CD40 Ligand is a membrane glycoprotein and differentiation antigen that is expressed on the surface of T cells, monocytes, basophils, eosinophils, platelets, dendritic cells, and endothelial cells. CD40 Ligand binds and activates the CD40 receptor on antigen-presenting cells. CD40 Ligand stimulates B cell proliferation, immunoglobulin class switching, and antibody secretion. CD40 Ligand also induces cytokine production and tumoricidal activity in peripheral blood monocytes. CD40 Ligand is a co-stimulator of activated T cell proliferation and inflammatory protein production.