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A dissociation between antibodies involved in the diagnosis of Trypanosoma cruzi infections and those participating in resistance against this parasite is reported. Mice immunized with different T. cruzi antigens (frozen-thawed culture forms, glutaraldehyde-fixed blood trypomastigotes, "metabolic antigens" from blood parasites, and surface glycoprotein from epimastigotes) present only antibodies detected by conventional immunofluorescence tests (IFA) using fixed parasites. However, mice chronically infected with T. cruzi harbor both reactive IFA antibodies and antibodies against living blood forms (ALBA) detectable by complement-mediated lysis (CML). Challenge of the various groups of animals with virulent T. cruzi show that only mice presenting ALBA are strongly resistant. In addition, the antibody lytic activity is lost in mice parasitologically cured after specific treatment. Those findings strongly suggest that …