It’s no accident of nature that human blood has split into a handful of distinct types: A, B, AB and O. People with type O blood are at less risk of dying from malaria but more vulnerable to cholera and stomach ulcers, suggesting that different diseases put different pressures on how blood evolved.
Malaria has probably killed more people than any other disease in human history. The malaria parasite invades red blood cells, and some malaria strains then snare other uninfected blood cells, forming “rosettes”.
No one knows exactly why they form, says Alexandra Rowe of the University of Edinburgh, UK. It was thought to be a way for the parasite to infect more cells, but it seems more likely that it may be a “cloaking device that hides infected cells from the immune system”, says Rowe.
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To form the rosettes – which cause damage by disrupting fine blood vessels – the malaria parasite uses sugars on the surface of red blood cells. People with type A or B blood have one of two kinds of these sugars on their cells while people with AB blood have both. People with type O have neither, and this appears to make it more difficult for the disease to form rosettes.
In Mali, Rowe found that children aged 3 or 4 with severe malaria were more likely to carry rosettes than children with milder malaria. Crucially, the less seriously affected children – with fewer or less-stable rosettes – were three times as likely to have type O blood as those with severe malaria (Proceedings of the National Academy of Sciences, vol 104, p 17471).
“If we can develop a drug or vaccine to reduce rosetting, mimicking the effect of blood group O, we may be able to save lives,” Rowe says.
“If we can develop a drug that mimics the effect of having blood group O, we may be able to save lives”
But if type O is so useful, why doesn’t everyone have it? Among people of European descent, who historically have had a lower risk of contracting malaria, less than half are type O, compared with more than half of Africans and nearly all of some Native American groups.
That might be because type O blood appears to make people more susceptible to other diseases, such as stomach ulcers caused by Helicobacter and gastroenteritis caused by noroviruses. Blood type O is also the most common type in Latin America. But there, cholera victims with type O blood are eight times more likely to have a severe form of the disease than a milder version.
