The relative phagocytosis index (RPI) is correlated with malaria exposure and boosted by infection. (A) The prevalence of antibodies promoting phagocytosis in children with and without concurrent asymptomatic parasitemia. Samples were considered positive for phagocytosis if the RPI exceeded the mean plus three standard deviations of a panel of 10 non-malaria exposed sera from Melbourne blood donors. (B) The RPI increased significantly with age in the Ngerenya cohort, Cuzick non-parametric test for trend across ordered groups, z = 7.86, P <0.001. (C) The RPI increased modestly with age among parasite positive children in the Chonyi cohort, Cuzick test for trend across ordered groups, z = 1.24, P = 0.214. The levels of antibodies promoting phagocytosis were higher (D) in children with asymptomatic parasitemia (parasite positive) than in those without (parasite negative), and (E) in children with exposure to parasites in the preceding six months (recent infection) than in those without (no recent infection). White boxes, parasite negative; grey boxes, parasite positive. (F) The RPI was higher in age-matched parasite positive children from the high transmission cohort (Chonyi, grey boxes), compared to the low transmission cohort (Ngerenya, white boxes). Ngerenya cohort, n = 287; Chonyi cohort, n = 109.