgM. Covalent interaction between SC and its ligand occurs to a variable degree in different species. The SC-IgA complex is endocytosed by the epithelial cell or hepatocyte, and transported across the cell into the external secretions by a microtubule-dependent vesicular transport mechanism.