Levels of specific anti-rHSP40 antibodies in the serum of pVAX1-HSP40-immunized mice. (A) The optical densities (OD₄₀₅) of total IgG prior to each of the three booster immunizations at 0, 2 and 4 weeks, and prior to the challenge at 6 week (i.e., two weeks after the third immunization) using ELISA. (B) Levels of IgG isotypes, IgG1 and IgG2a, two weeks after third immunization (i.e., 6 weeks after the primary immunization). There was no significant difference in the level of antibodies between immunized and control mice (p > 0.05).