Specific complement-fixing antibodies are produced in the serum of rabbits in response to injections of living or dead Plasmodium knowlesi.
Sera from rabbits receiving injections of either parasitized or normal monkey erythrocytes are parasiticidal in vitro for P. knowlesi. Because absorption of parasiticidal rabbit sera with normal monkey erythrocytes abolishes the parasiticidal effect, it is concluded that the effect is largely due to an antibody to the red cells. Normal rabbit serum is not parasiticidal.
Experiments on passive protection in monkey malaria with serum from rabbits which have received intraperitoneal injections of living or dead P. knowlesi yield no conclusive evidence that protective antibodies are formed.