What is the most common blood type in the world?

The most common blood type in the world is O positive. It combines type O in the ABO blood-group system with the Rh-positive factor, meaning red blood cells carry the D antigen.

Blood type is inherited through genes that determine which antigens appear on the surface of red blood cells. In the ABO system, people may have type A, B, AB, or O blood. The Rh system is separate: people are usually described as positive or negative depending mainly on whether the D antigen is present. O positive is therefore not simply another name for type O; it is one specific ABO-and-Rh combination.

Global frequencies vary considerably by country and ancestry. Type O is especially widespread in many regions, while type A is common in parts of Europe and type B has relatively high frequencies in parts of Asia and the Middle East. Even so, when ABO and Rh categories are combined worldwide, O positive is generally the largest single group.

O positive blood is important for transfusion services because it is common and can often be used for red-cell transfusions to other Rh-positive patients, provided ABO compatibility is also considered. The famous “universal donor” label usually refers to O negative red cells, not O positive.