Difference Between O Positive & O Negative Blood

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 2. 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 2. Type O blood has neither antigen.

Rh Factor

About O Positive Blood

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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 1. However, doctors would only use unmatched blood in an emergency.


The History of Blood Testing

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According to the Red Cross, O positive is the most common blood type across all ethnic groups 2. O negative blood is relatively rare.