Human beings are quite similar in so many ways, but when it comes to blood, there are definitely different types. Eight, to be exact. Our different blood types stem from the fact that we all have immune systems, and while they work equally well, they’re not all the same. The cells in your body are covered in antigens. These are special protein markers that function like QR-codes, saying these cells belong to you. When your immune system detects antigens that aren’t familiar, it tags them as foreign invaders and mark them for execution.

Now there are two main types of antigens that form on your red blood cells. They are called agglutinogens. They activate antibodies that knockout invaders and cause coagulation in the blood. The two kinds of agglutinogens are wisely given the discernible names A and B. So your red blood cells could have one kind of agglutinagen, or both, or you could have neither. And if you have any of these antigens on your blood cells, that means you don’t have any antibody for it. If you did, those antibodies would attack your own cells.

So, for example, if your red blood cells have A-antigen on them, then your blood type is A, and your antibodies would attack type B. If you have B-antigens on your blood cells, then you are type B, and you have antibodies for type A. If you have both, you are type AB, and you don’t have antibodies for either. And some people have neither kind of antigen. Their genes actually code for an agglutinogen that just doesn’t work, and they have antibodies for both A and B. These guys are called type O.

There’s another set of antigens on your blood cells that works in different ways from the agglutinogens. This is known as the Rhesus, or RH-system, and it’s actually a collection of 45 different antigens. But they’re all produced as a single group, so you either have all of them, or you have none of them. If your red blood cells have the RH antigens on them, we say that you’re Rh-positive, and if you don’t, you’re Rh-negative. So this is how we end up with eight different blood types. A, B, AB, and O, each with a positive and negative type.

Now, when it comes to transfusing blood from one person to another, compatibility is kind of important to preventing a serious, possibly life-threatening reaction. But finding the right match has less to do with which letter you have, than which letter you have the antibodies for. Which sounds confusing, but let me explain. So type AB people don’t have antibodies for either A or B, so they can accept A, B, AB, or O. But type O’s have both A and B antibodies, so they can only accept other O’s. And as for the RH antigens, people with positive blood types can accept either positive or negative blood. But negative types are safest taking only negative blood, because while they can tolerate positive blood, they can do it only once. After that, they’ll form antibodies against the RH antigens that they received, which means they can never accept positive blood again. The human body is so weird.