Dr. Harmatz will test the hypothesis that IgA immume complexes formed in blood may be transported into breast milk and may expose the neonatal intestinal immune system to antigen in the unique form of an IgA antibody-antigen complex. These studies will examine whether: 1. IgA immune complexes are excreted in breast milk: The presence of IgA-DNP10-BSA (dinitrophenyl bovine serum albumin) immune complexes will be identified by immunoprecipitation in breast milk after intravenous injection of IgA antibody and DNP10BSA. 2. IgA antibody and immune complexes bind to neonatoal intestine: The binding of radiolabelled IgA antibody and IgA immune complexes to purified brush border membrane will be examined. The receptor for IgA antibody will be characterized. 3. IgA antibody and IgA immune complexes are transported into the neonatal circulation: IgA antibody or IgA-DNP10-BSA immune complexes will be fed to infant mice. Immunoprecipitation will allow us to detect absorption of antibody or immune complexes into the circulation. 4. IgA immune complexes fed orally influence the neonatal systemic immune system: Newborn animals will be fed DNP10-BSA or IgA-DNP10-BSA immune complexes by gavage. Four weeks later they will receive parenteral immunization with either BSA or DNP-bovine gammaglobulin. IgG antibody responses to the parenteral immunization will be assessed. This project should provide information essential to understanding the development of sensitization or tolerance to environmental antigens in the neonatal immune system.