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Why do we have blood types?


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Each person’s immune system becomes familiar with his or her own blood type.

If people receive a transfusion of the wrong type of blood, however, their immune system responds with a furious attack, as if the blood were an invader. The exception to this rule is type O blood. It only has H antigens, which are present in the other blood types too. To a person with type A or type B, it seems familiar. That familiarity makes people with type O blood universal donors, and their blood especially valuable to blood centres.

Landsteiner reported his experiment in a short, terse paper in 1900. “It might be mentioned that the reported observations may assist in the explanation of various consequences of therapeutic blood transfusions,” he concluded with exquisite understatement. Landsteiner’s discovery opened the way to safe, large-scale blood transfusions, and even today blood banks use his basic method of clumping blood cells as a quick, reliable test for blood types.

But as Landsteiner answered an old question, he raised new ones. What, if anything, were blood types for? Why should red blood cells bother with building their molecular houses? And why do people have different houses?

Solid scientific answers to these questions have been hard to come by. And in the meantime, some unscientific explanations have gained huge popularity. “It’s just been ridiculous,” sighs Connie Westhoff, the Director of Immunohematology, Genomics, and Rare Blood at the New York Blood Center.

Basically, scientists still have no idea why we have blood types. 

Also blood type has nothing to do with diet.

El-Sohemy is an expert in the emerging field of nutrigenomics. He and his colleagues have brought together 1,500 volunteers to study, tracking the foods they eat and their health. They are analysing the DNA of their subjects to see how their genes may influence how food affects them. Two people may respond very differently to the same diet based on their genes.

“Almost every time I give talks about this, someone at the end asks me, ‘Oh, is this like the Blood Type Diet?’” says El-Sohemy. As a scientist, he found Eat Right 4 Your Type lacking. “None of the stuff in the book is backed by science,” he says. But El-Sohemy realised that since he knew the blood types of his 1,500 volunteers, he could see if the Blood Type Diet actually did people any good.

El-Sohemy and his colleagues divided up their subjects by their diets. Some ate the meat-based diets D’Adamo recommended for type O, some ate a mostly vegetarian diet as recommended for type A, and so on. The scientists gave each person in the study a score for how well they adhered to each blood type diet.

The researchers did find, in fact, that some of the diets could do people some good. People who stuck to the type A diet, for example, had lower body mass index scores, smaller waists and lower blood pressure. People on the type O diet had lower triglycerides. The type B diet – rich in dairy products – provided no benefits.

“The catch,” says El-Sohemy, “is that it has nothing to do with people’s blood type.” In other words, if you have type O blood, you can still benefit from a so-called type A diet just as much as someone with type A blood – probably because the benefits of a mostly vegetarian diet can be enjoyed by anyone. Anyone on a type O diet cuts out lots of carbohydrates, with the attending benefits of this being available to virtually everyone. Likewise, a diet rich in dairy products isn’t healthy for anyone – no matter their blood type.

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