Specialized Fc receptors (receptors, fc) for polymeric immunoglobulins, which mediate transcytosis of polymeric immunoglobulin a and immunoglobulin m into external secretions. They are found on the surfaces of epithelial cells and hepatocytes. After binding to immunoglobulin a, the receptor-ligand complex undergoes endocytosis, transport by vesicle, and secretion into the lumen by exocytosis. Before release, the part of the receptor (secretory component) that is bound to immunoglobulin a is proteolytically cleaved from its transmembrane tail. (From Rosen et al., The dictionary of Immunology, 1989)