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Reactions of antigens and antibodies are highly specific. Because of the high specificity, reactions between antigens and antibodies are suitable for identifying one by using the other. This is the basis of serologic tests. 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

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.

Cross-reaction can occasionally occur between related antigens and antibodies, limiting the usefulness of certain tests. Monoclonal antibodies excel in the identification of antigens because they consist of a single antigen-binding specificity (i.e., they lack cross-reacting antibodies found in polyclonal antibody preparations). Chapter 61 discusses the generation of specific antibodies, including monoclonal antibodies used for diagnostic purposes.


Many types of diagnostic tests are performed in ...

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