MadSci Network: Medicine
Query:

Re: What is sickle cell anemia?

Date: Sun Feb 27 14:22:55 2000
Posted By: Stephanie Murg, Undergraduate, Neurobiology, Center for Neurologic Disease at Harvard Medical School, Harvard University
Area of science: Medicine
ID: 950641824.Me
Message:

Sickle cell anemia (HbSS) is the most common variant of a group of 
inherited diseases that affect hemoglobin and results when an individual 
inherits a genetic mutation (substitution of valine for the normal glutamic 
acid in the 6th amino acid position of the beta globin chain of hemoglobin) 
from both parents. This substitution alters the hemoglobin molecule so it 
crystallizes and deforms the red cell into a sickle shape when the 
hemoglobin loses oxygen.

Clinical manifestations of sickle cell anemia result from increased blood 
viscosity and vascular obstruction by deformed, sickled red cells. The most 
common problem for sickle cell patients is the pain episode, a self 
limiting and reversible pain in the extremities, back, chest, and abdomen. 
A key factor affecting life span in sickle cell anemics is bacterial 
infection (or "sepsis"): death resulting from bacterial infection 
(particularly in the spleen) is most common in infants but patients at any 
age are at great risk.

In order to understand the advantages conferred by the sickle cell trait, 
it is necessary to understand a bit about malaria. Malaria is caused by a 
parasite usually spread by mosquitoes and leads to chills, sweating, fever, 
even death. After sustaining the bite of an infected mosquito, a person's 
red blood cells become like little factories in which the parasite can 
flourish. When the parasite enters a red blood cell containing sickle 
hemoglobin, that cell is more likely to become banana-(or sickle) shaped. 
Once it sickles, the red blood cell is recognized as abnormal and destroyed 
in the spleen. The malaria parasite is destroyed as well, and has less 
opportunity to make the person ill. 

In areas where malaria flourishes, people who lived long enough to have 
children tended to have the sickle cell trait: one normal beta-hemoglobin 
gene and one sickle beta-hemoglobin gene. They died of neither sickle cell 
disease (because both beta-hemoglobin genes were not sickle) nor malaria, 
and thus kept the sickle cell gene in the population. 

REFERENCES:
Motulsky AG. Frequency of sickling disorders in US blacks. N Engl J Med. 
1973;288:31

Sergeant G. Sickle cell disease. Oxford: Oxford University Press 1985.

Vichinski EP, Johnson R, and Lubin, B. Multidisciplinary approach to pain 
management in sickle cell disease. Am J Ped Hematol Oncol 1982 4:328.

Reid CD, Charache S, Lubin B, et al. Management and Therapy of Sickle Cell 
Disease. N.I.H. Publication No. 95-2117, 1995.

Barrett-Connor E. Bacterial infection and sickle cell anemia. Medicine 
1971;50:97-112. 

Embury SH, Hebble RP, Mohandas N, Steinbberg MH.(eds) Sickle Cell Disease: 
Basic Principles and Clinical Practice. Raven Press, Ltd., New York, 1991.





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