Blood type O is not the universal blood. Blood is mainly composed of blood cells and plasma. blood cells are composed of white blood cells, red blood cells and platelets. antigens on red blood cells and antibodies in plasma determine our ABO blood type. type A blood red blood cells contain A antigens and plasma contains anti-B antibodies. type B blood red blood cells contain B antigens and plasma contains anti-A antibodies. type AB blood cells have both antigens and plasma has neither. Type O red blood cells have neither A nor B antigens, but the plasma contains two types of antibodies, namely anti-A and anti-B antibodies. Since the plasma of type O blood contains anti-A and anti-B antibodies, if red blood cell preparations containing type O plasma are transfused into type A, B, or AB patients, they may cause various degrees of immune hemolytic transfusion adverse reactions. In addition, there are other blood group systems in the human body, such as the “panda blood” – Rh blood group system. If people with Rh blood type are transfused with incompatible blood, even if the ABO blood type is the same, the blood will not match, and in serious cases, the blood will be life-threatening due to hemolytic reactions. At present, when transfusing blood to patients, in principle, ABO blood group homotypic transfusion is used, and cross-matching tests are required. Homotypic blood transfusion is the most ideal and safest way, only when there is a lack of homozygous blood source for emergencies and when the amount of blood transfused is small, O blood is used instead of homozygous blood, thus it is clear that O blood is not the all-purpose blood.