Infections explain why our blood groups differ

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).

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 ...

Not far from A to B

CHEMISTS have finally found the crucial difference between the enzymes that dictate which blood group we inherit. These enzymes, called glycosyltransferases, adorn the surfaces of blood cells with specific sugars that distinguish the blood groups A, B and O.

Now a Canadian team jointly led by Stephen Evans of the University of Ottawa has discovered that just one of the enzyme's chain of 354 amino acids decides whether we end up with group A or B blood (Nature Structural Biology, DOI: 10.1038/nsb832). People with group O inherit enzymes that make simpler surface sugars.

From issue 2357 of New Scientist magazine, 24 August 2002, page 24

Enzymes convert all donor blood to group O

You're rushed into hospital and need a blood transfusion – but what is your blood group? In future, it may not matter, thanks to enzymes that scrub antigens from red blood cells, turning all donated blood into group O – which can be given safely to anyone.

The A and B antigens, which give blood groups their name, are sugars carried on the surface of red blood cells. Human red blood cells can carry one of these antigens, both, or neither; giving four blood groups: A, B, AB and O, respectively. Receiving mismatched blood can cause a life-threatening reaction, and errors are made in 1 in every 15,000 transfusions, on average.

In the 1980s, a team in New York, US, showed that an enzyme from green coffee beans could remove the B antigen from red blood cells. It proved too inefficient for practical use, but Henrik Clausen at the University of Copenhagen in Denmark and colleagues have now screened bacteria and fungi for more powerful enzymes. "The diversity you get in the bacterial kingdom is much higher," Clausen explains.

The researchers homed in on two enzymes.

One, from a gut bacterium called Bacteroides fragilis, removes the B antigen.The other, from Elizabethkingia meningosepticum – which causes opportunistic infections in people – targets the A antigen.The purified enzymes are highly efficient. For example, the B. fragilis enzyme is used up at only one-thousandth the rate of the coffee bean enzyme.Clausen's team is working with a company called ZymeQuest in Beverly, Massachusetts, US, which plans clinical trials to test whether the treated blood is safe and effective. If so, the technology should be in hot demand, because group O blood – the only safe option if there is any doubt about the recipient's blood group – is a precious commodity. "We're always in a shortage," says Richard Benjamin, chief medical officer with the American Red Cross in Washington DC, US.

Journal reference: Nature Biotechnology (DOI: 10.1038/nbt1298)