Blood groups: There are way more than you think. Here’s why they matter

You are probably aware of the two main blood group systems: the AB system and the rhesus system. Under the AB system, there are four main blood types A, B, AB and O. And each of these four types can be “rhesus positive” or “rhesus negative.” It may surprise you to hear, though, that there are many more blood group systems than just these two. Indeed, researchers at the University of Bristol recently completed the 44th blood group system called Er.