The blood type known as the universal donor is O negative (O-). Individuals with this blood type can donate red blood cells to people of all blood types because O- blood lacks A and B antigens on the surface of its red blood cells, making it less likely to provoke an immune response in recipients with different blood types. O- blood is especially critical in emergency situations when there isn't time to type a patient's blood. However, people with O- blood can only receive transfusions from other O- donors, highlighting the importance of diverse blood donation for medical needs.