CD22, also known as Lyb-8, is a heterodimeric transmembrane glycoprotein and a member of a structurally related group of immunoglobulin (Ig) superfamily domain-containing proteins called the sialoadhesin family. It is detected in the cytoplasm early in B cell development (late pro-B cell stage), appears on the cell surface concomitant with IgD, and is found on most IgM+IgD+ mature B lymphocytes. Expression is lost with terminal differentiation of B cells and is absent on plasma cells. Activation of B cells via cross-linking of surface Ig increases CD22 expression. CD22 associates with the B cell receptor (BCR) complex and mediates intercellular adhesion. Its intracellular domain is phosphorylated after antigen receptor cross-linking and is involved in negative regulation of B cell activation.