White blood cells called T lymphocytes play critical roles in immune defense against viruses, bacteria, fungi, protozoa, and cancer cells. They are also involved in allergies/asthma due to the development of an unwanted or excessive type of immune response to substances in our environment and in autoimmune diseases that result from the inappropriate attack of these cells on the body?s own tissues. The effector functions of T cells are mediated largely by proteins termed cytokines that either be expressed at the cell surface or secreted. Because T cells see foreign substances (antigens) in the form of peptide-major histocompatibility complex (MHC) molecule complexes on cell surfaces, we wish to know how such complexes interact with specific receptors to evoke the effector activities of mature T cells in the body, as well as regulate their growth, inactivation, or death. A central issue is the relationship between the amount of signaling received by the cell from receptors seeing peptide-MHC ligand and the functional response of each T cell, as well as the extent to which different cells, even with identical receptors, show different biological response patterns. We have used new methods for analyzing T cell recognition events and cytokine production by individual cells to address these issues. In the past year, we have shown that both mouse and human T-cells show a hierarchical organization of response thresholds to signaling - in other words, more antigen is needed to elicit one particular response as compared to another from the same T cell, so that as antigen concentration changes, so does the mix of effector molecules produced by the cells. This results in a changes in the quality as well as the quantity of an immune response as the amount of antigen changes in a host. These data are revealing the mechanisms involved in antigen-dependent T cell activation, showing the contribution of cell-to-cell heterogeneity in T cell immunity, and providing possible avenues for manipulation of specific immunity through the use of altered T cell receptor ligands. We have also begun to understanding the relationship between ligand density and T-cell receptor loss from the cell surface, as well as the relationship between this latter feature of T-cell receptor biology and the intracellular signals that lead to immune activation. Together, these observations on T-cell receptor downmodulation and activation support the hierarchical model and begin to provide a mechanistic basis for this property of T-lymphocytes.