Why do we have blood types?

When my parents informed me that my blood type was A+, I felt a strange sense of pride. If A+ was the top grade in school, then surely A+ was also the most excellent of blood types – a biological mark of distinction.

It didn’t take long for me to recognise just how silly that feeling was and tamp it down. But I didn’t learn much more about what it really meant to have type A+ blood. By the time I was an adult, all I really knew was that if I should end up in a hospital in need of blood, the doctors there would need to make sure they transfused me with a suitable type.

And yet there remained some nagging questions. Why do 40% of Caucasians have type A blood, while only 27% of Asians do? Where do different blood types come from, and what do they do? To get some answers, I went to the experts – to haematologists, geneticists, evolutionary biologists, virologists and nutrition scientists.

In 1900 the Austrian physician Karl Landsteiner first discovered blood types, winning the Nobel Prize in Physiology or Medicine for his research in 1930. Since then scientists have developed ever more powerful tools for probing the biology of blood types. They’ve found some intriguing clues about them – tracing their deep ancestry, for example, and detecting influences of blood types on our health. And yet I found that in many ways blood types remain strangely mysterious. Scientists have yet to come up with a good explanation for their very existence.

“Isn’t it amazing?” says Ajit Varki, a biologist at the University of California, San Diego. “Almost a hundred years after the Nobel Prize was awarded for this discovery, we still don’t know exactly what they’re for.”

Transfusion confusion

My knowledge that I’m type A comes to me thanks to one of the greatest discoveries in the history of medicine. Because doctors are aware of blood types, they can save lives by transfusing blood into patients. But for most of history, the notion of putting blood from one person into another was a feverish dream.

Renaissance doctors mused about what would happen if they put blood into the veins of their patients. Some thought that it could be a treatment for all manner of ailments, even insanity. Finally, in the 1600s, a few doctors tested out the idea, with disastrous results. A French doctor injected calf’s blood into a madman, who promptly started to sweat and vomit and produce urine the colour of chimney soot. After another transfusion the man died.

Such calamities gave transfusions a bad reputation for 150 years. Even in the 19th Century only a few doctors dared try out the procedure. One of them was a British physician named James Blundell. Like other physicians of his day, he watched many of his female patients die from bleeding during childbirth. After the death of one patient in 1817, he found he couldn’t resign himself to the way things were.

“I could not forbear considering, that the patient might very probably have been saved by transfusion,” he later wrote.

Blundell became convinced that the earlier disasters with blood transfusions had come about thanks to one fundamental error: transfusing “the blood of the brute”, as he put it. Doctors shouldn’t transfer blood between species, he concluded, because “the different kinds of blood differ very importantly from each other”.

Human patients should only get human blood, Blundell decided. But no one had ever tried to perform such a transfusion. Blundell set about doing so by designing a system of funnels and syringes and tubes that could channel blood from a donor to an ailing patient. After testing the apparatus out on dogs, Blundell was summoned to the bed of a man who was bleeding to death. “Transfusion alone could give him a chance of life,” he wrote.

Several donors provided Blundell with 14oz (0.4kg) of blood, which he injected into the man’s arm. After the procedure the patient told Blundell that he felt better – “less fainty” – but two days later he died.

Still, the experience convinced Blundell that blood transfusion would be a huge benefit to mankind, and he continued to pour blood into desperate patients in the following years. All told, he performed 10 blood transfusions. Only four patients survived.

While some other doctors experimented with blood transfusion as well, their success rates were also dismal. Various approaches were tried, including attempts in the 1870s to use milk in transfusions (which were, unsurprisingly, fruitless and dangerous).