Page 278 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 34 Laboratory Diagnosis
Complement Fixation
size and morphology.
If the antigen (the unknown virus in the culture fluid) and
the known antibody are homologous, complement will be
SEROLOGIC PROCEDURES
fixed (bound) to the antigen–antibody complex. This
makes it unavailable to lyse the “indicator” system, which is
1
composed of sensitized red blood cells.
diagnose current infection. Seroconversion is the term used
to describe the finding of antibody to a virus (or any
Hemagglutination Inhibition A rise in the titer of antibody to the virus can be used to
microbe) in a patient’s serum when the patient previously
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If the virus and antibody are homologous, the virus is
had no antibody. Stated another way, the patient’s serum has
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blocked from attaching to the erythrocytes and no hemag-
converted from antibody-negative to antibody-positive.
A serum sample is obtained as soon as a viral etiology is
glutination occurs. Only viruses that agglutinate red blood
cells can be identified by this method.
10 to 14 days later (convalescent-phase). If the antibody
Neutralization
titer in the convalescent-phase serum sample is at least
fourfold higher than the titer in the acute-phase serum
If the virus and antibody are homologous, the antibody
sample, the patient is considered to be infected. For exam-
bound to the surface of the virus blocks its entry into the cell.
ple, if the titer in the acute-phase serum sample is 1/4 and
This neutralizes viral infectivity because it prevents viral rep-
the titer in the convalescent-phase serum sample is 1/16 or
lication and subsequent CPE formation or animal infection.
greater, the patient has had a significant rise in antibody
titer and has been recently infected. If, however, the titer in
Fluorescent Antibody Assay
the convalescent-phase serum sample is 1/8, this is not a
If the virus-infected cells and the fluorescein-tagged anti-
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recent infection.
body are homologous, the typical apple-green color of fluo-
rescein is seen in the cells by ultraviolet (UV) microscopy.
It is important to realize that an antibody titer on a sin-
gle sample does not distinguish between a previous infec-
tion and a current one. The antibody titer can be determined
Radioimmunoassay
by many of the immunologic tests mentioned previously.
If the virus and the antibody are homologous, there is less
These serologic diagnoses are usually made retrospectively
antibody remaining to bind to the known radiolabeled virus.
because the disease has frequently run its course by the
time the results are obtained.
Enzyme-Linked Immunosorbent Assay
In certain viral diseases, the presence of IgM antibody is
In the ELISA test to identify a virus, known antibody is
ence of IgM antibody to core antigen indicates infection by
bound to a surface. If the virus is present in the patient’s
hepatitis B virus.
specimen, it will bind to the antibody. A sample of the anti-
Other nonspecific serologic tests are available. For
body linked to an enzyme is added, which will attach to the used to diagnose current infection. For example, the pres-
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example, the heterophil antibody test (Monospot) can be
bound virus. The substrate of the enzyme is added, and the
amount of the bound enzyme is determined.
used to diagnose infectious mononucleosis. In the hetero-
phile test, human serum is reacted with horse or sheep red
Immunoelectron Microscopy
patient has been infected with Epstein–Barr virus), then
If the antibody is homologous to the virus, aggregates of
agglutination of the red cells occurs. (See Chapter 37 for
virus–antibody complexes are seen in the electron
more information.)
microscope.
DETECTION OF VIRAL ANTIGENS
MICROSCOPIC IDENTIFICATION
Viruses can be detected and identified by direct micro-
fluids by various tests, but most often by an ELISA. Tests
scopic examination of clinical specimens such as biopsy Viral antigens can be detected in the patient’s blood or body
for the p24 antigen of human immunodeficiency virus
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material or skin lesions. Three different procedures can be
(HIV) and the surface antigen of hepatitis B virus are com-
used. (1) Light microscopy can reveal characteristic inclu-
mon examples of this approach.
sion bodies or multinucleated giant cells. The Tzanck
smear, which shows herpesvirus-induced multinucleated
Titer is a measure of the concentration of antibodies in the patient’s
giant cells in vesicular skin lesions, is a good example.
(2) UV microscopy is used for fluorescent antibody stain-
reaction in the test. See Chapter 64 for a discussion of titer and various
ing of the virus in infected cells. (3) Electron microscopy
serologic tests.
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