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Natural defence mechanisms (or innate factors) against malaria are most apparent in populations continually exposed to malaria parasites. For example, inherited conditions such as sickle cell anaemia and beta-thalassaemia, which cause deformities in red blood cells and are common in people from malarious regions, make it more difficult for malaria parasites to infect red blood cells.
Some people have red blood cells that lack proteins called Duffy antigens on their surface. These proteins act as receptors for Plasmodium vivax merozoites, so people without Duffy antigens are resistant to infection from this parasite.
Feature: Malaria versus the human genome
People residing in malaria-endemic regions acquire immunity to malaria through natural exposure to malaria parasites. Children living in areas of stable malaria transmission become infected early in life, and experience more severe disease symptoms during the first five years of life. But as immunity develops the disease becomes less severe and the number of parasites circulating in the blood declines. The acquired immune response to malaria is strain specific and is lost if a person moves away from a malaria endemic area.
Following infection with Plasmodium parasites, the immune system responds in a number of ways as it attempts to clear the parasite. Antibodies against schizont and merozoite antigens bind to infected red blood cells and to merozoites, and make them easier for macrophages and other immune cells to ingest. Antibodies also help other immune mediators, called complement proteins, to destroy parasites and they prevent merozoites infecting new red blood cells.
Macrophages which have taken up Plasmodium express parasite antigens on their surface, and other immune cells called T cells recognise these antigens and bind to them. The T cells become activated and release molecules called cytokines that promote further cell activation, parasite killing and antibody production.
The body's response to malaria parasites causes some of the disease symptoms. For example, malaria fever is associated with high levels of a molecule called tumour necrosis factor. This cytokine is released by macrophages when infected red blood cells rupture, and large numbers of parasites enter the blood stream.
Feature: Malaria and the human immune system
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[WTD023885] Immunity to malaria.doc