The CD40 ligand (CD40L/CD154) is a member of the TNF superfamily, a 39kDa glycoprotein that is mainly expressed in activated T cells. Soluble CD40L(sCD40L) with a molecular weight of 15-18 KDa is produced by proteolysis. CD40L binds to CD40 on the surface of antigen presenting cells (APC) to produce a variety of effects depending on the target cell type. As a co-stimulatory molecule, CD40L is particularly important for T cell subpopulation follicular T helper cells (Tfh cells). In Tfh cells, CD40L binds to CD40 on the surface of B cells to promote B cell maturation and function, thus promoting cell-to-cell connections. Deletion of the CD40L gene results in the inability of immunoglobulin type switching and is associated with hyperigm syndrome. Loss of CD40L also blocks germinal center formation, thus preventing an important process in the adaptive immune system called antibody affinity maturation.