It is important to note that the Blood Bank of Alaska never allows blood to expire. Systems are also in place to provide or receive assistance in the event of an emergency in another part of the country or the world. The average man has 12 pints of blood, the average woman has 9. Blood groups are distinctive molecules, called antigens, on the surface of red blood cells. A person's blood group antigens are determined by the genes that person has inherited.
Blood group antigens react with proteins called antibodies.
A person may develop antibodies against blood group antigens after contact with red blood cells from another person. This contact may occur as a result of transfusion or, for women, by pregnancy, when the mother may be exposed to small numbers of fetal red blood cells. This system is critical because people have strong antibodies in their plasma against ABO antigens. These antibodies occur naturally in everyone, even without contact with another person's red blood cells. The Rh factor is the principal antigen of the Rh blood group system.