Lymphocyte homeostasis is achieved by a balance between the production and death of lymphocytes. In particular, T lymphocytes differentiate and mature in the thymus and are released into the peripheral circulation, where they can travel to sites of encounter with antigen. Once in the circulation, T lymphocytes have varying life spans depending on whether they remain resting, become activated by antigen and replicate, or they become memory cells. The regulation of lymphocyte fate, especially the induction of programmed cell death, or apoptosis, has become a focus of intense molecular research, and much has been learned. In particular, the Fas receptor and other members of the tumor necrosis factor receptors, as well as their respective ligands, have emerged as key regulators of T lymphocyte apoptosis. We are studying genetic abnormalities of this death pathway, which we have found to underlie the human disease, autoimmune lymphoproliferative syndrome (ALPS). The study of ALPS has revealed that inhibiting the death of lymphocytes can lead to autoimmune consequences. Also, the homeostasis of T lymphocytes can be powerfully affected by viruses and other infectious agents. In particular, the human immunodeficiency virus can cause the attrition of CD4+ T lymphocytes by inducing the premature death of such cells. We are studying the molecular mechanism by which the HIV causes CD4+ T cell destruction.