639
CHAPTER SIXTEEN
Lymphatic System and Immunity
have allergies, fewer than 100 people a year die of anaphylac-
tic shock. The peanut allergy described in the chapter opening
vignette causes many of the symptoms of anaphylactic shock,
but usually not the sensation of the throat closing.
One theory of the origin of allergies, particularly anaphylactic shock, is
that they evolved at a time when insect bites and the natural substances
from which antibiotics such as penicillin are made threatened human
survival. Today, that once-protective response is an overreaction. The
observation that IgE protects against roundworm and F
atworm infec-
tions, in addition to taking part in allergic reactions, supports the idea
that this antibody class is a holdover from times past, when challenges
to the immune system di±
ered from what they are today.
Hypersensitivities that take one to three hours to
develop include
antibody-dependent cytotoxic reactions
(type II) and
immune complex reactions
(type III). In an
antibody-dependent cytotoxic reaction, an antigen binds to
a speciF c cell, stimulating phagocytosis and complement-
mediated lysis of the antigen. A transfusion reaction to mis-
matched blood is a type II hypersensitivity reaction. In an
immune complex or type III reaction, phagocytosis and lysis
cannot clear widespread antigen-antibody complexes from
the circulation. The complexes may block small vessels,
which damages the tissues that they reach.
Autoimmunity,
the loss of the ability to tolerate self-antigens, illustrates this
type of hypersensitivity reaction. It is discussed later in the
chapter in the section “Autoimmunity.”
The
tuberculin skin test
is used to detect individuals who have tuber-
culosis (TB) or who have had it (or a closely related infection) or been
exposed to it. The test introduces a tuberculin preparation called
purif
ed protein derivative
(PPD) into the super²
cial layers of the skin
(Mantoux test). If the person’s T cells have been sensitized to the anti-
gens of the mycobacteria that cause tuberculosis, an allergic reaction
occurs within forty-eight to seventy-two hours, and a localized region
of the skin and subcutaneous tissue hardens (indurates). The absence
of this reaction (negative result) signi²
es that the person’s T cells have
not previously encountered the mycobacterial antigens.
Allergic Reactions
Both allergic reactions and immune responses entail the sen-
sitizing of lymphocytes or the combining of antigens with
antibodies. An allergic reaction, however, is an immune
response to a nonharmful substance and can damage tissues.
An allergy is also called a hypersensitivity reaction. One form
of allergic reaction can occur in almost anyone, but another
affects only people with an inherited tendency toward exag-
gerated immune responses. The antigens that trigger allergic
responses are called
allergens
(al
er-jenz).
An
immediate-reaction
(type I or anaphylactic)
allergy
occurs within minutes after contact with an allergen. Persons
with this type of allergy have inherited the tendency to over-
produce IgE antibodies in response to certain antigens. IgE
normally comprises a tiny fraction of plasma proteins.
An immediate-reaction allergy activates B cells, which
become sensitized when the allergen is F
rst encountered.
Subsequent exposures to the allergen trigger allergic reac-
tions. In the initial exposure, IgE attaches to the membranes
of widely distributed mast cells and basophils. When a sub-
sequent allergen-antibody reaction occurs, these cells release
allergy mediators such as
histamine, prostaglandin D
2
, and
leukotrienes
(F g. 16.22)
. These substances affect physiology
by dilating arterioles and increasing vascular permeability,
both of which cause edema. They also cause contraction
of bronchial and intestinal smooth muscles, and increased
mucus production. The result is a severe infl
ammation reac-
tion responsible for the symptoms of the allergy, such as
hives, hay fever, asthma, eczema, or gastric disturbances.
Anaphylactic shock is a severe form of immediate-reaction
allergy in which mast cells release allergy mediators through-
out the body. The person may at first feel an inexplicable
apprehension, and then suddenly, the entire body itches and
breaks out in red hives. Vomiting and diarrhea may follow.
The face, tongue, and larynx begin to swell, and breathing
becomes difF cult. Unless the person receives an injection of
epinephrine (adrenalin) and sometimes a tracheotomy (an
incision into the windpipe to restore breathing), he or she
will lose consciousness and may die within five minutes.
Anaphylactic shock most often results from an allergy to peni-
cillin or insect stings. ±ortunately, thanks to prompt medical
attention and avoidance of allergens by people who know they
TABLE
16.9
|
Practical Classif
cation oF Immunity
Type
Mechanism
Result
Naturally acquired active immunity
Exposure to live pathogens
Stimulation of an immune response with symptoms of a disease
Arti²
cially acquired active immunity
Exposure to a vaccine containing weakened or dead
pathogens or their components
Stimulation of an immune response without the symptoms of a
disease
Arti²
cially acquired passive immunity
Injection of gamma globulin containing antibodies or
antitoxin
Short-term immunity without stimulating an immune response
Naturally acquired passive immunity
Antibodies passed to fetus from pregnant woman with
active immunity or to newborn through breast milk
from a woman with active immunity
Short-term immunity for newborn without stimulating an
immune response
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