CD40: A receptor molecule on the cell surface of all mature B cells (B lymphocytes), most B-cell malignancies, and monocytes, dendritic cells (in the nervous system), endothelial cells (within blood vessels), and epithelial cells. CD40 is a member of the tumor necrosis factor superfamily. Together with CD40 ligand, the molecule that binds to it, CD40 is an important contributor to the inflammatory processes that lead to atherosclerosis and thrombosis (clotting). The gene encoding CD40 is on chromosome 20.