CD22 

CD22 (Siglec 2) is a 135-kDa B-cell restricted antigen, and is an inhibitory coreceptor of the B-cell receptor (BCR). The extracellular portion of CD22 comprises seven immunoglobulin domains, and is responsible for binding α2,6Sia ligands. Interaction with these ligands in cis (on the B-cell surface) regulates the association of CD22 with the BCR and thereby modulates the inhibitory function of CD22. Interaction of CD22 to ligands in trans (on the surface of other cells) can regulate both B-cell migration as well as the BCR signaling threshold.
CD22 can also recruit the tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) to immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and inhibits BCR-induced Ca²⁺ signaling on normal B cells. CD22 is critical in maintaining a baseline level of B-cell inhibition and homeostasis in humoral immunity. CD22 is a target molecule in dysregulated B cells that cause autoimmune diseases and blood cancers^[1][2].