Page 276 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com • Increased corticosteroid levels suppress various host 265 mebooksfree.com
CHAPTER 33 Host Defenses
PEARLS

Interferons
defenses and predispose to severe viral infections, especially
• Viruses and double-stranded RNA are the most potent
disseminated herpesvirus infections. Malnutrition predis-
inducers of interferons. Many viruses induce interferons, and
poses to severe measles infections in developing countries.
many viruses are inhibited by interferons (i.e., neither the
The very young and the very old have more severe viral
induction of interferons nor its action is specific).
infections.
• Interferons act by binding to a receptor on the cell surface that Specific Defenses
mebooksfree.com mebooksfree.com mebooksfree.com • Active immunity to viral infection is mediated by both mebooksfree.com
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signals the cell to synthesize ribonuclease, protein kinase, and
oligo A synthetase in an inactive form. Double-stranded RNA
made by the infecting virus activates these proteins. Interferons
do not enter the cell and have no effect on extracellular viruses.
antibodies and cytotoxic T cells. It can be elicited either by
• Interferons inhibit virus replication by blocking protein synthe-
exposure to the virus or by immunization with a viral
sis, primarily by degrading mRNA and by inactivating elon-
vaccine.
gation factor-2.
• Passive immunity consists of antibodies preformed in
• Alpha and beta interferons have a stronger antiviral action
another person or animal.
than gamma interferon. The latter acts primarily as an interleu-
• The duration of active immunity is much longer than
kin that activates macrophages.
that of passive immunity. Active immunity is measured in
years, whereas passive immunity lasts a few weeks to a
Other Nonspecific Defenses
few months.
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mebooksfree.com mebooksfree.com mebooksfree.com • Passive immunity is effective immediately, whereas it takes mebooksfree.com
• Natural killer (NK) cells are lymphocytes that destroy cells
active immunity 7 to 10 days in the primary response (or
infected by many different viruses (i.e., they are nonspe-
3–5 days in the secondary response) to stimulate detectable
cific). NK cells do not have an antigen receptor on their surface,
unlike T and B lymphocytes. Rather, NK cells recognize and
amounts of antibody.
destroy cells that do not display class I MHC proteins on the
• Herd immunity is the protection of an individual that results
surface. They kill cells by the same mechanisms as do cytotoxic
from immunity in many other members of the population (the
T cells (i.e., by secreting perforins and granzymes).
herd) that interrupts transmission of the virus to the individual.
Herd immunity can be achieved either by immunization or by
• Phagocytosis by macrophages and the clearance of mucus by
the cilia of the respiratory tract are also important defenses.
natural infection of a sufficiently high percentage of the
Damage to these defenses predisposes to viral infection.
population.
mebooksfree.com mebooksfree.com mebooksfree.com ANSWERS mebooksfree.com mebooksfree.com
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SELF-ASSESSMENT QUESTIONS
(D) IgE can prevent viral infection by activating complement,
which leads to the production of the membrane attack complex.
1. Regarding the mode of action of interferon, which one of the fol-
(E) Interleukin-2 is important in protecting uninfected cells
lowing is the most accurate?
from viral infection by inhibiting the release of virus from
infected cells.
(A) It acts by inhibiting the virion protease.
(B) It acts by inhibiting the virion polymerase.
(C) It acts by inducing a ribonuclease that degrades viral mRNA.
(D) It acts by binding to the extracellular virion, thereby preventing
entry into the cell.
1. (C)
(E) It acts against viruses with a DNA genome but not against
viruses that have RNA as their genome.
2. Regarding immunologic aspects of viral diseases, which one of the
PRACTICE QUESTIONS: USMLE &
following is most accurate? 2. (C)
COURSE EXAMINATIONS
(A) Antibodies protect against some viral diseases by inhibiting the
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mebooksfree.com mebooksfree.com mebooksfree.com in the Basic Virology section of Part XIII: USMLE (National mebooksfree.com
synthesis of mRNA by the RNA polymerase in the virion.
Questions on the topics discussed in this chapter can be found
(B) IgG plays a major role in neutralizing virus infectivity during
the primary infection.
Board) Practice Questions starting on page 720. Also see Part
(C) IgA exerts an antiviral effect by preventing virus from
XIV: USMLE (National Board) Practice Examination starting
infecting the mucosal cells of the respiratory and gastroin-
on page 751.
testinal tracts.
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