CD22 is a B-lineage restricted 135 kDa glycoprotein whose cell surface expression is limited to resting and activated B lymphocytes. The physiological role of CD22 is still unknown. Targeted disruption of CD22 in mice results in a reduced level of surface IgM on peripheral B cells suggesting a role for CD22 in limiting antigen receptor signaling. CD22 is a member of the Ig gene superfamily that uniquely binds a sialic acid-dependent ligand. Recombinant human CD22 is a soluble 75.0 kDa protein which corresponds to the extracellular domain of CD22.