Chapter 21
The Immune System: Innate and Adaptive Body Defenses
765
21
2.
Te
adaptive (specific) defense system
is more like an elite
fighting force equipped with high-tech weapons to attack
particular
foreign substances. Te adaptive defense response,
which provides the body’s
third line of defense
, takes consider-
ably more time to mount than the innate defense response.
Although we consider them separately, the innate and adap-
tive systems always work hand in hand. An overview of these
two systems is shown in
Figure 21.1
. Small portions of this
diagram will reappear in subsequent figures to let you know
which part of the immune system we’re dealing with.
Although certain organs of the body (notably lymphoid or-
gans) are intimately involved in the immune response, the
im-
mune system
is a
functional system
rather than an organ system
in an anatomical sense. Its “structures” are a diverse array of mol-
ecules plus trillions of immune cells (especially lymphocytes) that
inhabit lymphoid tissues and circulate in body fluids.
Once, the term
immune system
was equated with the adap-
tive defense system only. However, we now know that the innate
and adaptive defenses are deeply intertwined. Specifically:
Te innate and adaptive systems release and recognize many
of the same defensive molecules.
Te innate responses are not as nonspecific as once thought.
Indeed, they have specific pathways to target certain foreign
substances.
Proteins released during innate responses alert cells of the
adaptive system to the presence of specific foreign molecules
in the body.
When the immune system is operating effectively, it protects
the body from most infectious microorganisms, cancer cells,
and (unfortunately) transplanted organs and graFs. It does this
both directly, by cell attack, and indirectly, by releasing mobiliz-
ing chemicals and protective antibody molecules.
PART 1
Innate Defenses
Because the innate defenses are part and parcel of our anatomy,
you could say we come fully equipped with innate defenses. Te
mechanical barriers that cover body surfaces and the cells and
chemicals that act on the initial internal battlefronts are in place at
birth, ready to ward off invading
pathogens
(harmful or disease-
causing microorganisms).
Many times, our innate defenses alone ward off infection by
destroying pathogens. In other cases, the adaptive immune sys-
tem is called into action to reinforce and enhance the innate
defenses. Either way, the innate defenses reduce the workload
of the adaptive system by preventing the entry and spread of
microorganisms in the body.
Surface Barriers:
Skin and Mucosae
Describe surface membrane barriers and their protective
functions.
Te body’s first line of defense—the
skin
and the
mucous membranes
,
along with the secretions these membranes produce—is highly ef-
fective. As long as the epidermis is unbroken, this heavily keratinized
epithelial membrane is a formidable physical barrier to most micro-
organisms that swarm on the skin. Keratin is also resistant to most
weak acids and bases and to bacterial enzymes and toxins. Intact
mucosae provide similar mechanical barriers within the body. Re-
call that mucous membranes line all body cavities that open to the
exterior: the digestive, respiratory, urinary, and reproductive tracts.
Besides serving as physical barriers, skin and mucous mem-
branes produce a variety of protective chemicals:
Acid.
Te acidity of skin, vaginal, and stomach secretions—
the
acid mantle
—inhibits bacterial growth.
Enzymes.
Lysozyme
—found in saliva, respiratory mucus, and
lacrimal fluid of the eye—destroys bacteria. Protein-digesting
enzymes in the stomach kill many different microorganisms.
Mucin.
Mucin
dissolved in water forms thick, sticky mucus
that lines the digestive and respiratory passageways. Tis
mucus traps many microorganisms. In contrast, the mucin
in watery saliva traps microorganisms and washes them out
of the mouth into the stomach where they are digested.
Defensins.
Mucous membranes and skin secrete small
amounts of broad-spectrum antimicrobial peptides called
defensins
. Defensin output increases dramatically in re-
sponse to inflammation when surface barriers are breached.
Using various mechanisms, such as disruption of microbial
membranes, defensins help to control bacterial and fungal
colonization in the exposed areas.
Other chemicals.
In the skin, some lipids in sebum and
derm-
cidin
in eccrine sweat are toxic to bacteria.
Innate
defenses
Surface barriers
Skin
Mucous membranes
Internal defenses
Phagocytes
Natural killer cells
Inflammation
Antimicrobial proteins
Fever
Humoral immunity
B cells
Cellular immunity
T cells
Adaptive
defenses
Figure 21.1
Overview of innate and adaptive defenses.
Humoral immunity (primarily involving B lymphocytes) and cellular
immunity (involving T lymphocytes) are distinct but overlapping areas
of adaptive immunity. For simplicity, the many interactions between
innate and adaptive defenses are not shown here.
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