CD22 is a 140-kDa member of the Siglec family of cell surface proteins. CD22 is an inhibitory coreceptor of the B-cell receptor (BCR), and plays a critical role in establishing signalling thresholds for B-cell activation. Like other coreceptors, the ability of CD22 to modulate B-cell signalling is critically dependent upon its proximity to the BCR, and this in turn is governed by the binding of its extracellular domain to alpha2, 6-linked sialic acid ligands. Manipulation of CD22 ligand binding in various experimental settings has profound effects on B-cell signalling.