Mucosal immunity is due in large part to the presence of a class of polymeric immunoglobulins in the secretions of epithelial surfaces vulnerable to the entry of environmental antigens. Synthesized in the interstitium, these antibodies are selectively and vectorially translocated across epithelial cells by a specific receptor. The receptor is ultimately sacrificed by proteolytic cleavage and a part of it remains associated with the antibody to protect against proteases present in the mucosal environment