The universal donor blood group is O negative (-O). This means that people with O negative blood can donate blood to anyone, regardless of the receiver’s blood type. Blood groups are determined by specific markers called antigens on the surface of red blood cells. People with type O blood do not have A or B antigens, so their blood is less likely to cause an immune reaction when given to others. Negative means there is no Rh factor antigen, which makes it even safer to give to all blood types. Because of this, O negative blood is very important in emergencies and hospitals when doctors do not have time to test for blood groups. It is also called the universal donor because it can be given to anyone without risk of serious reactions. However, people with O negative blood can only receive blood from other O negative donors.