CD40 Ligand, also known as TNFSF5, CD154, TRAP, and gp39, is a trimeric protein that is expressed in transmembrane and soluble forms. It is expressed by many immune cell types as well as by endothelial and smooth muscle cells of the vasculature. It binds to CD40 on antigen presenting cells, platelets, and endothelial and epithelial cells, leading to enhanced B cell activation and T cell dependent humoral immune responses. CD40 Ligand dysregulation contributes to immune pathology in AIDS, atherosclerosis, atherothrombosis, and restenosis.