WASp⁺ mature B cells exhibit a selective advantage in humans. (A,B) Expression of WASp in immature, mature, and IgM memory B cells in peripheral blood from a healthy person. Peripheral blood mononuclear cells from a healthy person were stained for CD19, CD24, CD38, CD10, CD27, and IgM in addition to WASp. (A) Based on these markers, the following B-cell subsets were identified: CD19⁺CD24^hiCD38^hi immature/transitional B cells, CD19⁺CD24^lowCD38^int naive mature B cells, and CD19⁺CD24^hiCD38^lowIgM⁺ IgM memory B cells. Transitional B cells express high levels of CD10, whereas memory B cells express high levels of CD27. (B) WASp expression and relative mean fluorescent intensity are shown for each of these B-cell subsets. Data shown are representative of more than 3 healthy persons. (C,D) Analysis of B cells in a WAS patient with a revertant mutation. B-cell subsets and relative WASp expression were determined in a WAS patient with a known revertant mutation. (C) B-cell subsets were identified as described for panel A. (D) WASp expression in immature, mature, and memory B cells (gated as in panel A) from the WAS patient.