Why do some people have blood type O?
Quick Answer
Blood type O happens when a person inherits versions of the ABO gene that do not add A or B markers to red blood cells. That leaves the cells with the basic H antigen, which is why they type as O.
The Full Story
The ABO blood group system was discovered in 1900 by Karl Landsteiner, and it depends on sugars attached to red blood cells. People with type O inherit ABO variants that do not produce an active enzyme to add the A or B sugar. As a result, their red blood cells do not display A or B antigens. Type O is common worldwide, but the exact frequency varies by population, and O negative blood is especially important in transfusions because it lacks A, B, and Rh(D) markers.
Key Facts
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