Endogenous type I IFN signaling regulates the production of antigen-specific IgG subtypes induced by mBSA plus IFA. Wild-type mice and mice unable to express functional type I receptor (IFNARKO) were immunized with mBSA plus IFA twice in a 1-week interval followed by intraarticular injection of mBSA in PBS 21 days later. Serum was collected at days 0, 14, 21, and 28 and analyzed for mBSA-specific antibodies with ELISA by using detection antibodies specific for (A) IgG1, (B) IgG2a, and (C) IgG2b. Data are expressed as the mean absorbance (450 nm) ± SEM; n ≥ 10. (*P < 0.05; **P < 0.01; ***P < 0.001; and ****P < 0.0001).