Blood Typing and Transfusions
A blood transfusion is the transfer of blood from one individual into the blood of another. In order for transfusions to be safely done, it is necessary for blood to be typed so that agglutination (clumping of red blood cells) does not occur. Blood typing usually involves determining the ABO blood group and whether the individual is Rh- (negative) or Rh+ (positive).
AB0 Blood Groups
AB0 blood typing is based on the presence or absence of two possible antigens, called type A antigen and type B antigen, on the surface of red blood cells. Whether these antigens are present or not depends on the particular inheritance of the individual.
A person with type A antigen on the surface of the red blood cells has type A blood; one with type B blood has type B antigen on the surface of the red blood cells. What antigens would be present on the surface of red blood cells if the person has type AB blood or type O blood? Notice in Figure 11.8 that a person with type AB blood has both antigens, and a person with type O blood has no antigens on the surface of the red blood cells.
Figure 11.8 Types of blood. In the ABO system, blood type depends on the presence or absence of antigens A and B on the surface of red blood cells. In these drawings, A and B antigens are represented by different shapes on the red blood cells. The possible anti-A and anti-B antibodies in the plasma are shown for each blood type. Notice that an anti-B antibody cannot bind to an A antigen, and vice versa.
It so happens that an individual with type A blood has anti-B antibodies in the plasma; a person with type B blood has anti-A antibodies in the plasma; and a person with type O blood has both antibodies in the plasma (Fig. 11.8). These antibodies are not present at birth, but they appear over the course of several months after birth.
Blood compatibility is very important when transfusions are done. The antibodies in the plasma must not combine with the antigens on the surface of the red blood cells, or else agglutination occurs. With agglutination, anti-A antibodies have combined with type A antigens, or anti-B antibodies have combined with type B antigens, or both types of binding have occurred. Therefore, agglutination is expected if the donor has type A blood and the recipient has type B blood (Fig. 11. 9). What about other combinations of blood types? Try out all other possible donors and recipients to see if agglutination will occur. Type O blood is sometimes called the universal donor because it has no antigens on the red blood cells, and type AB blood is sometimes called the universal recipient because this blood type has no antibodies in the plasma. In practice, however, there are other possible blood groups, aside from ABO blood groups, so it is necessary to physically put the donor's blood on a slide with the recipient's blood and observe whether the blood types match (no agglutination occurs) before blood can be safely given from one person to another. The use of blood substitutes does away with the problems of matching blood types.
Rh Blood Groups
The designation of blood type usually also includes whether the person has or does not have the Rh factor on the red blood cell. Rh- individuals normally do not have antibodies to the Rh factor, but they make them when exposed to the Rh factor. If a mother is Rh- and the father is Rh+, a child can be Rh+. The Rh+ red blood cells may begin leaking across the placenta into the mother’s cardiovascular system (Fig. 11.10), as placental tissues normally break down before and at birth. The presence of these Rh+ antigens causes the mother to produce anti-Rh antibodies. In a subsequent pregnancy with another Rh+ baby, the anti-Rh antibodies may cross the placenta and destroy the child’s red blood cells. This is called hemolytic disease of the newborn (HDN) because hemolysis continues after the baby is born. Due to red blood cell destruction, excess bilirubin in the blood can lead to brain damage and mental retardation or even death. The Rh problem is prevented by giving Rh- women an Rh immunoglobulin injection no later than 72 hours after giving birth to an Rh+ child. This injection contains anti-Rh antibodies that attack any of the baby’s red blood cells in the mother’s blood before these cells can stimulate her immune system to produce her own antibodies. This injection is not beneficial if the woman has already begun to produce antibodies; therefore, the timing of the injection is most important.
Figure 11.9 Blood transfusions. No agglutination (a) versus agglutination (b) is determined by whether antibodies are present that can combine with antigens.
Figure 11.10 Hemolytic disease of the newborn. Due to a pregnancy in which the child is Rh positive, an Rh-negative mother can begin to produce antibodies against Rh-positive red blood cells. In another pregnancy, these antibodies can cross the placenta and cause hemolysis of an Rh-positive child’s red blood cells.