FcRH1 expression by B-lineage cells in bone marrow and tonsils. (A) Bone marrow mononuclear cells from adult ribs were purified and 3-color immunofluorescent staining was performed. This pattern of FcRH1 expression by pre-B and B cells was confirmed for 3 additional bone marrow samples. (B) Tonsillar B cells purified using CD19 microbeads were stained with antibodies against CD38, IgD, and biotin-5A3 followed by streptavidin-APC. The different B-cell subpopulations were gated for FcRH1 analysis based on their CD38 and IgD expression. Note the biphasic pattern of FcRH1 expression by the pre-germinal center subpopulation. FcRH1 stained populations are shaded, and Ig isotype controls are unshaded.