Blood groups are determined by the presence or absence of certain antigens on the surface of red blood cells. The most important blood group system is the ABO system, which is determined by the presence or absence of two antigens: A and B. People with blood group A have antibodies against the antigen B, people with blood group B have antibodies against the antigen A. People with blood group AB have no antibodies to either antigen, and people with blood group O have both types of antibodies. There is also the D antigen which determines rhesus (Rh). People who have this antigen are said to be Rh positive, while those who do not are Rh negative. Blood transfusion refers to the process in which a person receives whole blood, or components of blood. A person with type O negative blood can donate red cells to any other blood type regardless of the rhesus, while a person with type O positive blood can donate red cells to any other ABO rhesus positive recipient. Those which have AB positive can receive blood from any other ABO regardless of the rhesus, whereas those with AB negative can receive blood from any other ABO with rhesus negative.
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