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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?

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marked as duplicate by De Novo, Bryan Krause Mar 8 at 15:41

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Yes, the recipient's red blood cells would be attacked by the donor's blood. However, there are not that many antibodies in the donor blood that would pose a major threat - and the donation is diluted by the many liters of recipient blood! Here is a complete answer, the part at the bottom is relevant to your question. $\endgroup$ – S Pr Mar 8 at 15:24
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    $\begingroup$ Good question! It just so happens we already have an answer for it! (See the above links, and thanks to @SPr for identifying it. $\endgroup$ – De Novo Mar 8 at 15:27