Blood is typed according to the antigens, or factors that promote an immune response, on the red blood cells. Matching the blood by type and Rh factor of a donor and recipient greatly reduces the possibility of a serious allergic reaction to the foreign blood.
Blood types include A, B, AB, and O. Types A and B have antigen A and antigen B, respectively, on the surface of their red blood cells, and Type AB blood has both A and B. Type O blood has neither antigen.
Blood is also classified by the presence or absence of another antigen called the rhesus or Rh factor. O positive blood has the Rh factor, but neither antigens A or B; and O negative blood lacks the Rh factor as well as antigens A and B.
People with an O negative blood type are sometimes called "universal donors" because their blood has no antigens that would cause an allergic reaction in a recipient of another blood type. However, doctors would only use unmatched blood in an emergency.
According to the Red Cross, O positive is the most common blood type across all ethnic groups. O negative blood is relatively rare.