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The human ABO blood group system is the first blood group system discovered, because it has regular blood group antibodies, it is the most important blood group system, divided into A, B, O and AB. Among them, only O-type people lack A and B antigens on red blood cells. Therefore, when the ABO blood type of the recipient is difficult to identify, the blood-matched O-type washed red blood cells can be transfused; and in the emergency of saving lives, there is insufficient blood of the same type or In case of deficiency, a small amount of blood type O can also be transfused to alleviate urgent needs.

However, type O plasma contains anti-A and anti-B antibodies, which can sensitize or agglutinate type A, B, and AB red blood cells, shorten their lifespan or immediately destroy them, which is a contraindication for blood transfusion. Therefore, type 0 whole blood was once called "dangerous omnipotent blood." Some people ignore the word "danger" and call type O blood "universal blood", which is wrong.