The Immune System: Innate and Adaptive Body Defenses
disorders including hepatitis C, genital warts, multiple sclerosis,
and hairy cell leukemia.
, or simply
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
innate and adaptive defenses.
so that tissue can be repaired. Once this is accomplished, heal-
ing usually occurs quickly.
In severely infected areas, the battle takes a considerable toll on
both sides, and creamy yellow
(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
. 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,
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.
Name the body’s antimicrobial substances and describe
A variety of
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
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
Infected cells can do little to save themselves, but some can se-
crete small proteins called
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
cause IFN protection is not
, 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
Viral nucleic acid
Infected by virus;
is killed by virus
from cell 1; interferon
induces synthesis of
Host cell 1
Host cell 2
genes switch on.
cell to turn on
genes for antiviral
proteins block viral
The interferon mechanism against viruses.