Question: Why do we have blood types

Ciarán O'Brien answered on 7 Nov 2014:

Short answer: Because evolution is messy and complicated.

Real answer: Blood cells have various little proteins studded into the surface, which can be signals to other cells or pumps to take in nutrients or expel waste. Blood also contains antibodies, which are tiny chemicals produced by white blood cells that attack anything foreign to the body. The antibodies are used to the proteins on the cell surfaces of your body, but not to those from someone else, which they’d see as foreign invaders to attack. So someone with blood type A will have blood cells with type A proteins, but no type A antibodies, while someone from type B will have type B proteins but no type B antibodies. Type AB is where blood cells have both A and B proteins, so that blood has no A/B antibodies, which is why type AB people can receive blood from anyone, but AB blood can only be given to other AB people. Type O is where the blood cells have neither protein on their surface, but antibodies against both, so they can only receive type O blood, but as type O blood cells don’t have any of these proteins on their surface, they won’t set off anyone’s immune system so type O people can give blood to anyone.

It’s a little more complicated than A, B, AB and O, there are actually 30 different blood types who can receive blood from some types but not others, but that’s how they all work, and I don’t want to bore you with the massive chart I’d need to show how all 30 blood types interact with eachother!

These differences are probably the result of a few hundred million years of evolution and mutation, but they’re harmless mutations in that no blood type is any better or worse than another. Researchers have found from studying the blood of people all over the world that Blood type A was far more common in Europe, while in Asia most people had type B or a slightly different kind of type A. The differences being separated by geography like that is a good indicator that the blood types evolved separately. It might have something to do with two slightly different kinds of early human, Neanderthal and Cro-Magnon, and how they evolved and intermingled. Like I said earlier, evolution is messy and complicated, so I doubt we’ll ever know for sure exactly how it happened, but this is the most likely story based on all the evidence scientists have gathered about blood types.

All of the information on what proteins your blood cells should have on their surfaces are coded in your DNA, half of which you got from your mother, and half from your father. Your blood is the determined by the blood groups your parents had. There are some handy charts here that can explain the ways your parent’s blood types can combine: http://mistupid.com/health/bloodinherit.htm

(I’ve edited this answer since Friday because I’m always mixing up blood types, so I sat down with a book on blood transfusions to make sure today)