Inhibition of P vivax invasion by anti-PvDBP II IgG Abs. (A) The mean inhibition (percentage ± % SD) of anti–PvDBP II IgG Abs (100 μg/mL) on the P vivax invasion efficiency. The inhibition was normalized to invasion efficiencies obtained in the presence of IgG Abs (100 μg/mL) purified from a rabbit prebleed serum. The actual mean invasion efficiencies (percentage ± % SD) in the prebleed controls are included below the corresponding isolate identifier. The assays were conducted in triplicate on 4 isolates in 2 separate experiments involving reticulocyte concentrates from 2 cord blood samples of different ABO type. (B) Polymorphisms in the predicted amino acid sequence of the PvDBP region II region in the parasites present in the 4 isolates used in the assay. The sequences were compared with that present in the P vivax reference strain (Sal 1). Nonsynonymous mutations at 3 residues (417, 437, and 503) that were previously suggested to interfere with the inhibition of PvDBP region II binding to DARC are enclosed in black frames. (C) Immunofluorescence microscopy with anti–P vivax DBPII IgG reacted with merozoites from a P vivax isolate (3 examples given) used in this assay.