The transport of serum proteins across the gastrointestinal tract of neonatal rats was used as a model for studying the mechanism of transcellular transport of proteins and, by inference, one of the factors controlling the rate of protein catabolism. The uptake of immunoglobulin G (IgG) molecules by intestinal cells and their transcellular transport involved specific saturable processes not shared by other classes of immunoglobulins. The first step in intestinal transport of IgG appears to include the formation of a complex of high molecular weight between these molecules and an IgG-specific receptor on enterocyte microvillous membranes. The Fc piece is the submolecular region of the IgG molecule that is required for interaction with the cell surface receptor and for specific uptake and transport of the whole molecule. In related studies, IgG complexes of high molecular weight were also formed in carcass homogenates after radioiodinated IgG had been administered intravenously to germ-free mice. Because of the IgG-specific concentration4atabolism effect these mice had a low serum concentration of IgG and a long survival of IgG. A major process in both the specific IgG transport between the mother and neonatal rat and the IgG concentration-catabolism effect may be competition for a limited number of saturable receptors on cell membranes that are specific for the IgG molecule. Such receptors would protect the immunoglobulin from catabolism and would be necessary for the specific cellular uptake and transport of this molecule.

The different classes of immunoglobulin molecules have different patterns of distribution and different rates of synthesis and catabolism. For example, in normal individuals the fractional catabolic rate of the immunoglobulins ranges from 6.3% of the intravascular pool for IgG to 72% for IgE (Waldmann & Strober 1969). Physiological factors have been described that affect the survival of different classes of immunoglobulin molecules in different ways. The effect of