Relationships between immunoglobulins and the intestinal epithelium.

The intestinal epithelium is intimately associated with immunoglobulins. This association may begin in neonatal life with the ingestion of large quantities of immunoglobulins in breast fluids. These ingested immunoglobulins probably have a local protective action in the intestinal lumen. In some mammalian species a large portion of the maternal immunoglobulins is translocated intact across the intestinal epithelium into the circulation, providing additional immunological protection. In rodents, the transepithelial translocation of IgG from breast fluids is initiated and critically dependent upon receptors on enterocyte surface membranes for the Fc region of IgG. Close epithelial-immunoglobulin relationships continue throughout life with the transfer of various classes of immunoglobulins across the epithelium into the intestinal fluids. In man and other mammalian species, IgA and IgM are selectively transported through enterocytes, principally in the crypts of intestinal glands. This transfer may involve binding of polymeric forms of these immunoglobulins to receptors on the abluminal surfaces of the enterocytes. The secretory component, a glycoprotein synthesized by enterocytes, may be such a receptor. IgE and IgG enter the gut lumen by mechanisms that are not defined but seem to be distinct from those involved in the translocation of IgA and IgM. Secreted antibodies in intestinal fluids and mucus bathe the luminal surfaces of intestinal epithelial cells but appear not to be firmly bound to their apical plasma membranes or glycocalyces. The intimate association of immunoglobulins with intestinal epithelial cells illustrates the close relationships that exist between the gut and lymphoid cells and their products. These relationships suggest the possibility that the gut epithelium is affected by a large variety of immunological reactions in health and disease; these possibilities, which have been explored only minimally, warrant much attention in the future. Studies on the binding, uptake, and intracellular transport of immunoglobulins by enterocytes could contribute much to the understanding of receptors for immunoglobulins on many other types of cells, such as lymphocytes, macrophages, mast cells, and the lining cells of placental or yolk sac membranes.