The blood type considered the universal donor is **O-negative (O-)**. Individuals with this blood type can donate their red blood cells to patients of all blood types without the risk of an immune reaction because their blood lacks A and B antigens, as well as the Rh factor. This characteristic makes O- blood especially valuable in emergency situations where blood type may not be immediately known. However, while O- can be given to anyone, individuals with this blood type can only receive O- blood in transfusions, making it important for blood banks to maintain a sufficient supply.