Chapter 21
The Immune System: Innate and Adaptive Body Defenses
771
21
disorders including hepatitis C, genital warts, multiple sclerosis,
and hairy cell leukemia.
Complement
Te term
complement system
, or simply
complement
, refers
to a group of at least 20 plasma proteins that normally circu-
late in the blood in an inactive state. Tese proteins include C1
through C9, factors B, D, and P, plus several regulatory proteins.
Complement provides a major mechanism for destroying
foreign substances in the body. Its activation unleashes inflam-
matory chemicals that amplify virtually all aspects of the in-
flammatory process. Activated complement also lyses and kills
certain bacteria and other cell types. (Luckily our own cells are
equipped with proteins that normally inhibit complement acti-
vation.) Although complement is a nonspecific defensive mech-
anism, it “complements” (enhances) the effectiveness of
both
innate and adaptive defenses.
so that tissue can be repaired. Once this is accomplished, heal-
ing usually occurs quickly.
Homeostatic Imbalance
21.1
In severely infected areas, the battle takes a considerable toll on
both sides, and creamy yellow
pus
(a mixture of dead or dying
neutrophils, broken-down tissue cells, and living and dead patho-
gens) may accumulate in the wound. If the inflammatory mecha-
nism fails to clear the area of debris, collagen fibers may be laid
down, which walls off the sac of pus, forming an
abscess
. Te ab-
scess may need to be surgically drained before healing can occur.
Some bacteria, such as tuberculosis bacilli, resist digestion
by the macrophages that engulf them. Tey escape the effects
of prescription antibiotics by remaining snugly enclosed within
their macrophage hosts. In such cases,
infectious granulomas
form. Tese tumorlike growths contain a central region of in-
fected macrophages surrounded by uninfected macrophages
and an outer fibrous capsule.
A person may harbor pathogens walled off in granulomas for
years without displaying any symptoms. However, if the person’s
resistance to infection is ever compromised, the bacteria may be
activated and break free, leading to clinical disease symptoms.
Antimicrobial Proteins
Name the body’s antimicrobial substances and describe
their function.
A variety of
antimicrobial proteins
enhance our innate de-
fenses by attacking microorganisms directly or by hindering
their ability to reproduce. Te most important antimicrobial
proteins are interferons and complement proteins
(Table 21.2)
.
Interferons
Viruses—essentially nucleic acids surrounded by a protein enve-
lope—lack the cellular machinery to generate A±P or synthesize
proteins. Tey do their “dirty work” in the body by invading tissue
cells and taking over the cellular metabolic machinery needed to
reproduce themselves.
Infected cells can do little to save themselves, but some can se-
crete small proteins called
interferons (IFNs)
(in
0
ter-fēr
9
onz) to
help protect cells that have not yet been infected. Te IFNs diffuse
to nearby cells, which they stimulate to synthesize proteins that
“interfere” with viral replication in still-healthy cells by blocking
protein synthesis and degrading viral RNA
(Figure 21.5)
. Be-
cause IFN protection is not
virus-specific
, IFNs produced against
a particular virus protect against other viruses, too.
Te IFNs are a family of immune modulating proteins pro-
duced by a variety of body cells, each having a slightly different
physiological effect. IFN alpha (α) and beta (β) have the antivi-
ral effects that we’ve just described and also activate NK cells.
Another interferon, IFN gamma (γ), or immune interferon, is
secreted by lymphocytes and has widespread immune mobiliz-
ing effects, such as activating macrophages. Because both mac-
rophages and NK cells can also act directly against cancerous
cells, the interferons have an indirect role in fighting cancer. Ge-
netically engineered IFNs have found a niche in treating several
Innate defenses
Internal defenses
Virus
New
viruses
Viral nucleic acid
DNA
mRNA for
interferon
Antiviral
mRNA
Nucleus
Infected by virus;
makes interferon;
is killed by virus
Binds interferon
from cell 1; interferon
induces synthesis of
protective proteins
Host cell 1
Host cell 2
Interferon
Interferon
receptor
1
2
3
4
5
Virus
enters cell.
Interferon
genes switch on.
Cell produces
interferon
molecules.
Interferon
binding stimulates
cell to turn on
genes for antiviral
proteins.
Antiviral
proteins block viral
reproduction.
Figure 21.5
The interferon mechanism against viruses.
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