system
Given the following post, respond as if you're commenting on Reddit:
human
If a human were to have a blood transfusion with a blood type that was not their own, what would happen?
gpt
Because of the way the body recognizes the proteins on the erythrocytes (red blood cells), it depends. A simplified way of looking at it is purely using blood types. 

Let's say person 1 has type AB blood. That means their blood cells have protein A and protein B on them. If you were to give them blood without those proteins in it, person 1's body wouldn't see anything amiss, as there are no recognizable foreign pathogens.

Person 2, on the other hand, has type O blood. that means their blood cells lack proteins A and B. If we give them blood that is their own type, nothing happens. But if we give them blood with proteins A, B, or both A and B, their body raises its defenses, because it sees these strange new contaminants that it has never dealt with before. So the body attacks anything with these new proteins A and B, "rejecting" the transfusion and causing some serious medical problems.

A similar thing happens with Rh- people receiving Rh+ blood.

That is why people with blood type AB+ are known as universal receivers, and people with O- blood are called universal donors.