The O blood group is known as the universal donor because the blood can be transfused successfully to blood groups O, A, B, and AB. However, the O blood group cannot receive blood from any other group except the O blood group. Blood groups are determined by the presence or absence of the A and B antigens. Blood group O does not have either the A or B antigens on red cells, but it contains both the A and B antibodies in the plasma. It should also be noted that not all blood group O’s are the same because of the Rh factor which determines whether the blood group is positive or negative. Thus, blood group O is further categorized into O positive and O negative with O negative being the true universal red cell donor.
The reason why blood group O can only receive from the same blood group is because the other blood groups contain antigens that are considered foreign by the recipient. The situation will force the O blood host to generate antibodies to fight the foreign antigen. The immune system effectively goes on attack mode and attacks the foreign blood which ultimately leads to organ failure and death.