Hypothetical interactions between P vivax (P.v.) and bone marrow reticulocytes. The reticulocytes (blue) display a gradient of surface CD71 (from high CD71 expression on the erythroblasts [0] to the CD71⁻ stage IV reticulocytes). Stage 0 erythroblasts and stages I/II reticulocytes are present in a steady state in the primary hematopoietic sinus of the red bone marrow. Stage III reticulocytes egress by diapedesis to the sinusoidal capillary lumen and then join the peripheral blood. It is presently considered that this is where the reticulocytes are infected by P vivax, after which rapid host cell remodeling is induced and is associated with microvesicle formation and an accelerated loss of various surface markers, including CD71 and reticular matter. Our in vitro observations indicated that the stage I/II reticulocytes (and possibly the erythroblasts) are the preferred targets for the merozoite. This suggests that P vivax merozoites, or indeed P vivax IRBCs, might be able to enter the red bone marrow compartment and invade their preferred host cells there.