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The presence and reproduction of pathogens may cause
an infection. Pathogens include bacteria, complex single-
celled organisms, fungi, and viruses. An infection may
not immediately cause symptoms. The body has innate
c) and adaptive (speciF
c) defenses against
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1. Species resistance
Each species is resistant to certain diseases that may
affect other species but is susceptible to diseases other
species may resist.
2. Mechanical barriers
a. Mechanical barriers include the skin and mucous
b. Intact mechanical barriers prevent entrance of
some pathogens.
c. Hair traps infectious agents; and fl
uids such as
tears, sweat, saliva, mucus, and urine wash away
organisms before they can F
rmly attach.
3. Chemical barriers
a. Enzymes in gastric juice and tears kill some
b. Low pH in the stomach prevents growth of some
c. High salt concentration in perspiration kills some
d. Interferons stimulate uninfected cells to synthesize
antiviral proteins that block proliferation of
viruses, stimulate phagocytosis, and enhance
activity of cells that help resist infections and stifl
tumor growth.
e. Defensins make holes in bacterial cell walls and
f. Collectins provide broad protection against a wide
variety of microbes by grabbing onto them, easing
g. Activation of complement proteins in plasma
stimulates infl
ammation, attracts phagocytes, and
enhances phagocytosis.
4. Natural killer (NK) cells
Natural killer cells secrete perforins, which destroy
cancer cells and cells infected with viruses.
5. Infl
a. Infl ammation is a tissue response to damage,
injury, or infection.
b. The response includes localized redness, swelling,
heat, and pain.
c. Chemicals released by damaged tissues attract
white blood cells to the site.
d. Clotting may occur in body fl
uids that accumulate
in affected tissues.
e. Connective tissue may form a sac around the
injured tissue and thus aid in preventing the
spread of pathogens.
2. Obstruction of lymph movement
a. Any condition that interferes with the fl ow of
lymph results in edema.
b. Obstruction of lymphatic vessels due to surgical
removal of lymph nodes causes edema in the
affected area.
1. Structure of a lymph node
a. Lymph nodes are usually bean-shaped, with
blood vessels, nerves, and efferent lymphatic
vessels attached to the indented region; afferent
lymphatic vessels enter at points on the convex
b. Lymph nodes are enclosed in connective tissue
that extends into the nodes and subdivides them
into nodules.
c. Nodules contain masses of lymphocytes and
macrophages and spaces through which lymph
fl ows.
2. Locations of lymph nodes
a. Lymph nodes aggregate in groups or chains along
the paths of larger lymphatic vessels.
b. They are in the cervical, axillary, supratrochlear,
and inguinal regions and in the pelvic,
abdominal, and thoracic cavities.
3. ±unctions of lymph nodes
a. Lymph nodes F
lter potentially harmful foreign
particles from the lymph before it is returned to
the bloodstream.
b. Lymph nodes are centers for the production of
lymphocytes that act against foreign particles.
c. They contain macrophages that remove foreign
particles from lymph.
1. Thymus
a. The thymus is a soft, bilobed organ within the
b. It slowly shrinks after puberty.
c. It is composed of lymphatic tissue subdivided
into lobules.
d. Lobules contain lymphocytes.
e. T lymphocytes leave the thymus and provide
f. The thymus secretes thymosins, which stimulate
maturation of T lymphocytes.
2. Spleen
a. The spleen is in the upper left portion of the
abdominal cavity.
b. It resembles a large lymph node encapsulated
and subdivided into lobules by connective
c. Spaces in splenic lobules are F
lled with blood.
d. The spleen, which F
lters foreign particles
and damaged red blood cells from the blood,
contains many macrophages and lymphocytes.
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