CD22 antigen appears early in B-cell lymphocyte differentiation at approximately the same stage as the CD19 antigen. Mediates B-cell B-cell interactions. May be involved in the localization of B-cells in lymphoid tissues. Binds sialylated glycoproteins; one of which is CD45. Preferentially binds to alpha-2,6-linked sialic acid. The sialic acid recognition site can be masked by cis interactions with sialic acids on the same cell surface. Upon ligand induced tyrosine phosphorylation in the immune response seems to be involved in regulation of B-cell antigen receptor signaling. Plays a role in positive regulation through interaction with Src family tyrosine kinases and may also act as an inhibitory receptor by recruiting cytoplasmic phosphatases via their SH2 domains that block signal transduction through dephosphorylation of signaling molecules. CD22 may be a useful marker for phenotyping mature leukemias, as CD22 membrane expression has been shown to be limited to the late differentiation stages between mature B cells (CD22+) and plasma cells (CD22-). CD22 is also strongly expressed in hairy cell leukemia.