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Which blood type can be given to almost anyone in an emergency as red cells?
O negative red cells lack A, B and Rh antigens, so they are the 'universal donor' type for red cells.
Blood types are defined by markers on the surface of red cells. The ABO system has A and B antigens; people with type A have A antigens, type B have B, AB have both, and O has neither. Separately, the Rh factor (the D antigen) makes a type positive or negative. People naturally carry antibodies against the ABO antigens they lack, so giving them red cells with the wrong antigen can trigger a dangerous immune reaction.
O negative red cells carry none of the A, B or Rh D antigens, so they are unlikely to provoke these reactions in most recipients. That is why hospitals and ambulances stock O negative when there is no time to determine a patient's type. The label applies to red cells only: plasma works in reverse, and the universal plasma donor type is AB, since AB plasma lacks anti-A and anti-B antibodies.
Practical limits exist. O negative is a minority blood type, roughly 7 percent of the US population according to commonly cited figures, so supplies are tight, and many hospitals switch patients to their own type, or to O positive for some patients, as soon as possible. Other, rarer antigens can still cause reactions, which is why in non-emergencies the donor and patient blood are matched and cross-checked before transfusion.