Quick answer
O negative (O−) is the universal red blood cell donor. Its red cells carry no A, B or RhD antigens, so a recipient's antibodies have nothing to attack. It is the type reached for in emergencies when there is no time to test the patient. For plasma, the roles reverse and AB is the universal donor.
Why O negative works
A transfusion reaction happens when antibodies in the recipient's plasma bind to antigens on the donor's red cells. Type O red cells have neither A nor B antigens, and Rh-negative cells lack the D antigen. With no major antigens present, there is nothing for anti-A, anti-B or anti-D antibodies to recognise, so O− red cells are compatible with all eight ABO/Rh types.
| Donor | Can give red cells to |
|---|---|
| O− | O−, O+, A−, A+, B−, B+, AB−, AB+ (all types) |
How hospitals use it
Trauma centres and emergency departments keep O− red cells ready for patients whose blood type is unknown, such as accident victims and people with severe bleeding. Once the patient's type is known, they are usually switched to type-specific blood to conserve O− supply. O− is also preferred for Rh-negative women of childbearing age and for some newborn transfusions.
Only about 7 percent of the U.S. population is O−, but demand is disproportionately high, which is why blood services frequently appeal for O− donors.
Limits of the “universal” label
“Universal” applies to red cells under the ABO and RhD systems only. There are more than 40 other blood group systems (Kell, Duffy, Kidd and others), and patients who have received many transfusions can carry antibodies to those antigens. Laboratories therefore still crossmatch whenever possible. O− whole blood is also not universal, because O plasma contains anti-A and anti-B antibodies.
The universal plasma donor is AB
Plasma compatibility runs opposite to red cells. AB plasma contains no anti-A or anti-B antibodies, so it can be given to recipients of any ABO type. O plasma, by contrast, contains both antibodies and is suitable only for type O recipients. See the plasma compatibility guide for details.