Members of the superfamily of ligands related to tumor necrosis factor (TNF) as well as the superfamily of receptors that recognize these ligands play critical roles in various aspects of mammalian lymphocyte development and immune function. Many of the receptors and their cognate ligands have well-known roles in the regulation of cell­cell interactions as well as in the proper development of lymphoid organs, skin and other systems. Disruption of the ligand­receptor interactions within this superfamily of molecules often leads to pathological conditions. Consequently, these are two of the most widely studied and pharmacologically interesting gene families in biology and medicine. The roles of TNF and TNF receptor-like molecules in the immune system are discussed. (c) 2002 Prous Science. All rights reserved.