Type O-negative is the universal blood type because it lacks the antigens that trigger immune reactions.

Blood typing is determined by the presence or absence of specific antigens on the surface of red blood cells. The ABO system, discovered by Karl Landsteiner in 1901, identifies A and B antigens, while the Rh factor, identified in 1940, determines if blood is positive or negative. Type O-negative blood is unique because it lacks A, B, and Rh (D) antigens entirely.When a person receives incompatible blood, their immune system produces antibodies to attack the foreign antigens, leading to a potentially fatal hemolytic transfusion reaction. Because O-negative blood has no antigens to be recognized as 'foreign,' it can be safely administered to patients of any blood type. This makes it the 'universal donor' and the gold standard for trauma care and emergency medicine.According to the American Red Cross, O-negative blood is in constant high demand despite only about 7% of the U.S. population possessing it. In air ambulances and emergency rooms, O-negative units are kept on standby for immediate use when there is no time to perform a cross-match. While O-negative can be given to anyone, people with O-negative blood can only receive O-negative blood themselves.