The advantage of blood type O is that it contains neither A nor B antigens and is known as a “universal transfusionist”. Blood type o can be used when patients with blood type A and blood type B need emergency transfusion. However, patients with blood type O can only receive blood type O transfusion, otherwise it may lead to hemolytic reaction. Blood type o is one of the more common in the world. o blood type o blood contains neither A antigen nor B antigen, and blood type A and B produce antibodies to both antigens. Therefore, when a patient with blood type A or B needs a blood transfusion but cannot find the same blood type in an emergency, blood type O can be used as a first aid. However, when a patient with blood type O needs blood transfusion, he/she cannot enter blood type A and B, but only blood type O. Otherwise, it may lead to hemolytic reaction of the patient.Patients with blood type Rh are relatively rare, but they can only enter the same type of blood for blood transfusion.