Dear Sarah,
At this very moment, several quarts of blood are circulating through your body at nearly 4 mph. But as you’ve pointed out, not everyone’s blood is the same.
Your question made me wonder exactly what we mean when we talk about blood types. I decided to ask my friend Amber Fyfe-Johnson, a researcher at Washington State University who studies cardiovascular diseases–diseases of the blood vessels– in kids.
Believe it or not, she said, there are more than 20 different blood groups. We’ll stick to the main one for now: ABO. There are 4 different types in this group: A, B, O, and AB.
You have trillions of blood cells. Each blood type refers to a specific marker on a red blood cell. It’s kind of like a little flag.
In the early 1900s, an Austrian doctor named Karl Landsteiner discovered three of the little flags. Today, we call these three flags A, B, and O.
These little markers make blood types compatible with each other. If a person with Type A blood is given Type B blood, his or her body sees the Type B surface flag as foreign and rejects it.
Meanwhile, Type O doesn’t have those surface markers. There is nothing on the surface of the red blood cell to reject. Type O blood can be transferred to pretty much anyone who needs it.
Fyfe-Johnson explained that the blood types we have today evolved a very long time ago. Type A is the most ancient blood type and has been found in hominids – or pre-humans. Scientists can use DNA from some blood cells found in fossils to help figure this out. Type O probably originated next, about 5 million years ago. Scientists are still trying to pinpoint when exactly each blood type evolved.
As is often the case, there are a few ways to think about the answer to your question.
One way to think about it is that our parents pass genetic information about our blood types down to us. It’s part of our DNA. Sometimes there’s a change, or mutation, in DNA.
“These different blood types evolved as a result of genetic mutations, but what caused certain blood types to be more successful is likely exposure to infectious diseases or other environmental pressures,” Fyfe-Johnson said.
The kinds of blood types that survive infections are often the ones that outlive the others.
For example, cells that are infected with a disease called malaria don’t stick to Type O or Type B red blood cells. Those with Type A blood who are infected with malaria are more likely to have clumps of cells form that can be harmful. Especially when they form in places like the brain or heart.
People with Type A blood are more likely to have serious complications or die as a result of malaria, whereas people with other blood types could survive. This happens with many kinds of diseases, she said.
“The short story is that blood types probably evolved as a way to fight infectious diseases or other environmental pressures,” she said. “Blood types that survived were more likely to be successful.”
In a way, it’s all about survival of the fittest blood.