Page 557 - Review of Medical Microbiology and Immunology ( PDFDrive )

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64
C
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Antigen–Antibody Reactions

in the Laboratory

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CHAPTER C ONTENT S

Introduction
Antiglobulin (Coombs) Test
Types Of Diagnostic Tests
Western Blot (Immunoblot)
Agglutination
Precipitation (Precipitin)
Fluorescence-Activated Cell Sorting (Flow Cytometry)
Radioimmunoassay (RIA)
Antigens
Enzyme-Linked Immunosorbent Assay (ELISA)
The ABO Blood Groups & Transfusion Reactions
Immunofluorescence (Fluorescent Antibody) Antigen–Antibody Reactions Involving Red Blood Cell
Rh Blood Type & Hemolytic Disease of the Newborn
Complement Fixation
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Self-Assessment Questions
Neutralization Tests
Practice Questions: USMLE & Course Examinations
Immune Complexes
INTRODUCTION
The results of many immunologic tests are expressed as
a titer, which is defined as the highest dilution of the speci-
Reactions of antigens and antibodies are highly specific. An
men (e.g., serum) that gives a positive reaction in the test.
antibody will react only with the antigen that induced it or
with a closely related antigen. Because of the great specificity,
example, 1/64 contains more antibodies (i.e., is a higher
reactions between antigens and antibodies are suitable for
identifying one by using the other. This is the basis of serologic Note that a patient’s serum with an antibody titer of, for
Table 64–1 describes the medical importance of sero-
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reactions. However, cross-reactions between related antigens
logic (antibody-based) tests. Their major uses are in the
can occur, and these can limit the usefulness of the test.
diagnosis of infectious diseases, in the diagnosis of
TABLE 64–1 Major Uses of Serologic (Antibody-Based) Tests
I. 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 con-
valescent 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.
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II. 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).
III. 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.

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