O negative is known as the universal red-cell donor in the ABO and Rh systems.
Red blood cells of type O lack A and B antigens on their surfaces, and Rh-negative cells lack the D antigen used to define Rh-positive status. Because these cells do not display those three major antigens, O-negative red cells are often used urgently when a patient's blood type is not yet known.
The phrase “universal donor” needs careful qualification. It refers mainly to red-cell transfusion under the ABO and Rh systems, not to every blood component or every clinical situation. Hospitals still perform compatibility testing whenever possible, and additional blood-group antigens can matter in transfusion medicine.
AB positive is commonly called the universal plasma donor because AB plasma lacks anti-A and anti-B antibodies. Plasma compatibility is therefore different from red-cell compatibility, a distinction that causes many common quiz errors.