CD40 (48 to 50 kDa) is a transmembrane glycoprotein mainly expressed on the surface of B cells and also expressed on monocytes, dendritic cells, and thymic epithelium. CD40 is a member of the tumor necrosis factor (TNF) receptor superfamily, which includes the low affinity nerve growth factor (NGF) receptor and CD95/Fas. CD40 is the receptor for CD40 ligand. CD40 ligand (CD40L, CD154, gp39, and TRAM) belongs to the TNF gene family and is expressed more widely than CD40 predominantly on activated CD4+ T cells. Following interaction with CD40 ligand, CD40 mediates a number of major immunoregulatory functions, central to the control of thymus dependent humoral immunity and may be critical in the development of cell mediated immune responses. Other biological actions include B cell homotypic adhesion, proliferation, immunoglobulin isotype switch, and secretion. Activation of CD40 has also been shown to inhibit the growth of certain B cell lymphomas and to induce the death of transformed cells of mesenchymal or epithelial origin.