CD22 is a 130 kDa type I transmembrane glycoprotein, also known as Siglec-2 and BL-CAM. It is a member of the immunoglobulin superfamily (sialoadhesin subgroup). CD22 is expressed in the cytoplasm of pro-B and pre-B cells, and on the surface of mature and activated B cells, but is absent on plasma cells. It is part of the B-cell receptor (BCR) complex and associates with signaling molecules such as SHP-1, Syk, Lck, Lyn, and phospholipase Cγ1. The primary function of CD22 is believed to be the limitation of antigen receptor signaling by modulating the threshold for B-cell activation. Studies have shown that CD22 can bind to CD45RO and CD75, although the identity of its natural ligand(s) remains a subject of debate.
