Frequency of FO, MZ, and ABC subsets in young and old C57BL/6 female mice. Flow cytometry was performed on splenocytes from young (3–5 months) and old (18–24 months) mice by staining with antibodies against AA4.1, CD43, B220, CD21, and CD23, and gating on live, mature B220+ cells. (A) Representative chromatograms are shown. Numbers represent percentage of cells in the indicated populations. (B) Number of B cells in each subset from young and old mice. Each symbol represents a mouse, with the mean number of each B cell population shown with a bar (n = 18–33 mice for each subset and age). P values were determined using an unpaired, equal variance Student t test.