Drepanocytosis
The fibers of polymerised haemoglobin S distort the normal shape of erythrocytes (red blood cells) which, instead of biconcave discs, become similar to a crescent or a sickle. This morphology gives their names to cells and the generated pathology:
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| From the Greek δρεπανον: sickle. | From the Latin falx, falcis: sickle. |
Heterozygous individuals have a mixture of haemoglobin A and haemoglobin S. Haemoglobin A prevents polymerisation, so the anaemia is moderated. In homozygous individuals, haemoglobin S polymerises freely; in the narrowest capillaries the erythrocytes, which cannot deform, stop flowing; in addition, haemoglobin fibers may even break the erythrocytes (image), producing the most severe form of the disease. Furthermore, haemoglobin S is more stable in its deoxygenated form, so oxygen supply to tissues is poorer.
In contrast, haemoglobin S confers resistance against a kind of malaria. Along evolution, this "advantage" has lead to drepanocytosis (with heterozygous genotype) having an incidence of up to 40% in some African regions where malaria is endemic.