CD40 is a type I membrane protein found on the surface of B cells and primary carcinomas. It belongs to the TNF-R family. CD40 serves as the receptor for CD40 ligand (CD40L), a cytokine expressed on the cell surface of T cells. CD40 plays a critical role in B cell proliferation, antibody class switching, modulation of apoptosis in the germinal center throμgh interaction with T cells expressing CD40L, and activation of CD4+ T cells. In non-hemopoitic cells such as epidermal basal cells, CD40 ligation serves as the signal for inhibition of cell growth and induction of differentiation. The cytoplasmic domain of CD40 interacts with TRAF6, a TNFR associated factor homolog that mediates signaling from CD40. Ligation of CD40 with its ligand preferentially induces stress-activated protein kinases in B lymphocytes.