CD22 is detected in the cytoplasm early in B cell development (late pro-B cell stage), appears on the cell surface simultaneously with surface IgD, and is found on most mature B cells. Expression is lost with terminal differentiation of B cells and is absent on plasma cells. Activation of B cells via surface Ig increases CD22 expression [1]. It is more strongly expressed on prolymphocytic leukemia and HCL than in chronic lymphocytic leukemia. B cell lineage ALL express membrane and cytoplasmic CD22. CD22 forms a loose complex with the BcR cell antigen receptor (BcR) [2]. The cytoplasmic domain is tyrosine phosphorylated upon ligation of the BCR and associates via SH2 domains with the tyrosine phosphatase SHP-1, the tyrosine kinase Syk and phospholipase C-g1 [3,4]. CD22 down-modulates the B cell activation threshold, presumably through its association with SHP-1 and other signaling molecules [2,3]. Mice deficient in CD22 show exaggerated antibody responses to antigen and have raised levels of autoantibodies [1,5]. CD22 can also mediate cell adhesion through its interaction with cell surface molecules bearing the appropriate sialoglycoconjugates, but only when these conjugates are not on the CD22 bearing cell itself [2,6].Synonyms: B-cell receptor CD22, B-lymphocyte cell adhesion molecule, BL-CAM, Leu-14, SIGLEC2, Sialic acid-binding Ig-like lectin 2, Siglec-2