O- (O negative) blood is one of the eight main blood types in the ABO blood group system. It is characterized by the absence of A and B antigens on the surface of red blood cells, making it a universal donor type. This means that individuals with O- blood can donate to any other blood type, which is crucial in emergency situations when there is no time to determine a patient's blood type. People with O- blood can only receive blood from other O- donors. This blood type is relatively rare, found in about 7% of the population. Due to its universal donor status, O- blood is often in high demand in blood banks and hospitals, especially during emergencies or natural disasters, where immediate transfusions may be necessary.