Free heme is an endogenous danger signal to provoke innate immunity. Active innate immunity is a precondition of an effective adaptive immune response. However, heme catabolites, CO, biliverdin and bilirubin trigger immunosuppression. Furthermore, free heme induces the expression of heme oxygenase-1 to reinforce the production of CO, biliverdin and bilirubin. As such, free heme can drive two antagonistic mechanisms to affect adaptive immunity. What is the outcome of animal immune response to an antigen in the presence of free heme? The question remains to be explored. Here we report the immunization results by intraperitoneal injection of the formulations containing BSA and heme. When the used heme concentrations were about less than 1μM, the production of anti-BSA IgG and IgM was unaffected; when the used heme concentrations were about more than 1μM but less than 5μM, the production of anti-BSA IgG and IgM was enhanced; when the used heme concentrations were about more than 5μM, the production of anti-BSA IgG and IgM was suppressed. The results demonstrate that heme can modulate adaptive immunity (at least humoral immunity) by the mode of double concentration-thresholds. If heme concentrations are below the first threshold, there is no effect on adaptive immunity; if between the first and second thresholds, there is promotive effect; if over the second threshold, there is inhibitive effect. A hypothesis is also presented here to explain the mode.