Page 273 - Review of Medical Microbiology and Immunology ( PDFDrive )

mebooksfree.com
mebooksfree.com

mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com 6. Fever mebooksfree.com mebooksfree.com
mebooksfree.com
PART III Basic Virology
262
antibody begins to appear in the blood several days after
infection.
Elevated body temperature may play a role in host
Alpha interferon has been approved for use in patients
defenses, but its importance is uncertain. Fever may act
with condyloma acuminatum and chronic active hepatitis
in two ways: (1) The higher body temperature may
caused by hepatitis B and C viruses. Beta interferon is used
directly inactivate the virus particles, particularly envel-
in the treatment of multiple sclerosis. Gamma interferon
reduces recurrent infections in patients with chronic gran-
veloped viruses; and (2) Replication of some viruses is
ulomatous disease (see Chapter 68). Interferons are also
reduced at higher temperatures; therefore, fever may
used clinically in patients with cancers such as Kaposi’s oped viruses, which are more heat-sensitive than nonen-
inhibit replication.
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com The mucociliary clearance mechanism of the respiratory mebooksfree.com
sarcoma and hairy cell leukemia.
7. Mucociliary Clearance
2. Natural Killer Cells
Natural killer (NK) cells are an important part of the innate
tract may protect the host. Its damage (e.g., from smoking)
defenses against virus-infected cells. They are called “natu-
results in an increased frequency of viral respiratory tract
ral” killer cells because they are active without the necessity
infections, especially influenza.
of being exposed to the virus previously and because they
are not specific for any virus. NK cells are a type of T lym-
8. Circumcision
phocyte but do not have an antigen receptor. They recog-
nize virus-infected cells by the absence of class I MHC
(major histocompatibility complex) proteins on the surface
by three sexually transmitted viruses: HIV, human papil-
of the virus-infected cell. They kill virus-infected cells by There is evidence that circumcision prevents infection
lomavirus (HPV), and herpes simplex virus type 2
secreting perforins and granzymes, which cause apoptosis
(HSV-2).
mebooksfree.com mebooksfree.com mebooksfree.com Several factors influence host defenses in a nonspecific or mebooksfree.com
mebooksfree.com
mebooksfree.com
of the infected cells. (See page 516 for more information.)
9. Factors That Modify Host Defenses
3. Phagocytosis
Macrophages, particularly fixed macrophages of the reticu-
multifactorial way:
loendothelial system and alveolar macrophages, are the
(1) Age is a significant variable in the outcome of viral
important cell types in limiting virus infection. In contrast,
infections. In general, infections are more severe in neo-
polymorphonuclear leukocytes are the predominant cellu-
nates and in the elderly than in older children and young
lar defense in bacterial infections.
older people than in younger adults, and herpes simplex
4. α-Defensins
virus infections are more severe in neonates than in
adults.
α-Defensins are a family of positively charged peptides with adults. For example, influenza is typically more severe in
mebooksfree.com mebooksfree.com mebooksfree.com exacerbate eye damage. It is not clear how these effects are mebooksfree.com
(2) Increased corticosteroid levels predispose to more
mebooksfree.com
mebooksfree.com
antiviral activity. (They also have antibacterial activity; see
severe infections with some viruses, such as varicella-zoster
Chapter 8.) They interfere with human immunodeficiency
virus; the use of topical cortisone in herpetic keratitis can
virus (HIV) binding to the CXCR4 receptor and block
entry of the virus into the cell. The production of α-defensins
mediated, because corticosteroids can cause a variety of
may explain why some HIV-infected individuals are long-
pertinent effects, namely, lysis of lymphocytes, decreased
term “nonprogressors.”
recruitment of monocytes, inhibition of interferon produc-
tion, and stabilization of lysosomes.
5. Apolipoprotein B RNA-Editing
(3) Malnutrition leads to more severe viral infections
Enzyme (APOBEC3G)
(e.g., there is a much higher death rate from measles in
APOBEC3G is an important member of the innate host
tion causes decreased immunoglobulin production and
defenses against retroviral infection, especially against HIV.
phagocyte activity as well as reduced skin and mucous
APOBEC3G is an enzyme that causes hypermutation in developing countries than in developed ones). Poor nutri-
membrane integrity.
mebooksfree.com mebooksfree.com mebooksfree.com SPECIFIC DEFENSES mebooksfree.com mebooksfree.com
mebooksfree.com
retroviral DNA by deaminating cytosines in both mRNA
and retroviral DNA, thereby inactivating these molecules
and reducing infectivity. HIV defends itself against this
innate host defense by producing Vif (viral infectivity pro-
tein), which counteracts APOBEC3G, thereby preventing
There is evidence for natural resistance to some viruses in
certain species, which is probably based on the absence of
hypermutation from occurring.
mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com