CD21^lo transitional B cells are the predominant B-cell population in the blood of B-cell–deficient XLA patients. (A) PBMCs from a healthy donor and an XLA patient were stained with anti-CD20, -CD27, -CD10, and -CD21 mAbs. (Left panel) The frequency of CD20⁺ B cells; these cells were divided into transitional, naive, and memory B-cell subsets based on differential expression of CD10 and CD27 (middle panel). The transitional B cells were further divided into CD21^lo and CD21^hi subsets. The histograms presented for CD21 expression (right-hand panel) were obtained by gating on the CD20⁺CD10⁺CD27⁻ B cells (which appear in the lower right-hand quadrant of the contour plot depicted as the middle panel). The overlay solid gray histogram represents the fluorescence of lymphocytes incubated with an isotype control mAb. Flow-cytometric analysis of transitional B-cell subsets detected in donors and XLA patients was performed on the same day with identical instrument settings. The values in each plot are the mean percentages of each of the indicated B-cell subsets from all studied healthy donors and patients. (B) Summary of the frequency of total B cells, transitional B cells, and transitional B-cell subsets in healthy donors (HD) and XLA (n = 5).