O Positive vs. O Negative Blood Types: Understanding the Differences

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.

Type O

• 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.

Rh Factor

• 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.

Donation

• 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.

Frequency

• According to the Red Cross, O positive is the most common blood type across all ethnic groups. O negative blood is relatively rare.