Anemia is not a simple product of hemolysis in sickle cell anemia, as demonstrated by the marrow expansion of only five times normal, compared with the 10 times normal seen in thalassemia major. One reason for this is the right shift of the oxygen equilibrium curve described above: this type of oxygen binding curve results in increased oxygen delivery to the tissues. This explains why erythropoietin increases only below 9 g of hemogobin. In addition, the response to erythropoietin is blunted (that is, it is inappropriately low for the anemia) below the threshold of 9 g of hemoglobin. The reason for this is not understood, but the fact that age increases the deficiency of erythropoietin suggests that 'silent' renal damage might be involved in a reduction in the erythropoietin response. Another contributor to the anemia (a reduction of erythropoietin response) is the increase in 2,3-diphosphoglycerate (2,3-DPG) found in sickle red cells.

The hemolysis is also complex: actually there are three red cell populations in sickle cell anemia, the very dense red cells, which live only 4-5 days; the F cells (these are sickle cells containing Hb F), which have close to a normal lifespan; and the rest of the cells, which fall in between. Coexistence of a thalas-semia and high Hb F reduces hemolysis considerably.

Interestingly, the level of Hb F also defines the properties of progenitors as well as those of the circulating hemopoi-etic cytokines. Low-Hb F sickle cell anemia patients have an increased number of circulating burst-forming units—ery-throid (BFU-E), which are in active cycle. In addition, these patients have constitutively circulating granulocyte macrophage colony-stimulating factor (GM-CSF) and Steel factor, unlike normal individuals and high Hb F sicklers. In contrast, the low-Hb F patients have IL-3 circulating in addition to a hemopoietic inhibitory factor, TNF-a.

Sickle cell anemia patients are susceptible to an acute anemia superimposed on their chronic anemia, due to infection with parvovirus B19. This can produce a life-threatening drop in hemoglobin, which is generally self-limited (7-15 days). This results from its ability to infect BFU-E through the blood group P system that serves as receptor. Treatment is by transfusion of red cells in most cases.

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