CD40 ligand, also known as CD40-L, T-cell antigen Gp39, TNF-related activation protein, Tumor necrosis factor ligand superfamily member 5, CD154, CD40L and TNFSF5, is a single-pass type II membrane protein which belongs to the tumor necrosis factor family. CD40L ( CD154 / TNFSF5 ) is specifically expressed on activated CD4+ T-lymphocytes. The soluble form of CD40L ( CD154 / TNFSF5 ) is derived from the membrane form by proteolytic processing. N-linked glycan is a mixture of high mannose and complex type. Glycan structure does not influence binding affinity to CD40. Release of soluble CD40L from platelets is partially regulated by GP IIb/IIIa, actin polymerization, and an matrix metalloproteinases (MMP) inhibitor-sensitive pathway. CD40L ( CD154 / TNFSF5 ) mediates B-cell proliferation in the absence of co-stimulus as well as IgE production in the presence of IL-4. It is involved in immunoglobulin class switching. Defects in CD40L are the cause of X-linked immunodeficiency with hyper-IgM type 1 (HIGM1), also known as X-linked hyper IgM syndrome (XHIM). It is an immunoglobulin isotype switch defect characterized by elevated concentrations of serum IgM and decreased amounts of all other isotypes.