Why O negative blood is crucial in transfusion medicine? O negative blood is known as the universal donor for red blood cells. But why is that? O negative blood lacks A, B, and Rh antigens. This means it doesn't trigger an immune response when transfused into any patient, regardless of their blood type. In emergency situations, when there's no time to type and crossmatch, O negative blood can be a lifesaver. It's especially vital for trauma patients, newborns, and anyone with rare blood types. B