WHY people have different blood groups is something of a mystery. But a new analysis suggests that different groups evolved to give populations a balanced defence against viruses and bacteria.
People have either blood group A, B, AB, or O, with each type occurring at different frequencies in populations around the world.
Now Robert Seymour and his colleagues at University College London have used a mathematical model to show that this diversity is caused by selection pressures imposed on human populations by viral and bacterial infections (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2004.2674).
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Their model reveals that if viral infections dominate a population, blood type O will be most common, whereas if bacterial infections are more common, then A and B blood types will be more frequent. These results closely resemble the balance of blood types seen in today’s populations, Seymour says.
Viral infections increase the frequency of type O due to the antibodies in our blood. Type A people have anti-B antibodies, type Bs have anti-A antibodies, type Os have both, and type ABs have none.
This affects the transmission of viruses. Team member Kenth Gustafsson found recently that measles viruses pick up a chemical signature specific to each blood type as they burst out of infected cells. They do this by copying patterns of sugar molecules attached to cell surface proteins. This makes the viruses susceptible to attack by antibodies in people with a different blood type.
Gustafsson found that viruses emerging from type A and B cells are neutralised by type O blood, because it contains antibodies to fight against molecules carrying signatures of type A and B. But type A or B blood could not neutralise viruses emerging from type O cells (Blood, vol 99, p 2477).
When the team plugged these findings into their model, they found that type O people are good at transmitting viruses to people with A or B blood types. But type O people are better protected from viruses that had infected type A, B or AB individuals.
This means that if viruses were the sole scourge, only type O people would survive in the long run. “But that isn’t the case,” Seymour says. “So we have to account for some advantage that As, Bs, and ABs have over the Os.”
It turns out that bacterial infections restore the balance. Bacteria have to adhere to the sugars on some epithelial cells, and these sugars are specific to each blood group. The model shows that bacteria will evolve to attach to sugars specific to the majority blood type in a population and attack these hosts. So bacteria tend to infect people with type O in populations where that group is most common.
“It’s a compelling model,” says John Martinko, an expert on ABO blood groups at the Southern Illinois University at Carbondale. But he says the model does not answer a key question. ABO blood groups evolved at least 13 million years ago in common ancestors of great apes and humans, and the sugar molecules associated with the blood types have not changed much since. If blood groups are a defence against infections, it’s a mystery why ABO molecules are so rigidly conserved when bacteria and viruses are constantly mutating.