Plasmodium falciparum: Rapid assay for in vitro inhibition due to human serum from residents of malarious areas

Tightly synchronized cultures of Plasmodium falciparum were exposed to various dilutions of human sera from healthy adults living in malarious areas before and after merozoite invasion was completed. All “immune” sera inhibited merozoite invasion to a greater or lesser degree, depending upon the dilution of serum added to the cultures. Additionally, most sera demonstrated moderate to severe retardation of parasite development, even when merozoite invasion was completed in nonimmune serum before developing parasites were exposed to immune serum dilutions. Both types of inhibition were discernable on Giemsa-stained thin films, and these observations were highly correlated with incorporation of ³H]hypoxanthine. Large numbers of serum samples could be examined using microliter quantities of immune serum in microtiter plate cultures processed with a cell harvester.