CD22 expression in purified B-CLL cells. CLL cells were immunoseparated to more than 96% purity. They were stained with FITC-labeled monoclonal antibodies to CD22 or a nonbinding control and were analyzed by flow cytometry. Histograms show fluorescence of an anti-CD22 antibody (shaded), superimposed with that of an isotype control (solid line) from all 22 patients. A threshold level of fluorescence intensity was identified (shown as a horizontal line), such that fewer than 2% of cells stained with the control antibody displayed fluorescence above this level. The number of cells with anti-CD22 fluorescence above this threshold is indicated. Samples, in which less than 50% of CLL cells had anti-CD22 fluorescence above the threshold, were considered low expressing.