The immune response involves acomplex set of reactions on the part of T lymphocytes, B lymphocytes, macrophages and other antigen-presenting cells (APC), and cells of the inflammatory system. The principal regulatory cells of the immune system are the helper/inducer T lymphocytes. Many, if not all, of the effects of helper/inducer T cells are mediated by their production of a set of polypeptides, often designated lymphokines (Lk). Lk are potent, pleiotropic factors that control a wide range of functions including activation, growth, and differentiation of the cells of the immune system, among which are the T cells themselves. Indeed, two T cell-derived Lk, interleukin-2 (IL-2) and interleukin-4 (IL-4) function as autocrine growth factors, regulating the growth of the T cells that produce them.

Cyclosporin A, one of the most valuable of the immunosuppressive drugs, mediates much of its function by inhibiting transcription of Lk genes; this strongly suggests that Lk production is a key element in immune responses in vivo. Not only do Lk influence virtually every aspect of the biology of lymphocytes and other cells of the hematopoietic system, they have great promise as pharmacologic agents. Several are in the midst of clinical trials to determine their potential efficacy. T Cell Receptor Specificity Leads to “Targeted” Lk Production Lk production is an induced property of activated T cells. Physiologically, the induction of Lk gene transcription and