The blood type that is the universal donor is type O negative. A universal donor blood type means that supposedly, anyone who has this type of blood can donate to anyone else, regardless of blood type. Type O negative blood is theoretically capable of being transfused to any person.
Humans have four blood types: A, B, AB and O. The surface of red blood cells contain substances called antigens. Type A blood has A antigens, and type B blood has B antigens. Type AB has both types of antigens, but type O has neither antigen.
The plasma, or liquid part of blood, contains antibodies, substances that attack anything that has a specific antigen on it. Type A blood contains B antibodies, type B blood has A antibodies, type AB has neither antibody, and type O has both antibodies.
In addition, these blood types are designated negative or positive, according to whether or not an antigen called the Rh factor is present. In actuality, humans then have eight different blood types when the Rh factor is included. Since type O negative has neither antigen, no type of blood attacks it, so it can be transfused to almost anyone.