CD40 Ligand, also known as TNFSF, a type II membrane protein with homology to TNF, is transiently expressed on activated T cells and known to be important for B cell Ig production and for activation and differentiation of monocytes and dendritic cells. Mature human CD40 Ligand consists of a 22 amino acid (aa) cytoplasmic domain, a transmembrane segment, and an 215 aa extracellular region . It is primarily expressed on activated CD4+ T lymphocytes, and also found in other types of cells, like NK cells, mast cells, basophils and eosinophils. In total CD40 ligand has three binding partners: CD40, α5β1 integrin and αIIbβ3. CD40 Ligand dysregulation on T cells and antigen presenting cells contributes to the immune deficiency associated with HIV infection and AIDS. CD40 ligation by CD40 Ligand promotes B cell activation and T cell-dependent humoral responses.