Yesterday I made an offhand remark that the Red Cross was so persistent in stalking me because I was O-. Here’s why that’s important:
Your red blood cells can have two different proteins on them (A or B), or both, or neither. If you receive a transfusion with red blood cells with a protein you don’t have, your immune system will mark it as a foreign invader and attack it. If you get too much of these cells, you will die. Parallel to this is another protein Rh factor, which you either have (Rh+) or you don’t (Rh-). A- means you have A proteins, but not B or Rh factor. If you are Rh- and receive Rh+ red blood cells, same problem.
O- red blood cells have no immune-triggering proteins on it, and so can be given to anyone. That makes O- blood extremely useful when you don’t have time to type a patient, or have limited carrying capacity. Moreover, O- red blood cells are the only type O- patients can accept. So O- is often the first to run out.
You might think this makes donations from AB+ people pointless, but that is not the case. Their red blood cells are of limited use, but their blood plasma (which is the part that contains the antigens that trigger attacks on foreign matter) contains no antigens to any blood type. Meanwhile everything looks like an invader to my O- plasma, and it will react accordingly. This also means you can only do whole blood donations between complete type matches. A B+ person can take my red blood cells, but would need plasma from someone else.
And that is why typed-matched blood is preferred and donations of all types are necessary.