Type O blood is defined as containing neither A nor B antigens on the surface of red blood cells. Blood type O is a type of blood, and blood can be categorized as type A, B, AB and O. One of the purposes of distinguishing blood type O is to avoid hemolytic reactions when transfusion is required in clinical practice. One of the purposes of distinguishing blood type is to avoid hemolytic reaction when blood transfusion is required for clinical purposes. People with blood type O are commonly known as universal transfusionists, but they are not free to be transferred to people with other blood types. Because other blood types contain anti-A and anti-B antibodies in the plasma, a large amount of O-type blood may lead to different degrees of hemolysis of red blood cells in the recipient, so O-type blood cannot be arbitrarily transfused to other people. In addition, people with type O blood cannot receive transfusion of other blood types, which is prone to hemolytic reaction. For more information, please consult a hematologist.