If for some reason the amount of blood in the body is severely reduced, through hemorrhage (excessive bleeding) or disease, the body cells suffer from lack of oxygen and nutrients. One possible measure to take in such an emergency is to administer blood from another person into the veins of the patient, a procedure called transfusion. Care must be taken in transferring blood from one person to another, however, because the patient’s plasma may contain substances, called antibodies or agglutinins, that can cause the red cells of the donor’s blood to rupture and release their hemoglobin. Such cells are said to be hemolyzed, and the resulting condition can be dangerous. Certain proteins, called antigens or agglutinogens, on the surface of the red cells cause these incompatibility reactions. There are many types of such proteins, but only two groups are particularly likely to cause a transfusion reaction, the so-called A and B antigens and the Rh factor.
The AB0 Blood Type Group
There are four blood types involving the A and B antigens: A, B, AB, and 0. These letters indicate the type of antigen present on the red cells. If only the A antigen is present, the person has type A blood; if only the B antigen is present, he or she has type B blood. Type AB red cells have both antigens, and type 0 have neither. Of course no one has antibodies to his or her own blood type antigens, or their plasma would destroy their own cells. Each person does, however, develop antibodies that react with the AB antigens he or she is lacking. (The reason for the development of these antibodies is not totally understood, because people usually develop antibodies only when they have been exposed to an antigen.) It is these antibodies in the patient’s plasma that can react with antigens on the donor’s red cells to cause a transfusion reaction.
Testing for Blood Type Blood sera containing antibodies to the A or B antigens are used to test for blood type. These antisera are prepared in animals using either the A or the B antigens to induce a response. Blood serum containing antibodies that can agglutinate and destroy red cells with A antigen is called anti-A serum; blood serum containing antibodies that can destroy red cells with B antigen is called anti-B serum. When combined with a blood sample in the laboratory, each antiserum causes the corresponding red cells to clump together in a process known as agglutination. The blood’s agglutination pattern when mixed separately with these two sera reveals its blood type (Fig. 9-9). Type A reacts with anti-A serum only; type B reacts with anti-B serum only. Type AB agglutinates with both, and type 0 agglutinates with neither A nor B. A blood specimen from any person who has had a prior blood transfusion or a pregnancy is tested further for the presence of any less common antibodies. Both the red cells and the serum are tested separately for any possible cross-reactions with donor blood.
Figure 9-9 Blood typing. Labels at the top of each column denote the kind of antiserum added to the blood samples. Anti- A serum agglutinates (causes to clump) red cells in type A blood, but anti-B serum does not. Anti-B serum agglutinates red cells in type B blood, but anti-A serum does not. Both sera agglutinate type AB blood cells, and neither serum agglutinates type 0 blood.
Blood Compatibility Heredity determines a person’s blood type, and the percentage of people with each of the different blood types varies in different populations. For example, about 45% of the white population of the United States have type 0 blood, 40% have A, 11% have B and only 4% have AB.
The percentages vary within other population groups. In an emergency, type 0 blood can be given to any AB0 type because the cells lack both A and B antigens and will not react with either A or B antibodies. People with type 0 blood are called universal donors. Conversely, type AB blood contains no antibodies to agglutinate red cells, and people with this blood type can therefore receive blood from any AB0 type donor. Those with AB blood are described as universal recipients. Whenever possible, it is safest to give the same blood type as the recipient’s blood.
The Rh Factor
More than 85% of the United States’ population has another red cell antigen group called the Rh factor, named for Rhesus monkeys, in which it was first found. Rh is also known as the D antigen.
People with this antigen are said to be Rh positive; those who lack this protein are said to be Rh negative. If Rh-positive blood is given to an Rh-negative person, he or she may produce antibodies to the “foreign” Rh antigens. The blood of this “Rh-sensitized” person will then destroy any Rh-positive cells received in a later transfusion. Rh incompatibility is a potential problem in certain pregnancies. A mother who is Rh negative may develop antibodies to the Rh protein of an Rh-positive fetus (the fetus having inherited this factor from the father). Red cells from the fetus that enter the mother’s circulation during pregnancy and childbirth evoke the response. In a subsequent pregnancy with an Rh-positive fetus, some of the anti-Rh antibodies may pass from the mother’s blood into the blood of her fetus and destroy the fetus’s red cells. This condition is called hemolytic disease of the newborn (HDN). An older name is erythroblastosis fetalis). HDN is now prevented by administration of immune globulin Rho(D), trade name Rho-GAM, to the mother during pregnancy and shortly after delivery. These preformed antibodies clear the mother’s circulation of Rh antigens and prevent stimulation of an immune response. In many cases, a baby born with HDN could be saved by a transfusion that replaces much of the baby’s blood with Rh-negative blood.