The CD40 antigen is a single chain glycoprotein. It is known to be a member of the tumor necrosis factor / nerve growth factor superfamily and shows a significant homology to the Hodgkin's disease associated antigen, CD30. CD40 is present on all B cells except plasma cells. It is also found on some epithelial cells, carcinomas and lymphoid dendritic cells. The precise function of the CD40 antigen is unknown but it appears to be involved in the transduction of regulatory signals for cellular functions such as B cell proliferation and differentiation.