CLL IGs belonging to subset 4 bind with different intensities to memory and naïve B cells. Tonsil mononuclear cells were incubated with CLL subset 4 mAbs 183, 240 and 342, and with a panel of commercial antibodies reactive with membrane antigens that distinguish human B cell subsets. (A) Two subpopulations (dim and bright) of CD19+ cells can be identified based on CLL subset 4 mAb binding. mAb141, which uses IGHV4-34 but does not belong to subset 4, recognizes only one population. (B) After gating specifically on CD19+ cells, it is clear that the CD19+/CLL mAbbright fraction is predominantly CD27−/IgD+/CD38low (naïve B cells) and the CD19+/CLL mAbdim cells are mainly CD27+/IgD−/CD38l°w/high (memory B cells). In contrast, CD19+ cells bound by mAb141 are predominantly CD27−/IgD+/CD38l°w cells (naïve B cells).