Opsonic activity of ascitic fluids from Plasmodium falciparum-infected Saimiri monkey: positive correlation with protection in passive transfer assay.

An investigation into the protective activity of ascitic fluids from Saimiri monkeys infected with Plasmodium falciparum and the role played by opsonins in that activity was undertaken. P. falciparum-parasitized blood was collected from splenectomized Saimiri (when parasitaemia reached at least 20% or more) and used in an in vitro phagocytic assay including ascitic fluid and cultures of peripheral blood monocytes from normal Saimiri. Under the conditions of this in vitro assay, we found that ascitic fluid phagocytosis-promoting factors were opsonic rather than cytophilic. The opsonic activity was highly specific for parasitized red blood cells and was effective against all stages of development of the parasite. A highly positive relationship between in vitro opsonizing activity and in vivo protective capacity of immune ascitic fluid was found.