The blood type known as the universal donor for red blood cell transfusions is O negative.
O-negative red blood cells lack A and B antigens and also lack the RhD antigen. Because these three major markers are absent from their surfaces, the cells are less likely to be attacked by antibodies in recipients with A, B, AB, or Rh-positive blood. For that reason, O negative is often used in emergencies when there is no time to complete blood typing.
“Universal donor” is a practical shorthand, not an absolute guarantee. Human blood contains many other antigen systems, and unusual antibodies can still create incompatibility. Hospitals therefore perform blood typing, antibody screening, and crossmatching whenever circumstances allow.
The term applies specifically to red blood cells. Plasma compatibility works differently: AB plasma is generally considered the universal plasma donor because it lacks anti-A and anti-B antibodies. O-negative units are also relatively limited, so blood services try to reserve them for patients who need them most, including certain emergency cases and patients who could become pregnant.