The universal donor blood type is O negative (O-). This blood type can be given to any other blood type in transfusions because it lacks A, B, and Rh antigens that could trigger an immune response in the recipient. O negative blood is especially important for emergency transfusions when there is no time for blood typing. Due to its compatibility with all major blood types, O negative blood is in high demand and often referred to as the universal blood type for red blood cell transfusions.