Antigen distribution: CD22 is detected in the cytoplasm early in B cell development (late pro-B cell stage), appears on the cell surface simultaneously with surface IgD, and is found on most mature B cells. Expression is lost with terminal differentiation of B cells and is absent on plasma cells. Activation of B cells via surface Ig increases CD22 expression [1]. CD22 reacts with most B cell leukemias including Hairy Cell Leukemia (HCL) and B cell lymphomas. Cellular Expression: Pan B cells, (surface and cytoplasmatic staining).