infected by the infectious agents that cause measles, mumps,
gonorrhea, and syphilis, but other animal species are not.
Similarly, humans are resistant to certain forms of malaria
and tuberculosis that affect birds. However, new infl uenza
strains that affect humans may come from birds, especially
In San Francisco in 1982, Simon Guzman became one of the first
recorded individuals to succumb to AIDS and was the ±
rst to die from
a parasitic infection known only in sheep—
infection, which causes relentless diarrhea, illustrates a hallmark of
AIDS: alteration of species resistance.
Mechanical Barriers
The skin and mucous membranes lining the passageways
of the respiratory, digestive, urinary, and reproductive sys-
tems create
mechanical barriers
that prevent the entrance
of some infectious agents. As long as these barriers remain
intact, many pathogens are unable to penetrate them.
Another protection is that the epidermis sloughs off, remov-
ing superF cial bacteria with it. In addition, the mucus-coated
ciliated epithelium, described in chapter 19 (p. 742), that
lines the respiratory passages entraps particles and sweeps
them out of the airways and into the pharynx, where they
are swallowed. Hair traps infectious agents associated with
the skin and mucous membranes and sweat and mucus rinse
away microorganisms. Tears, saliva, and urine also wash
away organisms before they become F
rmly attached. These
barriers provide a
rst line oF deFense.
The rest of the non-
speciF c defenses discussed in this section are part of the
ond line oF deFense.
Chemical Barriers
Enzymes in body fl uids provide a
chemical barrier
to patho-
gens. Gastric juice, for example, contains the protein-splitting
enzyme pepsin and has a low pH due to hydrochloric acid in
the stomach. The combined effect of pepsin and hydrochloric
acid kills many pathogens that enter the stomach. Similarly,
tears contain the enzyme lysozyme, which destroys certain
bacteria on the eyes. The accumulation of salt from perspira-
tion also kills certain bacteria on the skin.
onz) are hormonelike peptides
that lymphocytes and fibroblasts produce in response to
viruses or tumor cells. Once released from a virus-infected
cell, interferon binds to receptors on uninfected cells, stimu-
lating them to synthesize proteins that block replication of
a variety of viruses. Thus, interferon’s effect is nonspeciF
Interferons also stimulate phagocytosis and enhance the
activity of other cells that help to resist infections and the
growth of tumors.
Other antimicrobial biochemicals are defensins and
are peptides produced by neutro-
phils and other types of granular white blood cells in the
organisms because their structure is far simpler than that of
a living cell and they must infect a living cell to reproduce.
An infection may be present even though an individual feels
well. People infected with the human immunodeficiency
virus (HIV), which causes AIDS, often live for many years in
good health before becoming ill. However, impairment of the
immune system begins as soon as the virus enters T cells.
The human body can prevent entry of pathogens or
destroy them if they enter. Some mechanisms are general and
protect against many types of pathogens, providing
They function the same way regard-
less of the pathogen or the number of exposures. These
mechanisms include species resistance, mechanical barriers,
chemical barriers (enzyme action, interferon, and comple-
ment), natural killer cells, infl
ammation, phagocytosis, and
fever. Other defense mechanisms are very precise, target-
ing speciF c pathogens with an
or immunity. These more directed responses are carried
out by specialized lymphocytes that recognize foreign mol-
ecules (nonself antigens) in the body and act against them.
Innate and adaptive defense mechanisms work together to
protect the body against infection. While the innate defenses
respond rapidly, slower-to-respond adaptive defenses begin
as well.
Species Resistance
Species resistance
refers to the fact that a species may be
resistant to diseases that affect other species because its
cells do not have receptors for the pathogen or its tissues do
not provide the temperature or chemical environment that
a particular pathogen requires. ±or example, humans are
Major Organs of the
Lymphatic System
In groups or chains
along the paths of larger
lymphatic vessels
Filter foreign particles and
debris from lymph; produce and
house lymphocytes that destroy
foreign particles in lymph; house
macrophages that engulf and
destroy foreign particles and
cellular debris carried in lymph
In the mediastinum
posterior to the upper
portion of the body of the
Houses lymphocytes;
erentiates thymocytes into
T lymphocytes
In upper-left portion of the
abdominal cavity, inferior
to the diaphragm, posterior
and lateral to the stomach
Blood reservoir houses
macrophages that remove foreign
particles, damaged red blood
cells, and cellular debris from the
blood; contains lymphocytes
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