Humans have four different blood types: A, B, AB and O. Each of these blood types is also labeled positive or negative, depending on whether the red blood cells carry the Rh factor on their surfaces. Rh negative blood may attack Rh positive blood, should they meet.
Blood types are inherited characteristics and are determined by the genes carried on chromosome 9 of humans. The red blood cells in blood carry identifying factors called antigens on their surfaces. People with blood type A carry A antigens on their red blood cells, type B carries B antigens, type AB carries both antigens and type O carries neither. Blood plasma, the watery part of blood, contains antibodies that attack specific blood cell antigens. Type A blood carries anti-B antibodies, B blood carries anti-A, AB carries neither and O carries both.
Type AB blood is labeled the universal recipient because a person with type AB blood can receive any of the other types; it has no antibodies to attack the red blood cells of donors. Type O blood is named the universal donor because it usually can be given to anyone without fear of the O blood being attacked; it contains no antigens to trigger an attack.