Categorization of B cells in the bone marrow and spleen. (A) B-cell populations in the bone marrow and spleen analyzed on the basis of surface IgM and IgD expression. Categorizations depend on analysis of CD21 expression as well (Figure 4A shows the CD21 levels in the bone marrow). F-I refers to IgD^hiIgM^loCD21^int follicular B cells and F-II to IgD^hiIgM^hiCD21^int follicular B cells. NF/T1 refers to IgM^hiIgD^loCD21^lo B cells in the bone marrow and IgM^hiIgD^lo/− CD21^lo B cells in the spleen. Fraction E refers to IgM⁺IgD⁻ B cells in the bone marrow. MZP B cells are IgM^hiIgD^hiCD21^hi presumed MZ precursors, and MZ B cells are IgM^hiIgD^lo/− CD21^hi B cells both of which are present only in the spleen (for gating strategy, see Hardy et al,⁸  Cariappa et al,¹²  Li et al,²⁷  and Hendriks et al²⁸ ). (B) Regression analysis of BrdU decay data (from a continuous labeling experiment) reveals that NF/T1 B cells and fraction E B cells have distinct half-lives.