Quick answer
O negative (O−) is the universal red blood cell donor under the ABO and RhD systems. Its red cells carry no A, B or RhD antigens, so a recipient's anti-A, anti-B and anti-D antibodies have nothing to attack. It is the type reached for in emergencies when there is no time to test the patient. Compatibility still depends on other blood group systems and crossmatching, and for plasma the roles reverse: AB is the universal plasma donor.
O negative vs O positive: who can receive each?
| Donor type | Compatible red cell recipients | Universal red cell donor? |
|---|---|---|
| O negative (O−) | O−, O+, A−, A+, B−, B+, AB−, AB+ | Yes, under ABO/RhD rules |
| O positive (O+) | O+, A+, B+, AB+ | No; generally Rh-positive recipients only |
These rules describe red blood cells. The universal plasma donor is AB. Whole blood and individual transfusion decisions need separate clinical assessment.
Check your donor and recipient
Choose two blood types to see their red cell or plasma compatibility and the reason for the result.
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 A, B or RhD antigens present, this avoids incompatibility involving those three antigens in the simplified model. O− red cells can still carry other blood-group antigens, so a universal-donor label does not mean a unit is automatically suitable for every patient.
| 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. Other blood group systems (including Kell, Duffy and Kidd) also matter, 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.