O blood is not a universal blood, because the principle of blood transfusion is to transfuse the same type of blood, only in the last resort in order to save the patient’s life, can a small amount of slow transfusion O blood, because the transfusion mainly depends on whether the donor red blood cells and the recipient’s serum antibodies agglutination reaction, O blood, because there is no A antigen on the red blood cell membrane, there is no B antigen, so, will not produce with the recipient’s serum agglutination reaction. However, the recipient’s red blood cells are considered to react with the donor’s serum during the transfusion. In contrast, people with type O blood contain both anti-A and anti-B antibodies in their serum, and there is a risk of agglutination when transfused to other blood types. Although, this agglutination reaction occurs to a lesser extent, it is still possible for an agglutination reaction to occur causing hemolysis, so O blood type is not a universal blood.