Reactions of antigens and antibodies are highly specific. An antibody will react only with the antigen that induced it or with a closely related antigen. Because of the great specificity, reactions between antigens and antibodies are suitable for identifying one by using the other. This is the basis of serologic reactions. However, cross-reactions between related antigens can occur, and these can limit the usefulness of the test.
The results of many immunologic tests are expressed as a titer, which is defined as the highest dilution of the specimen (e.g., serum) that gives a positive reaction in the test. Note that a patient’s serum with an antibody titer of, for example, 1/64 contains more antibodies (i.e., is a higher titer) than a serum with a titer of, for example, 1/4.
Table 64–1 describes the medical importance of serologic (antibody-based) tests. Their major uses are in the diagnosis of infectious diseases, in the diagnosis of autoimmune diseases, and in the typing of blood and tissues prior to transplantation.
Table 64–1Major Uses of Serologic (Antibody-Based) Tests |Favorite Table|Download (.pdf) Table 64–1 Major Uses of Serologic (Antibody-Based) Tests
Diagnosis of infectious diseases
When the organism cannot be cultured (e.g., syphilis and hepatitis A, B, and C).
When the organism is too dangerous to culture (e.g., rickettsial diseases).
When culture techniques are not readily available (e.g., HIV, EBV).
When the organism takes too long to grow (e.g., Mycoplasma).
One problem with this approach is that it takes time for antibodies to form (e.g., 7–10 days in the primary response). For this reason, acute and convalescent serum samples are taken, and a fourfold or greater rise in antibody titer is required to make a diagnosis. By this time, the patient has often recovered and the diagnosis becomes a retrospective one. If a test is available that can detect IgM antibody in the patient’s serum, it can be used to make a diagnosis of current infection. In certain infectious diseases, an arbitrary IgG antibody titer of sufficient magnitude is used to make a diagnosis.
Diagnosis of autoimmune diseases
Determination of blood type and HLA type
Known antibodies are used to determine ABO and Rh blood types.
Known antibodies are used to determine class I and class II HLA proteins prior to transplantation, although DNA sequencing is also being used.
Microorganisms and other cells possess a variety of antigens and thus induce antisera containing many different antibodies (i.e., the antisera are polyclonal). Monoclonal antibodies excel in the identification of antigens because cross-reacting antibodies are absent (i.e., monoclonal antibodies are highly specific).
TYPES OF DIAGNOSTIC TESTS