Blood groups and blood types are classifications based on the levels of antibodies and inherited antigens in red blood cells. The International Society of Blood Transfusion acknowledges 36 human blood group systems and several hundred antigens. The two most important and common blood group systems are the ABO and Rh systems. The former determines blood type while the latter determines if the blood possesses the D antigen. This information is integral to blood transfusions.
The ABO blood group system is the one about which most people are aware. This system classifies blood based on the presence of the A and B antigens and antibodies. In this group, there are four types of blood. Type A contains the A antigen and the B antibody. Type B contains the B antigen and the A antibody. For type AB, the blood needs both A and B antigens, but neither antibody. Type O is the opposite -- neither antigen but both antibodies. The ABO blood group system is the single most important classification system in human blood transfusions. A mismatch in ABO typing can cause serious reactions and even death in the recipient.
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