Crosslinking of cell surface receptors is recognized as an important signal-transducing event in immune responses, but little is known about the nature of the interactions that occur or the factors that control these processes. In the work proposed, two different systems will be examined to address these fundamental questions. The crosslinking of IgE-receptor complexes on the surface of mast cells and basophils that leads cellular degranulation in the allergic response has been extensively studied by experimentalists and theorists, but the kinetics and thermodynamics of receptor crosslinking by simple bivalent haptens have not been directly measured. Our laboratory has developed fluorescence binding methods and functionally active bivalent haptens for this purpose, and systematic analyses will be carried out to examine the roles of IgE-receptor density and other parameters on the crosslinking events and also to examine the relationships between the rate and extent of receptor crosslinking and cell activation. The structural nature of the active crosslinked aggregate will be examined by the use of a series of long, rigid bivalent haptens of variable lengths that will be tested for their ability to trigger cellular degranulation, and this will indicate whether crosslinked receptors must interact directly in order for a transmembrane signal to be delivered. Fluorescence depolarization experiments on receptor-bound IgE will be continued to help interpret both the thermodynamic and structural studies, and changes in membrane potential resulting from crosslinking will be studied using fluorescence methods to investigate the functional significance of these events in signal transduction. The interaction of a cytotoxic T lymphocyte (CTL) with a target cell containing the appropriate histocompatibility antigens on its surface leads to the specific lysis of that target in a process that appears to play a role in host defense against viral infections as well as in possible tumor cell rejection. The T cell surface components that are involved in the cell-cell interactions which take place in this process have been recently delineated using monoclonal antibodies and cloned human CTL, but the nature of the interactions of these components with each other and with components on the target cells are poorly understood. The work proposed will initiate a study of these interactions using a variety of fluorescence methods to characterize the lateral mobilities and distributions of important T cell surface components both before and after crosslinking of these components either by monoclonal antibodies or by their interactions with target cells.