Phagocytosis assay validity. (A) Phagocytosis of whole merozoites is specific to malaria-immune sera and efficiently inhibited by treatment with cytochalasin D. (B) The IgG fraction purified from serum mediates phagocytosis in a concentration dependent fashion, in assays using cultured THP-1 cells and freshly isolated PBMCs. (C) Flow cytometry histogram overlay contrasting the phagocytosis in monocytes from human PBMCs when freshly isolated merozoites are opsonized with purified IgG from malaria-immune adults (grey line) with unopsonized merozoites (black line). (D) Equivalent levels of phagocytosis obtained when merozoites were stained with the pH dependent dye pHrodoTM or with ethidium bromide, indicating internalization of merozoites into acidic phago-lysosomes. Experiments were conducted using malaria-immune IgG (MIG). PBMCs. peripheral blood mononuclear cells.