The universal blood donor type is O negative (O-). This blood type is highly valuable in medical emergencies because it can be safely given to patients of any blood type without the risk of severe transfusion reactions. O negative blood is particularly critical for trauma patients or during surgeries when there may not be time to determine a patient's blood type. While only a small percentage of the population has O negative blood, its general compatibility makes it essential for blood banks to maintain adequate supplies. Donating O negative blood helps save lives and is a vital community service.