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Blood: Physiology and Circulation
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irreversible distortion of the red cells. The sickle-shaped cells become clogged in small blood vessels, causing obstruction of the microcirculation, which in turn results in damage to and destruction of various tissues. The Hb S gene is distributed geographically in a broad equatorial belt in Africa and less often in other parts of the continent and the Americas, where at least 8 percent of black Americans, or 1 in every 12, carry the sickle cell trait. The actual disease is less common (about 1 in 500 black Americans). The persistence of Hb S has been explained by the fact that heterozygous persons are resistant to malaria. When the red cells of a person with the sickle cell trait are invaded by the malarial parasite, the red cells adhere to blood vessel walls, become deoxygenated, assume the sickle shape, and then are destroyed, the parasite being destroyed with them.
Blood smear in which the red cells show variation in size and shape typical of sickle cell anemia. (A) Long, thin, deeply stained cells with pointed ends are irreversibly sickled. (B) Small, round, dense cells are hyperchromic, because a part of the membrane is lost during sickling. (C) Target cell with a concentration of hemoglobin on its centre. (D) Lymphocyte. (E) Platelets. EncyclopĂŚdia Britannica, Inc. 188