This question already has an answer here:
How come a person with blood group O can donate to a person with blood group AB? Since there are A and B antibodies in the O blood group blood surely this would cause agglutination in the blood of the recipient?
My first hypothesis was that the antibodies and red blood cells were separated through use of a centrifuge or something but blood transfusion has been used for hundreds of years before this machine was even invented? What is going on here? Or was it just that before blood types were discovered only people with identical blood groups could donate/recieve from one another?