In vitro and in vivo studies on protective and inhibitory antibodies against Plasmodium falciparum in the Saimiri monkey.

Immunoglobulins (Ig) were purified from serum or ascitic fluids of Saimiri monkeys infected with Plasmodium falciparum and with differing levels of functional immunity. Their protective activity was assessed in studies of passive transfer of immunity in vivo, and their neutralizing activity was measured in studies of parasite inhibition in vitro in cultures of human or Saimiri RBC. Although protective and inhibitory antibodies were detected in different Ig preparations, the levels of these activities were not directly correlated. Some Ig preparations, showing high protective activity, provide little or no inhibition of the parasite in vitro. Conversely, inhibitory activity was present in Ig preparations unable to confer protection in vivo. These results indicate that protective antibodies are active through a more complex immune mechanism than simple neutralization probably involving cellular immunity.