The transfer of immune complexes from the lumen of the small intestine to the bloodstream in sucking rats.

The epithelial cells along the small intestine of the sucking rat display specific receptors for IgG. These permit the uptake and transcellular transport of IgG, usually presented in milk, from the gut to the bloodstream. Antigen already combined with IgG may also gain access to the bloodstream by this route, but no data exists to show whether this could be sufficient to immunize or to induce tolerance in the young rat to a future exposure to antigen. Monoclonal IgG1 antibody to horseradish peroxidase was used in order to quantitate such a transfer of antigen, using the electron microscope and also studies with radiolabelled antigen and/or antibody. It was found that much less intact antigen was transferred to the bloodstream than would have been calculated from the quantity of antibody transported. Also, IgG1 combined with antigen was more likely to be broken down intracellularly than free IgG1. This was compared with the transport across rat hepatocytes of monoclonal polymeric IgA anti-horseradish peroxidase. The transfer of polymeric IgA from blood to bile was the same whether or not the IgA was complexed with antigen. However, as for the IgG system, less antigen was transported intact into bile than might have been expected theoretically.