When and Why is Blood Typing Done?
Fans of the popular television show ER know how important blood type is in an emergency. 'Start the O-neg,' shouts Doctor Green, and the team swings into action. Green calls for type O, Rh-negative blood in life-and-death situations when there's no time for blood typing. Since that type contains no A, B, or Rh antigens, it can - in theory - be safely given to all.
Blood carries antibodies against antigens that are not its own. Antibodies are defenses of the immune system. They destroy proteins the body recognizes as foreign. For example, if you have type A blood, you carry no antibodies against the A antigen, but your blood makes antibodies against the B antigen. Receive a type B or type AB transfusion and your own blood will attack the B antigens in the donation. The locking of antibody to antigen causes red blood cells to rupture and clump. Circulating clumps of red blood cells are a life-threatening emergency in themselves.
People with type AB+ blood carry all three antigens and none of the antibodies. They can receive blood from any other type but can donate only to their own type. People with type O- blood make no A, B, or Rh antigens, so they can give blood to all other types without fear of dangerous clumping. (The number of anti-A, anti-B, and anti-Rh antibodies in their blood is small and will do little harm in the recipient.) In practice, O-negative is given far less often than ER would have you believe. For one thing, blood typing can be done in minutes. For another, transfusing blood matched to the patient's own type is far safer than giving O-negative to everyone.
