CD22, or cluster of differentiation-22, is a molecule belonging to the SIGLEC family of lectins. It is found on the surface of mature B cells and to a lesser extent on some immature B cells. CD22 functions as an inhibitory receptor for B cell receptor (BCR) signaling. It is also involved in the B cell trafficking to Peyer's patches in mice. In mice, it has been shown that CD22 blockade restores homeostatic microglial phagocytosis in aging brains. Because CD22 is restricted to B cells, it is an excellent target for immunotherapy of B cell malignancies. There are several mechanisms by which this can be achieved, namely monoclonal antibodies, bispecific antibodies, antibody-drug conjugates, radioimmunoconjugates or CAR-T cells.