The proposed research may be considered in four areas: 1) Having shown that specific immunization decreases intestinal uptake of macromolecular antigens, the local antibody response of rats orally and parenterally immunized with horseradish peroxidase (HRP) and bovine serum albumin (BSA) will be further characterized as to immunoglobulin class using radioimmunodiffusion and rabbit anti-rat Iga, Igb, IgA, Igl antisera. Using an ammonium sulfate precipitation technique, quantitative antibody activity in intestinal secretions, mucosal extracts, solubilized brush border membrane and sera from immunized animals will be determined. Results can be used to determine the antibody class involved in decreased uptake and to localize the site of antibody interference with antigen absorption. 2) Antigens are taken up by a pinocytotic mechanism requiring first cellular binding and then invagination. The characteristics of binding of I-HRP 125 and BSA 125 antigens to purified brush border membranes will be determined with regard to energy requirements, competitive inhibition, enhancement with charged polypeptides and effect of immunization (secretory and systemic antibodies). 3) Data from above studies will be applied to the study of physiologic macromolecules (bacterial enterotoxins such as cholera toxin and food proteins). Normal brush border binding and uptake characteristics will be noted morphologically using HRP-conjugated antibodies and compared to immunized animals. Uptake in control and immunized animals will be determined using radiolabeling techniques. 4) In vivo studies using the rat lymph fistula model will be done to confirm in vitro binding and uptake studies with and without immunization. BIBLIOGRAPHIC REFERENCES: Walker, W.A., Wu,M., Isselbacher, K.J. and Bloch, K.J.: Intestinal uptake of macromolecules: IV. The effect of pancreatic duct ligation on breakdown of antigen and antigen-antibody complexes on the intestinal surface. Gastroenterology 69: (in press), 1975; Walker, W.A.: Immunologic mechanisms controlling macromolecular penetration in the small intestine. Microbiology (in press), 1975.