protein_function: Mediates B-cell B-cell interactions. May be involved inthe localization of B-cells in lymphoid tissues. Binds sialylatedglycoproteins; one of which is CD45. Preferentially binds toalpha-2,6-linked sialic acid. The sialic acid recognition site canbe masked by cis interactions with sialic acids on the same cellsurface. Upon ligand induced tyrosine phosphorylation in theimmune response seems to be involved in regulation of B-cellantigen receptor signaling. Plays a role in positive regulationthrough interaction with Src family tyrosine kinases and may alsoact as an inhibitory receptor by recruiting cytoplasmicphosphatases via their SH2 domains that block signal transductionthrough dephosphorylation of signaling molecules.
CD22 or cluster of differentiation-22, is a molecule belonging to the SIGLEC family of lectins. This gene is mapped to 19q13.2. It is found on the surface of mature B cells and to a lesser extent on some immature B cells. Generally speaking, CD22 is a regulatory molecule that prevents the overactivation of the immune system and the development of autoimmune diseases. It is a negative regulator of antigen receptor signaling whose onset of expression at the mature B cell stage may serve to raise the antigen concentration threshold required for B cell triggering. CD22 functions as an inhibitory receptor for B cell receptor (BCR) signalling. This gene can downmodulate signaling through the IgM and IgD B-cell receptors(BCRs), but not through the IgG BCR, because the IgG cytoplasmic tail prevents CD22 phosphorylation and actually enhances IgG-BCR signaling.
