Historically, people w/ type AB blood were called universal recipients. What is the rationale for this term?

Answer 1

While we now know that there are more factors then A, B and O surface antigens on blood cells, this is the oversimplyfied explaination:

There are A surface proteins and B surface proteins that exist on blood cells. O is the lack of A or B.

A body rejects protein that it normally doesn't have. A person that has AB blood can tolerate blood that has protein of it's own type AB, blood that has A because they have A protein on their blood, and they can accept B blood because they have B protein on their blood, and they can tolerate O blood because the O blood doesn't have any A or B on it for the recipient's body to reject.

In contrast, people with type O blood can only accept blood of type O.

Again in actuality, matching donated blood to a recipient is more complicated than just A, B and O.

Answer 2

The ABO blood grouping system is based on the presence or absence of two antigens (A and B) in the cell membrane of a red blood cell (RBC.)

If your RBCs carry only the A antigen, then your blood type is A. If they carry the B type, then your blood type is B. If they carry both, then you are a AB and if neither, your type is O.

The way the immune system works is by recognising foreign antigens and "attacking" them by causing agglutination and then cell death. Obviously there has to be a mechanism to recognise and tolerate "self" antigens. So if your blood type is A, then your body doesn't react to the A antigen, but because the B antigen is foreign, it will react to that. Therefore if B type blood was injected into you, you will suffer an anaphylactic reaction which in severe cases can lead to death. The same obviously applies to B type blood which will react with A type blood.

If your blood type is AB, your immune system is tolerant of both A and B antigens and therefore will accept, A type blood, B type blood and even O type blood (which contains neither A nor B antigen.) This is why AB type blood is called the "universal recipient." In the same way, O type blood is called the "universal donor" since it can donate blood to any other type without causing an immune reaction.

The Wikipedia article I cite should give you more detail if you need it.

Answer 3

If you go back in time then there was no concern for Rh factor or other factors. Lets pretend that A is raisins and B is chocolate chips that could be put in a cookie. Lets say that the only way you could flavor cookies is with raisins and chocolate chips. If you liked both raisins and chocolate chips (you are AB) then you would like all cookies (you can universally receive cookies from anyone).
Now as you find out more then you discover that there are nuts(Rh factor) which explains why you really did not like all cookies. If you are AB+ then you are still universal recipient if you dont worry about further antigens.