CD40/LMP1⁺//CD40^−/− mice show normal B- and T-cell distribution and normal serum immunoglobulin titers. (A) Total numbers and percentages of B220⁺ B and CD5^high T cells in both the spleen (SP) and inguinal lymph nodes (iLNs). Data presented are means (± SEM) of 4 to 5 mice per group tested in independent experiments. (B) Nonimmunized mice (5 per group) were analyzed for the concentrations of the total serum immunoglobulin titers of the indicated isotypes by ELISA. CD40^−/− mice showed a defect in CSR to IgG1, IgG2a, and IgG2b, which could be rescued in CD40/LMP1⁺//CD40^−/− mice.