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.