SciShow explains what blood types are and which is compatible to which in case a transfusion is needed. There are four blood groups A, B, AB, and O and each of these groups can have a negative or positive Rh, making humans beings that have in a total of 8 blood types A Rh+, A Rh-, B Rh+, B Rh- and so on.
We have different blood types, because we have different immune systems. Our immune system protects the cells of our bodies because it recognizes the antigens, the proteins the cells are covered with like a nametag, and attacks cells that does not have our antigens. If the immune systems sees antigens that do not belong to us, then it triggers the release of antibodies to catch and destroy those foreign cells.
The red blood cells have two main types of antigens: agglutinogens A and B. They activate antibodies that attack invaders. If you have aggulinogen A, then you do not have antibodies for A, but you have antibodies for B and so on. This is why A cannot accept blood from anyone except A type as the immune system would attack blood type B, for example, and destroy it.
Here is a table from the video to better understand the issue:
We also have another set of antigens in the blood cells: Rh antigens, or Rhesus. If you have them, you are Rh positive, if not, then you’re Rh negative. This is how you get 8 blood types: A, B, AB, O AND Rh -/+.
So, given the antibodies we have in our bodies based on the antigens on our blood cells, tranfusion needs to make sure we are compatible. So, AB, which has no antibodies for other blood types can accept A, B, AB and O, but O can accept only from O as it has antibodies A and B.
Image, credit Scishow:
Also, given Rh antigens, Rh positive can accept Rh+ adn Rh-, but Rh- can accept only from Rh-.
Image credit SciShow: