Liver plays an important role in the translocation of IgA into the upper gastrointestinal tract. However, the amount of IgA transported and the mechanisms involved is highly variable among species. Outcomes of experimental studies suggested that large amounts of polymeric IgA (pIgA) are cleared from the plasma by hepatocytes, which produce the polymeric immunoglobulin receptor, secretory component (SC), and express it on their sinusoidal plasma membranes. These circulating pIgA binds to SC, which is internalized into endocytic vesicles and transported across the hepatocyte to the bile canalicular membrane, where the pIgA is released into bile in complex with a portion of the SC, as secretory sIgA (sIgA).

Moreover, studies showed that humans have much less hepatic transport of circulating IgA, at least in part as SC is present only in biliary epithelium, and there is comparatively more local production of IgA within hepatobiliary tissues. Nevertheless, certain IgA1 myeloma proteins appear to bind to and enter human hepatocytes through an asialoglycoprotein receptor. These differences in human and experimental models have biological significance of the biliary secretion of IgA, including the disposal of circulating IgA-antigen complexes into bile.

Source: Brown WR, Kloppel TM. The role of the liver in translocation of IgA into the gastrointestinal tract. Immunol Invest. 1989 Jan-May;18(1-4):269-85.