According to the Red Cross, blood type O is the universal blood donor. Group O blood does not contain either A or B antigens on its red blood cells. Therefore, it can be donated to any patient without complications.
Blood types are determined by the presence or absence of antigens on red blood cells, states the Red Cross. Red blood cells can have A or B antigens on their surfaces. The different antigens present determine the blood type of the patient. For example, blood type A only has the A antigen present on its red blood cells, while type AB has A and B antigens present. Blood type O has no antigens on its red blood cells and is, therefore, a universal donor.