In a lot of important ways, human beings are pretty similar to one other.   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 the same.   All of the cells in your body are covered in antigens -- these are special protein markers that function sort of like name tags, saying “These cells belong to you.”   When your immune system detects antigens that aren’t familiar, it triggers the release of antibodies that tag foreign invaders and mark them for termination!   Now, there are two main types of antigens that form on your red blood cells. They’re called agglutinogens; they activate antibodies that smother invaders like a team of rugby players and cause coagulation in the blood.    The two kinds of agglutinogens are wisely given the discernible names of A and B.    So, your blood cells can have one kind of agglutinogen, or both, or you could have neither.    And if you have ANY of these antigens on your blood cells, that means that you don't have any antibody for it. If you did, those antibodies would attack your own cells.    On the flip side, you DO have antibodies that would attack the type of antigen that you DON’T have.    So, for example, if your red blood cells have A antigens on them, then your blood type A. And your antibodies would attack type B.   If you have B antigens on your blood cells, then you’re type B, and you have antibodies for type A.   If you have BOTH, you’re 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 the antibodies for both A and B. These guys are called type O.    Now, just to keep things interesting, 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 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 what letter you have, than what 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 Os have both A & B antibodies, so they can only accept other Os.    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 only do it 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!   Thank you so much for watching this Quick Question, especially to our Subbable subscribers who keep these answers coming!   If you have a quick question, let us know on Facebook and Twitter or in the comments below, and don’t forget to go to YouTube.com/scishow and subscribe!