CD40 ligand (CD40L, CD154) is a member of the TNF superfamily of molecules and is a CD40L is a 39 kDa glycoprotein that is primarily expressed on activated T cells. 15-18 kDa soluble forms of CD40L (sCD40L) is produced by proteolytic cleavage. CD40L binds to CD40 on antigen-presenting cells (APC), which leads to many effects depending on the target cell type. CD40L acts as a costimulatory molecule and is particularly important on a subset of T cells called T follicular helper cells (TFH cells). On Tfh cells, CD40L promotes B cell maturation and function by engaging CD40 on the B cell surface and therefore facilitating cell-cell communication. A defect in this gene results in an inability to undergo immunoglobulin class switching and is associated with hyper IgM syndrome. Absence of CD40L also stops the formation of germinal centers and therefore prohibiting antibody affinity maturation, an important process in the adaptive immune system.