Ch. 18 - Practical Applications of Immunology

Tortora - Microbiology: An Introduction 14th Edition

Tortora14th EditionMicrobiology: An IntroductionISBN: 9780138200398Not the one you use?Change textbook

Tortora 14th Edition

Ch. 18 - Practical Applications of Immunology

Problem 3

Chapter 18, Problem 3

Label the components of the direct and indirect FA tests in the following situations. Which test is direct? Which test provides definitive proof of disease?

Verified step by step guidance

Step 1: Understand the principle of the Direct and Indirect Fluorescent Antibody (FA) tests. The Direct FA test detects the presence of antigen in a sample by using a fluorescently labeled antibody that binds directly to the antigen. The Indirect FA test detects antibodies in the patient's serum by using an unlabeled primary antibody and a fluorescently labeled secondary antibody that binds to the primary antibody.

Step 2: For the Direct FA test, label the components as follows: the fluorescently labeled antibody (primary antibody) binds directly to the antigen present in the sample. There is no secondary antibody involved in this test.

Step 3: For the Indirect FA test, label the components as follows: the primary antibody (unlabeled) binds to the antigen, and then a fluorescently labeled secondary antibody binds to the primary antibody. This amplifies the signal and allows detection of antibodies rather than antigens.

Step 4: Identify which test is direct and which is indirect based on the labeling: the test where the fluorescent label is on the primary antibody is the Direct FA test, and the test where the fluorescent label is on the secondary antibody is the Indirect FA test.

Step 5: Determine which test provides definitive proof of disease. The Direct FA test detects the actual antigen (pathogen) in the sample, providing direct evidence of infection, whereas the Indirect FA test detects antibodies, which indicate exposure but not necessarily active disease.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Direct Fluorescent Antibody (FA) Test

The direct FA test uses fluorescently labeled antibodies that bind directly to specific antigens on a pathogen in a sample. This test detects the presence of the microorganism itself, providing rapid identification. It is called 'direct' because the antibody is directly conjugated to the fluorescent dye.

Indirect Fluorescent Antibody (FA) Test

The indirect FA test involves two steps: first, an unlabeled primary antibody binds to the antigen, then a fluorescently labeled secondary antibody binds to the primary antibody. This method amplifies the signal and is often used to detect antibodies in patient serum, indicating exposure rather than direct presence of the pathogen.

Definitive Proof of Disease in FA Tests

Definitive proof of disease typically comes from detecting the pathogen itself, which is achieved by the direct FA test. While the indirect test shows immune response (antibodies), it does not confirm active infection. Therefore, the direct FA test provides conclusive evidence of disease by visualizing the actual microorganism.

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Related Practice

Textbook Question

Use the following choices to answer questions 1 and 2:

a. Hemolysis

b. Hemagglutination

c. Hemagglutination inhibition

d. No hemolysis

e. Precipitin ring forms

Patient’s serum, influenza virus, sheep red blood cells, and anti-sheep red blood cells are mixed in a tube. What happens if the patient has antibodies against influenza?

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Textbook Question

The examples in questions 1 and 2 are:

a. Direct tests

b. Indirect tests

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Textbook Question

How are monoclonal antibodies produced?

Textbook Question

Use the following choices to answer questions 1 and 2:

a. Hemolysis

b. Hemagglutination

c. Hemagglutination inhibition

d. No hemolysis

e. Precipitin ring forms

Patient’s serum, Chlamydia, guinea pig complement, sheep red blood cells, and anti-sheep red blood cells are mixed in a tube. What happens if the patient has antibodies against Chlamydia?

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Textbook Question

Use the following choices to answer questions 4 and 5:

a. Anti-Brucella

b. Brucella

c. Substrate for the enzyme

Which is the third step in a direct ELISA test?

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Textbook Question

Define the following terms, and give an example of how each reaction is used diagnostically:

a. Viral hemagglutination

b. Hemagglutination inhibition

c. Passive agglutination

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