CD40 ligand (CD40L) is a 33 kDa type II membrane glycoprotein expressed mainly on the cell surface of activated T lymphocytes, but also exists as a soluble form extracellularly. CD40L is the ligand for CD40, a member of the TNF superfamily, which is expressed on the cell surface of B cells, macrophages/monocytes, dendritic cells, vascular endothelial cells, and epithelial cells. CD40L plays an important role in B cell proliferation, antibody class switching, modulation of apoptosis in the germinal center through interaction with B cells expressing CD40, and activation of CD4+ T cells. Mutation within the CD40L gene is linked to hyper IgM syndrome, an X linked immunodeficiency disease that is characterized by elevated level of serum IgM and decreased level of other isotypes.