CD40L is a member of the tumor necrosis factor (TNF) family expressed on activated T cells after T cell antigen receptor ligation. It binds to the CD40 receptor on В cells and dendritic cells and thereby provides a critical helper T cell' signal necessary for germinal center formation, isotype class switching, and production of, for example. IgG and IgE antibodies. CD40L may also transmit a signal back to T cells, to promote short-term T cell proliferation. CD40L and CD40 are not simply restricted to cells regulating immune responses. The CD40L—CD40 signaling pathway may also play an important role in the regulation of epithelial cell, fibroblast, and smooth muscle cell proliferation.