The protective activity of serum and fractionated serum from rats against Babesia divergens.

The rat adapted strain of bovine Babesia, B. divergens was used as a model to investigate the mechanisms of immunity to this parasite. The participation of humoral factors in acquired immunity to B. divergens was investigated in splenectomised rats. Antibodies to B. divergens were detected by IFA test in sera collected during infection and at different times after recovery. The protective activity of the same serum was tested in vivo by passive transfer and compared with the antibody levels as measured by IFA test. The role of antibody in protection was confirmed after fractionating IgM and IgG from immune and hyper-immune sera. The protective activity of sera collected immediately after recovery was mainly due to IgM antibodies. The protective activity of sera collected 3-4 days after recovery and of hyper-immune sera was mainly due to IgG antibodies. In hyper-immune serum IgM antibodies were partially protective.