p50-/- mice accumulate MZ B cells with age that have the capacity to bind high levels of IgM-IC A. FACS profile of splenocytes from 2 and 6 month old B6 and 6 month p50-/- mice reveal the presence of normal numbers of MZ B cells in spleens of the p50-/- mice. Histograms illustrate binding of IgM-IC to p50-/- MZ B cells as described in Figure 2A. B. In vitro IgM-IC binding assay. Binding to MZ B cells (shaded profiles); binding to FO B cells (clear profiles).