How does the sickle cell trait offer protection against malaria?
In the 1940s, E.A.Beet, a British medical officer stationed in Northern Rhodesia (now Zimbabwe), observed that blood from malaria patients who had sickle cell trait had fewer malarial parasites than blood from patients without the trait. Following this observation, a physician in Zaire reported that there were fewer cases of severe malaria among people with sickle cell trait than among those without it.
In 1954, Anthony Allison, continued to build on these observations and hypothesized that sickle cell trait offered protection against malaria. He suggested that those with the trait did not succumb to malaria as often as those without it; but, when they did, their disease was less severe. It is now known that, when invaded by the malarial parasite, normally stable red cells of someone with the sickle cell trait can sickle in a low oxygen environment (like the veins). The sickling process destroys the invading organism and prevents it from spreading through the body. This apparent ability of a genetic condition to protect carriers is particularly important in infants. Thus, in regions repeatedly devastated by malaria, people who carry the sickle cell trait will have a greater chance for survival than other individuals.
In the following years, evidence began to collect in support of this theory as well as some against it. When studies were restricted to young people, the hypothesis held - the sickle cell trait did offer protection to children but not to adults since they were unable to develop antibodies to the malarial parasite. However, even though their immunity was partial, it did help them to survive but offered little additional advantage. Since the youngsters were not able to produce antibodies to the malarial parasite until their immune systems matured, it was the pre-immune malarial patients whose survival was protected by sickle cell trait. For them as well, although protection was only...