chapter 17 diagnosing infections
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Chapter 17
Diagnosing Infections
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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17.1 Survey of Microbial Disease
Methods of identifying unknown microbes fall into three categories:
1. Phenotypic – observable microscopic and macroscopic characteristics
2. Genotypic – genetic make up
3. Immunological – serology; antibody-antigen reactions
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Figure 17.1 Sampling sites
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Figure 17.2 Scheme of specimen isolation and identification
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Figure 17.5
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17.5 Immunological Methods
• Serology – in vitro diagnostic testing of serum– Antibodies have extreme specificity for
antigens
• Visible reactions include precipitates, color changes, or the release of radioactivity
• Tests can be used to identify and to determine the amount of antibody in serum – titer
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Figure 17.7 Basic principles of serological testing using antibodies and antigens
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Agglutination and Precipitation Reactions
• Agglutination testing – antibody cross links whole-cell antigens, forming complexes that settle out and form visible clumps– Blood typing, some bacterial and viral diseases
• Precipitation tests – soluble antigen is made insoluble by an antibody – VDRL– Most precipitation reactions are carried out in agar
gels
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Figure 17.8 Specificity and sensitivity in immune testing
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Insert figure 17.10Cellular\molecular view
Figure 17.9 Agglutination and precipitation reactions
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The Western Blot for Detecting Proteins
• Electrophoretic separation of proteins, followed by an immunoassay to detect these proteins
• Highly specific and sensitive• Sites of specific antibody binding will appear as
a pattern of bands • Second test used to verify HIV status
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Figure 17.12 The Western blot procedure
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Complement Fixation• Lysin mediated hemolysis• Test uses four components
– Antigen, antibody, complement, and sensitized sheep RBCs
• Steps of test1. Test antigen reacts with test antibody
2. Contents of tube from (1.) are mixed with sheep RBCs• Complement used up in first stage, no hemolysis
• Unfixed complement, hemolysis
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Figure 17.13 Complement fixation test
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Immunoassays • Extremely sensitive to detect trace antigens and
antibodies• Radioimmunoassay (RIA) – antigens or
antibodies labeled with radioactive isotopes• Enzyme-linked immunosorbent assay
(ELISA) – enzyme-antibody complex produces a colored product when an enzyme-substrate reaction occurs– Indirect– Capture
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Figure 17.15 Methods of ELISA testing
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In vivo Testing
• Antigens are introduced directly into the body to determine the presence or absence of antibodies– Tuberculin skin test, allergy testing
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Figure 17.17 Diagnosing viral infections
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