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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 (or, in other words, the smallest concentration) of the specimen (e.g., serum) that still 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

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 ...

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