THE LIVING WORLD
Unit Six. Animal Life
27. How the Animal Body Defends Itself
A person’s blood type indicates the class of antigens found on the red blood cell surface. There are several groups of red blood cell antigens, but the major group is known as the ABO system. In terms of the antigens present on the red blood cell surface, a person may be type A (with only A antigens), type B (with only B antigens), type AB (with both A and B antigens), or type O (with neither A nor B antigens).
The immune system is tolerant to its own red blood cell antigens. A person who is type A, for example, does not produce anti-A antibodies. However, people with type A blood do make antibodies against the B antigen, and people with blood type B make antibodies against the A antigen. People who are type AB develop tolerance to both antigens and thus do not produce either anti-A or anti-B antibodies. Those who are type O make both anti-A and anti-B antibodies.
If type A blood is mixed on a glass slide with serum from a person with type B blood, the anti-A antibodies in the serum cause the type A blood cells to clump together, or agglutinate (this is shown in the upper right panel of figure 27.17). These tests allow the blood types to be matched prior to transfusions, so that agglutination will not occur in the blood vessels, where it could lead to inflammation and organ damage.
Figure 27.17. Blood typing.
Agglutination of the red blood cells is seen when blood types are mixed with sera containing antibodies against the A and B antigens. Note that no agglutination would be seen if type O blood (not shown) was used.
Rh Factor. Another group of antigens found in most red blood cells is the Rh factor (Rh stands for rhesus monkey, in which these antigens were first discovered). People who have these antigens are said to be Rh-positive, whereas those who do not are Rh-negative. There are fewer Rhnegative people because the Rh-positive allele is clinically dominant to the Rh-negative allele and is more common in the human population. The Rh factor is of particular significance when Rh-negative mothers give birth to Rhpositive babies.
Because the fetal and maternal blood are normally kept separate across the placenta (see chapter 31), the Rh-negative mother is not usually exposed to the Rh antigen of the fetus during pregnancy. At the time of birth, however, a varying degree of exposure may occur, and the Rh-negative mother’s immune system may become sensitized and produce antibodies against the Rh antigen. If the woman does produce antibodies against the Rh factor, these antibodies can cross the placenta in subsequent pregnancies and cause hemolysis of the Rh-positive red blood cells of the fetus. The baby is therefore born anemic with a condition called erythroblastosis fetalis, or hemolytic disease of the newborn.
Erythroblastosis fetalis can be prevented by injecting the Rh-negative mother with an antibody preparation against the Rh factor within 72 hours after the birth of each Rh-positive baby. The injected antibodies inactivate the Rh antigens and thus prevent the mother from becoming actively immunized to them.
Monoclonal antibodies are antibodies that are specific to one antigen. Because they provide a very sensitive assay, mono- clonals are often commercially prepared for use in clinical laboratory tests. Modern pregnancy tests, for example, use particles that are covered with monoclonal antibodies produced against a pregnancy hormone (abbreviated hCG—see chapter 31) as the antigen. In the blood pregnancy test, these particles are mixed with a sample from a pregnant woman. If the sample contains a significant level of the hCG hormone, it reacts with the antibody and causes a visible agglutination of the particles, indicating a positive test result. Over-the-counter pregnancy tests work in a similar way. hCG in a pregnant woman’s urine binds to the monoclonal antibodies within the testing strip and indicates a positive result.
Key Learning Outcome 27.9. Agglutination occurs because different antibodies exist for the ABO and Rh factor antigens on the surface of red blood cells. Monoclonal antibodies are commercially produced antibodies that react against one specific antigen.