Ability of RBCs to cross the spleen, dispersion of individual values, and possible variability of the splenic detection threshold in the context of P falciparum infection. (A) The speculative distribution of mature RBCs with different levels of splenic “crossability” at homeostasis. Approximately 1% of the circulating RBC population loses its ability to cross the spleen each day. When the spleen crossability of a RBC is below the splenic retention threshold, the corresponding RBC is retained in the cords of the red pulp. This retention is expected to trigger phagocytosis, followed by iron recycling into erythropoiesis. Aging of a RBC is associated with a progressive reduction of RBC deformability that influences RBC crossability.⁹⁹  A link between deformability (assessed by ektacytometry) and spleen crossability has been established in thalassaemia.¹⁰⁰  (B) 1. The elongation of ring-infected RBCs is reduced¹⁸  with a wide dispersion of individual values, as assessed using optical tweezers.³²  Deformability values of rings span the splenic retention threshold, as shown by the retention of rings in an ex vivo human spleen-perfusion system.⁶  2. The splenic crossability of a proportion of the ring population is therefore below the splenic retention threshold. This subpopulation is retained and (at least partially) phagocytosed, therefore unable to sequester in the vasculature of other organs. 3. The subpopulation that is still able to circulate is available for maturation and subsequent cytoadherence-based sequestration. Despite their gross inability to cross the spleen, cytoadherent mature forms are protected from splenic retention by sequestration in the microcirculation. 4. The biomass of sequestered mature forms generated at each cycle thus depends on the proportion of circulating rings and determines the number of rings produced at the next reinvasion wave.⁵  The proportion of rings allowed to circulate through the spleen is therefore a determinant of the in vivo multiplication factor of the parasite biomass. (C) Influence of a left shift in the crossability of RBCs. Several RBC disorders/polymorphisms are associated with a reduced RBC deformability or decreased ability of RBCs to cross the spleen (1). This abnormality may induce a similar shift in the deformability of ring-infected RBCs. The subpopulation of rings retained is therefore greater, and the subpopulation allowed to circulate until maturation and sequestration is smaller, giving rise to a smaller sequestered biomass of mature forms. The number of rings generated at the next reinvasion cycle is also smaller, leading to a reduced in vivo multiplication factor. This process may account for part of the protection from severe malaria induced by the HbAS trait (see text). (D) In an anemic condition, when a greater number of normal RBCs are retained in the spleen, the spleen clearance zone will be shifted to the right. If the splenicretention threshold is higher (1) (ie, the spleen retains RBCs with a higher crossability), the subpopulation of rings retained is greater (2), and the subpopulation allowed to circulate until maturation and sequestration is smaller (3), giving rise to a smaller sequestered biomass of mature forms. The number of rings generated at the next reinvasion cycle is also smaller, leading to a reduced in vivo multiplication factor. A more stringent spleen retention threshold also induces the retention of a greater proportion of normal RBCs, thereby favoring the occurrence of anemia. (E) Uninfected RBCs in the blood of patients or in culture usually have a reduced deformability (1). The population distribution curve of uninfected RBCs is therefore shifted to the left, a phenomenon predicted to increase the proportion of uninfected RBCs retained in the spleen, and thereby RBC loss and subacute/acute anemia. The existence of this third subpopulation of RBCs explains how anemia and a decrease of the deformability of circulating RBCs can be associated.^6,34  Whether this process affecting uninfected RBCs is unimodal or multimodal is not known.