617
CHAPTER SIXTEEN
Lymphatic System and Immunity
squamous epithelial cells called endothelium. These thin walls
allow tissue fl uid (interstitial fl uid) from the interstitial space
to enter the lymphatic capillaries. Fluid inside a lymphatic
capillary is called
lymph
(limf). Special lymphatic capillaries
(
lacteals
) in the lining of the small intestine absorb digested
fats, then transport the fats to the venous circulation.
Lymphatic Vessels
The walls of
lymphatic vessels
are similar to those of veins,
but thinner. Each is composed of three layers: an endothelial
lining, a middle layer of smooth muscle and elastic ± bers,
and an outer layer of connective tissue. Also like those
veins below the heart, the lymphatic vessels have semilunar
valves, which help prevent backfl ow of lymph.
Figure 16.3
shows one of these valves.
The larger lymphatic vessels lead to specialized organs
called
lymph nodes
(limf no
¯dz). After leaving the nodes, the
vessels merge into larger lymphatic trunks.
Lymphatic Trunks and Collecting Ducts
The
lymphatic trunks,
which drain lymph from the lym-
phatic vessels, are named for the regions they serve. For
example, the
lumbar trunk
drains lymph from the lower
limbs, lower abdominal wall, and pelvic organs; the
intes-
tinal trunk
drains the abdominal viscera; the
intercostal
and
bronchomediastinal trunks
drain lymph from portions of the
thorax; the
subclavian trunk
drains the upper limb; and the
16.1
INTRODUCTION
The
lymphatic
(lim-fat
ik)
system
is a vast collection of cells
and biochemicals that travel in lymphatic vessels, and the
organs and glands that produce them. The lymphatic system
includes a network of vessels that assist in circulating body
fl uids, so it is closely associated with the cardiovascular sys-
tem. Lymphatic vessels transport excess fl
uid away from the
interstitial spaces in most tissues and return it to the blood-
stream
(f g. 16.1)
. Without the lymphatic system, this fl uid
would accumulate in tissue spaces. The organs of the lym-
phatic system also help defend the body against infection by
disease-causing agents, or
pathogens
(path
o-jenz).
16.2
LYMPHATIC PATHWAYS
The
lymphatic pathways
begin as lymphatic capillaries that
merge to form larger lymphatic vessels. These, in turn, lead
to larger vessels that unite with the veins in the thorax.
Lymphatic Capillaries
Lymphatic capillaries
are microscopic, closed-ended tubes.
They extend into the interstitial spaces, forming complex
networks that parallel the networks of the blood capillaries
(f g. 16.2)
. The walls of lymphatic capillaries are similar to
those of blood capillaries. Each consists of a single layer of
PEANUT ALLERGY
one has eaten one by two years of age. This is suF
cient exposure to set the stage
for later allergy in genetically predisposed individuals. That the average age of ±
rst
allergic reaction to peanuts is fourteen months suggests that the initial exposure—
necessary to “prime” the immune system for future response—happens through
breast milk or in the uterus. Countries where peanuts are rarely eaten, such as
Denmark and Norway, have very low incidence of peanut allergy.
The dry roasting of peanuts in the United States may make the three gly-
coproteins that evoke the allergic response more active. In China, where pea-
nuts are equally popular but are boiled or fried, allergy is rare. However, children
of Chinese immigrants in the United States have the same incidence of peanut
allergy as other children, supporting the idea that method of preparation con-
tributes to allergenicity.
Peanut allergy is probably impossible to eradicate. Creating a genetically
modi±
ed peanut that lacks allergens is diF
cult because at least three of its pro-
teins would have to be removed—it would no longer be a peanut. The best
approach is for people who know they are allergic to avoid peanuts. Always ask if
it is an ingredient in a food. Parents of small children with peanut allergy should
read all food labels meticulously and carry an injectible “pen” of epinephrine in
case the child reacts. A person in the midst of an allergic response to peanuts
should remain in the emergency department for a few hours, because a danger-
ous late-phase response can occur three to ten hours after the initial symptoms
of rash, diF
culty breathing, and/or gastrointestinal upset.
T
he young woman went to the emergency department for
sudden onset of difficulty breathing. She was also flushed
and had vomited. An astute medical student taking a quick
history from the woman’s roommates discovered that she
had just eaten cookies from a vending machine in their dorm.
Suspecting that the cookies may have contained peanuts, the student
alerted the attending physician, who treated the woman for suspected pea-
nut allergy—giving oxygen, an antihistamine, a steroid drug, and epineph-
rine. She recovered.
Peanut allergy is common and on the rise in certain westernized coun-
tries. In the United States, about 1% of children under the age of ±
ve and 2%
of the population over ten years of age have had allergic reactions to pea-
nuts. About 30,000 people react each year, and about 200 die.
Peculiarities of peanuts and our fondness for them may explain why
peanut allergy prevalence is increasing. Three glycoproteins in peanuts are
allergens, causing the misdirected immune response that is an allergy. These
glycoproteins are highly concentrated in the peanut, and when ingested,
they disturb the intestinal lining in such a way that they enter the circulation
rapidly, without being digested. Many allergens confront cells of the immune
system beneath the intestinal lining.
Compounding the rapidity with which peanut allergens ²
ood the blood-
stream is that people in the United States eat many peanuts—virtually every-
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