Chapter 21
The Immune System: Innate and Adaptive Body Defenses
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21
antibodies or T cells. Antibody-associated reactions cause
im-
mediate
and
subacute hypersensitivities
. T cells cause
delayed
hypersensitivities.
Immediate Hypersensitivities
Immediate hypersensitivities
, also called
acute
or
type I hy-
persensitivities
, are simply what most of us would call
allergies
(
allo
5
altered;
erg
5
reaction). An
allergen
is an antigen that
causes an allergic reaction. Allergic reactions begin within sec-
onds aFer contact with the allergen and last about half an hour.
±e initial meeting with an allergen produces no symptoms
but it sensitizes the person. APCs digest the allergen and present
its fragments to CD4 cells as usual. In susceptible individuals,
an abnormally large number of these CD4 cells differentiate into
IL-4-secreting T
H
2 cells. IL-4 stimulates B cells to mature into
IgE-secreting plasma cells, which spew out huge amounts of an-
tibody specific for that allergen. When the IgE molecules attach
to
mast cells
and
basophils
, sensitization is complete.
Later encounters with the same allergen trigger an aller-
gic reaction, in which the allergen promptly binds and cross-
links the IgE antibodies on the surfaces of the mast cells and
basophils. ±is event induces an enzymatic cascade. Mast cells
and basophils release a flood of
histamine
and other inflamma-
tory chemicals that together induce the inflammatory response
typical of allergy
(Figure 21.21)
.
Allergic reactions may be local or systemic (bodywide). Mast
cells are abundant in connective tissues of the skin and beneath
the mucosa of respiratory passages and the gastrointestinal tract,
and these areas are common sites of local allergic reactions.
Histamine causes blood vessels to dilate and become leaky,
and is largely to blame for the best recognized symptoms of al-
lergy: runny nose, itching reddened skin (hives), and watery eyes.
When the allergen is inhaled, symptoms of
asthma
appear because
smooth muscle in the walls of the bronchioles contracts, constrict-
ing those small passages and restricting air flow. When the aller-
gen is ingested in food or via drugs, gastrointestinal discomfort
(cramping, vomiting, or diarrhea) occurs. Over-the-counter an-
tiallergy drugs contain antihistamines that counteract these effects.
±e systemic response known as
anaphylactic shock
is fairly
rare. It typically occurs when the allergen directly enters the
blood and circulates rapidly through the body—for example, a
bee sting, spider bite, or injection of a foreign substance (such as
penicillin or other drugs that act as haptens).
±e mechanism of anaphylactic shock is essentially the same
as that of local responses, but when mast cells and basophils are
enlisted throughout the body, the outcome is life threatening.
±e bronchioles constrict (and the tongue may swell), making
it difficult to breathe, and the sudden vasodilation and fluid loss
from the bloodstream may cause circulatory collapse (hypoten-
sive shock) and death within minutes. Epinephrine is the drug
of choice to reverse these histamine-mediated effects.
Subacute Hypersensitivities
Like the immediate types,
subacute hypersensitivities
are
caused by antibodies (IgG and IgM rather than IgE). However,
Mast cell with
fixed IgE
antibodies
Granules
containing
histamine
IgE
Outpouring of fluid
from capillaries
Release of mucus
Constriction of small
respiratory passages
(bronchioles)
Sensitization stage
Subsequent (secondary)
responses
Antigen
Histamine
Mast cell granules
release contents
after antigen binds
with IgE antibodies
Adaptive defenses
Humoral immunity
1
Antigen (allergen)
invades body.
2
Plasma cells produce
large amounts of class
IgE antibodies against
allergen.
6
Histamine causes blood vessels to dilate and become leaky,
which promotes edema; stimulates secretion of large amounts
of mucus; and causes smooth muscles to contract. (If respiratory
system is site of antigen entry, asthma may ensue.)
5
Antigen combines
with IgE attached
to mast cells (and
basophils), which triggers
degranulation and release
of histamine (and other
chemicals).
4
More of same
antigen invades body.
3
IgE antibodies
attach to mast cells in
body tissues
(and to
circulating basophils).
Figure 21.21
Mechanism of an acute allergic (immediate
hypersensitivity) response.
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