What blood type is known as the universal donor for red blood cells?

Blood types come down to molecules on the surface of red cells. Type A cells carry the A antigen, type B carry B, AB carry both, and O carry neither. A separate marker, the Rhesus D antigen, decides whether you are positive or negative. Your immune system makes antibodies against the antigens you lack, so giving type A blood to a type B patient triggers a dangerous reaction in which the transfused cells clump and break apart. O negative red cells carry no A, no B and no Rh D antigen, so there is nothing on them for a recipient's antibodies to attack, and they can be given to almost anyone. That makes O negative the stock most in demand for emergencies where there is no time to type a patient. The logic reverses for plasma, which contains the antibodies rather than the antigens, so AB plasma is the universal one there.

Only a small percentage of people are O negative, which is why blood services appeal for them so often.