CD22 is a 130 kD type I transmembrane glycoprotein also known as Siglec-2 and BL-CAM and is a member of the immunoglobulin superfamily (sialoadhesion subgroup). CD22 is expressed in the cytoplasm of pro-B and pre-B cells, and on the surface of mature B and activated B cells, but not on plasma cells. CD22 is present in the B cell receptor complex and associates with SHP-1, Syk, Lck, Lyn, and phospholipase Cγ1. A primary function of CD22 is thought to be in limiting antigen receptor signaling by modulating B cell activation threshold. CD22 has been shown to bind to CD45RO and CD75, although the natural ligands for this molecule remain controversial.