Human CD154 (CD40 Ligand) is a member of the tumor necrosis factor (TNF) family and is expressed on the surface of activated T cells. It can undergo proteolytic cleavage into a 19kD immunologically active soluble form. Interaction of CD154 and CD40 is essential for isotype switching in B cells.  Known genetic defects that alter this interaction lead to impaired immune system function.  Increased levels of  CD154  has been associated with autoimmune disorders including SLE, CLL and eosinophilic fasciitis. CD154 has been reported to be expressed on vascular endothelial cells, smooth muscle cells, macrophages and activated platelets indicating a role for the CD40-CD154 immunoregulatory signaling in arthrosclerosis and cardiovascular disorders.