This project is directed to the separation, purification, and characterization of the functions of and mechanisms of interaction among macrophages, thymus-derived (T cells), and precursors of antibody- producing cells (B cells) in the development and regulation of humoral immune responses in vitro. Future studies will focus on mechanisms of antigen presentation to T and B cells by macrophages, the mechanisms by which phytomitogen-activated T cells and their products suppress antibody responses, the function and mechanism of action of helper T cells in the switch from IgM to IgG antibody synthesis, the mechanism by which immune response genes regulate antibody responses to antigens controlled by such genes, and the role of leukocyte alloantigens in development and regulation of antibody responses.