Using data from different kinds of experiment on cell cultures and animals, they show how the Plasmodium falciparum parasite secretes RIFIN, and how the protein makes its way to the surface of the blood cell, where it acts like glue. The team also demonstrates how it bonds strongly with the surface of type A blood cells, but only weakly to type O.

It suggests that the selective pressure imposed by malaria may contribute to the variable global distribution of ABO blood groups in the human population. According to principal investigator Mats Wahlgren, professor at Karolinska Institutet's department of microbiology, tumour and cell biology, the finding is "conceptually simple".

However, since RIFIN is found in many different variants, it has taken the research team a lot of time to isolate exactly which variant is responsible for this mechanism."We can now explain the mechanism behind the protection that blood group O provides against severe malaria, which can, in turn, explain why the blood type is so common in the areas where malaria is common," Wahlgren added.

In Nigeria, for instance, more than half of the population belongs to blood group O, which protects against malaria. It has long been known that people with blood type O are protected against severe malaria, while those with other types, such as A, often fall into a coma and die. Malaria infects 200 million people a year, 600,000 of whom, primarily children under five, fatally.

Malaria, which is most endemic in sub-Saharan Africa, is caused by different kinds of parasites from the plasmodium family, and effectively all cases of severe or fatal malaria come from the species known as Plasmodium falciparum.The study was published in the journal Nature Medicine.

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