FO B cells from both aged and young mice can give rise to ABCs after transfer into replete hosts. MACS-sorted CD23⁺ B cells (8 × 10⁶) from aged or young mice were labeled with CFSE, then injected separately into young congenic C57BL/6 hosts. Recipient mice were analyzed 1 month after transfer. (A) Percentages and numbers of donor cells 1 month after transfer. Top plots show live lymphocytes, CD19⁺B220⁺ gated, of donor (CD45.2⁺) and host (CD45.1⁺) phenotype. Absolute numbers of donor cells (bottom graph) were not significantly different (n.s.). Similar results were found in 5 experiments. (B) Surface-staining phenotype of CD45.2⁺ B cells before (left) and 1 month after transfer (right) shows ABCs (CD21/35⁻CD23⁻⁾ and MZ (CD21/35^hiCD23⁻) subsets of donor origin after 1 month. (C) CFSE dilution histograms of B cells from aged (filled gray) and young (bold line) donors recovered 1 month after transfer. (D) The dilution of CFSE in B cells from aged and young donors accompanied by surface expression of CD21/35, CD23, and IgM.