Antigen-specific antibody responses in wild-type and ATF6α-deficient mice. Age- and gender-matched Atf6a+/+ and Atf6a−/− mice were immunized intraperitoneally with haptenated antigens. Blood samples were obtained at various intervals, and serial dilutions of sera were assayed by hapten-specific ELISA. Absorbance signals ≥ 2 times background (pre-immune sera) were used to determine titers of anti-NP antibodies. (A and B) For the T cell-dependent antigen NP5-KLH, mice received a primary immunization and a booster immunization 3 weeks later. Serum samples were collected at 3, 6 and 27 weeks (wks) post-primary immunization and analyzed for titers of (A) IgM and (B) IgG anti-NP antibodies (mean ± S.D., n = 4). (C) For the T cell-independent antigen NP-Ficoll, serum samples were harvested 3 weeks post-immunization and analyzed for titers of IgM and IgG3 anti-NP antibodies (mean ± S.D., n = 6).