Serum titers of total IgG, IgG1, and IgG2a isotype antibodies against ApxN and P97C in mice immunized with ApxN, P97C, and Ap97 vaccines. Blood samples were obtained at 0, 14, 28, 42, and 63 Days after the first immunization. A. Total IgG titers against ApxN. B. Total IgG titers against P97C. C. IgG isotype titers against ApxN. D. IgG isotype titers against P97C. IgG1 and IgG2a titers were evaluated at 3 weeks after the last immunization (Day 63). The numbers above the columns indicate the mean IgG2a/IgG1 ratios (C and D). Data represent the mean ± SD (n = 5). *P < 0.05 and **P < 0.01, significantly different from the corresponding anti-PBS serum controls (unpaired t-test).