A and B, Flow cytometry demonstrated absence of B cells in lymphoid tissues in B−/− rats. Heterozygous (B+/−) and wild type (WT) rats demonstrated similar B cell numbers. Systemic levels of IgM (C) and IgG (D) were undetectable in B−/− rats as measured by ELISA. WT and heterozygous rats had similar levels of circulating IgM and IgG. Similarly, splenocytes did not produce IgM or IgG as determined by ELISPOT analysis (E and F, respectively). * P < 0.05, ** P < 0.001, *** P < 0.0001 for WT compared to B−/−.