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Chapter 64: Antigen–Antibody Reactions in the Laboratory

INTRODUCTION

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
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TABLE 64–1 Major Uses of Serologic (Antibody-Based) Tests

  1. 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.

  2. Diagnosis of autoimmune diseases

    • Antibodies against various normal body components are used (e.g., antibody against DNA in systemic lupus erythematosus, antibody against human IgG [rheumatoid factor] in rheumatoid arthritis).

  3. 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.

EBV = Epstein-Barr virus; HIV = human immunodeficiency virus; HLA = human leukocyte antigen.

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.

TYPES OF DIAGNOSTIC TESTS

Many types of diagnostic tests are performed in ...

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