Page 275 - Review of Medical Microbiology and Immunology ( PDFDrive )

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264
PART III Basic Virology
releasing proteolytic enzymes called granzymes into the
infected cell, which degrade the cell contents; and (3) by
activating the FAS protein, which causes programmed cell
death (apoptosis).
Not all virus infections induce antibodies. Tolerance
to viral antigens can occur when the virus infection
develops in a fetus or newborn infant. The model system
in which tolerance has been demonstrated is lympho-
cytic choriomeningitis (LCM) infection in mice. If LCM
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virus is inoculated into a newborn mouse, the virus
replicates widely, but no antibodies are formed during
the lifetime of the animal. The virus is recognized as
“self,” because it was present at the time of maturation of
the immune system. If LCM virus is given to an adult
mouse, antibodies are formed normally. There is no
example of total tolerance to a virus in humans; even in
congenital rubella syndrome, in which the virus infects
the fetus, some antibody against rubella virus is made.
However, virus production and shedding can go on for
months or years.
Suppression of the cell-mediated response can occur
during infection by certain viruses. The best-known exam- FIGURE 33–3 Herd immunity. Immunization of the nine
ple is the loss of tuberculin skin test reactivity during
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people (tan color) can protect the one unimmunized person (red
measles infection. Infection by cytomegalovirus or HIV
color) by interrupting transmission. Immunization levels of 90%
can also cause suppression. Some viruses can “downregu-
late” (reduce) the amount of class I and class II MHC pro-
individual.
tein made by cells, which may be a mechanism by which
these viruses suppress cell-mediated immunity.
2. Passive Immunity
virtue of the other members of the population (the herd)
Transfer of human serum containing the appropriate anti-
being incapable of transmitting the virus to that indi-
bodies provides prompt short-term immunity for indi-
immunizing a population with a vaccine that interrupts
viduals exposed to certain viruses. The term passive refers
transmission, such as the live, attenuated polio vaccine,
to the administration of preformed antibodies. Two types
but not with a vaccine that does not interrupt transmis-
of immune globulin preparations are used for this pur- vidual (Figure 33–3). Herd immunity can be achieved by
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pose. One has a high titer of antibody against a specific
sion, such as the killed polio vaccine (even though it
virus, and the other is a pooled sample from plasma
protects the immunized individual against disease). Note
donors that contains a heterogeneous mixture of antibod-
that herd immunity occurs with the live polio vaccine
ies with lower titers. The immune globulins are prepared
primarily because it induces secretory IgA in the gut,
which inhibits infection by virulent virus, thereby pre-
by alcohol fractionation, which removes any viruses in the
venting its transmission to others. In addition, the live
serum. The three most frequently used high-titer prepara-
virus in the vaccine can replicate in the immunized per-
tions are used after exposure to hepatitis B, rabies, and
son and spread to other members of the population,
varicella-zoster viruses. Low-titer immune globulin is
used mainly to prevent hepatitis A in people traveling to
thereby increasing the number of people protected. How-
areas where this infection is hyperendemic.
concerned is the induction of IgA, which prevents
Two specialized examples of passive immunity include
the transfer of IgG from mother to fetus across the placenta
and the transfer of IgA from mother to newborn in ever, the important feature as far as herd immunity is
Herd immunity can be achieved by natural infection as
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colostrum.
well as vaccines. For example, if a viral disease, such as
measles, occurred in approximately 90% of a group, and if
those who recovered from the disease had sufficient immu-
3. Herd Immunity
“Herd immunity” (also known as “community immu-
as a source of virus for others, then the remaining 10% of
the group are protected by herd immunity.
nity”) is the protection of an individual from infection by
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