Sickle Cell Anemia
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Blood disease that is fatal in humans being homozygous carriers of the defect gene. On the other hand individuals being heterozygous for the gene responsible for sickle cell haemoglobin (HbS), in which a substitution of valine for glutamic acid occurs in the beta-chain of the molecule, are strongly (90 %) protected against severe Malaria tropica (due to Plasmodium falciparum). This effect is based on the fact that P. falciparum may not develop into mature schizonts after having entered the red blood cell due to the low oxygen tension and leakage of potassium from host cells during sequestration in capillaries, thus considerably reducing the pathologic effects, e.g., in the brain.