system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens if a person is given the wrong type of blood for a transfusion.
gpt
Blood has anti other blood sensors on it. Type A has anti-B, type O has anti A and anti B, type B has anti A and type AB has neither. So following these rules, Type A can receive from only A or O. Type B can receive B or O.  Type O can only receive O. Type AB can receive O, A, B, and AB.  This is why type O is considered the universal donor while type AB is the universal receiver.  If they are not a blood match, the blood will attack the foreign blood. The reason someone is getting a transfusion is because they need blood. Adding bad blood triggers an auto response from the blood to destroy the foreign blood. This exacerbates the lack of blood issue by causing existing blood cells to die and destroying the transfused blood cells as well. Now the body not only has survive whatever trauma but also has to clean up the giant mess in the bloodstream now with multiple clots that can reach cause heart attack or stroke. Most likely it kills the patient if not caught immediately.  
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Edit: Rh factor is another aspect of the blood. This is either you have Rh factor (+) or you don't (-).  Those who don't, don't want it and those who do don't care if they don't get more.