Type I IFNR deletion abrogates the T-dependent antibody response. B6, BXD2, and BXD2-Ifnar−/− mice were immunized with either NP-CGG or NP-Ficoll. Seventeen hours later, the mice were either treated with placebo or CpG-A as described in Materials and Methods. Sera were collected on day 17 after immunization. IgM (A), IgG2b (B), IgG2c (C) titers of anti-NP2, as measured by O.D. units, were assayed by ELISA (N=6, *p<0.05, **p<0.01 for CpG-A treated vs untreated BXD2 mice, *p<0.05, **p<0.01, ***p<0.001 for B6 or BXD2-Ifnar−/− vs. BXD2 mice).