Quantification of RSP2 erythroid precursor and binding of complement C3. (A) Flow cytometry analysis showing that the percentage of HEL 92.1.7 RSP2⁺ cells depends on parasite load. Results of 3 representative experiments are shown. Dilutions of 0.5% to 8% trophozoites were used, and the mean percentage (± SD) of erythroblasts cocultured with FCR3^CSA that were RSP2⁺ was determined after reinvasion. (B) Labeling of RSP2⁺ HEL erythroblasts with mAb B4 (FL1 RSP2) or with a human anti-C3 antibody by incubation with mAb B4 and C3⁺ human serum (FL1 C3). IgG2a was used as an isotypic control (gray) for RSP2, and C3 immunostaining was used to assess the specificity of C3 binding to B4 (IgG2a).