Page 68 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 8 Host Defenses
(3) Splenectomy removes an important source of both
on whether it is a primary or secondary response. However,
phagocytes and immunoglobulins, which predisposes to
passive immunity has the important disadvantage that the
sepsis caused by three encapsulated pyogenic bacteria:
S. pneumoniae, Neisseria meningitidis, and Haemophilus
antibody concentration decreases fairly rapidly as the pro-
teins are degraded, and so the protection usually lasts for
influenzae. S. pneumoniae causes approximately 50% of
episodes of sepsis in splenectomized patients. Patients with
sickle cell anemia and other hereditary anemias can autoin-
antibodies can be lifesaving in certain diseases that are
caused by powerful exotoxins, such as botulism and teta-
farct their spleen, resulting in a loss of splenic function and
a predisposition to sepsis caused by these bacteria. only a month or two. The administration of preformed
nus. Serum globulins, given intravenously, are a prophylac-
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(4) People who have diabetes mellitus, especially those
tic measure in patients with hypogammaglobulinemia or
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who have poor glucose control or episodes of ketoacidosis,
bone marrow transplants. In addition, they can mitigate the
symptoms of certain diseases such as hepatitis caused by
have an increased number of infections and more severe
infections compared with people who do not have diabetes.
The main host defense defect in these patients is reduced
bacterial diseases with an invasive form of pathogenesis.
Active adaptive immunity is protection based on expo-
neutrophil function, especially when acidosis occurs.
sure to the organism in the form of overt disease, subclini-
Two specific diseases highly associated with diabetes are
cal infection (i.e., an infection without symptoms), or a
malignant otitis externa caused by Pseudomonas aerugi-
vaccine. This protection has a slower onset but longer dura-
nosa and mucormycosis caused by molds belonging to the
tion than passive immunity. The primary response usually
genera Mucor and Rhizopus. In addition, there is an increased
incidence and increased severity of community-acquired
An important advantage of active immunity is that an
pneumonia caused by bacteria such as S. pneumoniae and
anamnestic (secondary) response occurs (i.e., there is a
S. aureus and of urinary tract infections caused by organ- takes 7 to 10 days for the antibody to become detectable.
rapid response [approximately 3 days] of large amounts of
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isms such as Escherichia coli and C. albicans. Candidal
antibody to an antigen that the immune system has previ-
vulvovaginitis is also more common in diabetic patients.
ously encountered). Active immunity is mediated by both
Diabetic patients also have many foot infections because
atherosclerosis compromises the blood supply and necrosis
(1) Antibodies protect against organisms by a variety of
of tissue occurs. Skin infections, such as ulcers and celluli-
mechanisms—neutralization of toxins, lysis of bacteria in
tis, and soft tissue infections, such as necrotizing fasciitis,
the presence of complement, opsonization of bacteria to
are common and can extend to the underlying bone, caus-
facilitate phagocytosis, and interference with adherence of
ing osteomyelitis. S. aureus and mixed facultative anaerobic
bacteria and viruses to cell surfaces. If the level of IgG
bacteria are the most common causes.
drops below 400 mg/dL (normal = 1000–1500 mg/dL), the
Fever
staphylococci increases.
Infection causes a rise in the body temperature that is
Because antibodies, especially IgG, are detectable for
attributed to endogenous pyrogen (IL-1) released from risk of pyogenic infections caused by bacteria such as
days to weeks after infection, they are thought not to play
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macrophages. Fever may be a protective response because a
a major role in combating the primary infection at the
variety of bacteria and viruses grow more slowly at elevated
initial site of infection (usually the skin or mucous mem-
temperatures.
semination of the organism to distant sites in the body
and against a second infection by that organism at some
ADAPTIVE (SPECIFIC) IMMUNITY
future time.
(2) T cells mediate a variety of reactions, including
Adaptive immunity results either from exposure to the
cytotoxic destruction of virus-infected cells and bacteria,
organism (active immunity) or from receipt of preformed
activation of macrophages, and delayed hypersensitivity.
antibody made in another host (passive immunity).
Passive adaptive immunity is a temporary protection
phages, are the main host defense against mycobacteria
against an organism and is acquired by receiving serum
such as M. tuberculosis and systemic fungi such as Histoplasma
containing preformed antibodies from another person or T cells, especially Th-1 cells (see Chapter 58) and macro-
and Coccidioides. T cells also help B cells to produce anti-
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animal. Passive immunization occurs normally in the form
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body against many, but not all, antigens.
of immunoglobulins passed through the placenta (IgG) or
breast milk (IgA) from mother to child. This protection is
Table 8–3 describes the essential host defense mecha-
very important during the early days of life when the child
has a reduced capacity to mount an active response.
humoral immunity against pyogenic bacteria and exotoxins
Passive immunity has the important advantage that its
and cell-mediated immunity against several intracellular
protective abilities are present immediately, whereas active
bacteria.
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