Allergies Are Caused by an
Overreaction to an Antigen
An example of an overactive immunological memory is
an
allergy
, or a
h yp e rsen sitivity
. Allergies develop when an
individual is exposed to a type of antigen, referred to as
an
allergen
. Examples of allergens
include pet dander, foods, pollen,
antibiotics, poison ivy,
and bee
venom. Upon initial exposure to
the allergen, the lymphatic system
produces IgE antibodies that bind
to the surfaces of mast cells and
basophils. Later, upon reexposure
to the allergen, these cells secrete
histamine
,
prostaglandins,
and
other chemicals, causing vasodila-
tion, contraction of airway smooth
muscle,
and
increased
mucus
secretion. These factors contrib-
ute to the allergy symptoms of
runny nose, sneezing, congestion, and difficulty breath-
ing. A severe, life-threatening allergic reaction in which
the airways constrict so much that the person has extreme
difficulty breathing is called
anaphylactic shock
. Ana-
phylactic shock can be treated by injecting epinephrine to
dilate the airways and strengthen the heart.
Immunological memory forms the basis of vaccinations.
Let’s turn next to vaccinations and see how they work.
There Are Many Ways
to Develop Immunity
You can acquire immunity to a pathogen in a number of
ways. In some of the mechanisms, you are exposed to the
antigen, and your body develops antibodies in response
to it. These processes result in
active immunity
. Other
mechanisms involve receiving premade antibodies that
your body can use to defend itself against the disease-
causing agent. This is called
passive immunity
. Active
immunity stays with you for long periods (or life), as the
memory cells and long-lasting antibodies remain with you.
However, passive immunity is fleeting. Once the antibod-
ies degrade, so does the immunity because there are no
immune cells to produce new antibodies.
In
n a tu ra lly acquired p a ssive im m u n ity ,
antibodies are
passed from mother to baby across the placenta (dur-
ing fetal development) or via the breast milk.
A r tific ia lly
acquired p a ssiv e im m u n ity
is the result of intravenous in-
jection of anti-serum or anti-toxin solutions containing
Types of adaptive immunity Table 12.2
Type
How acquired
Naturally acquired
Following exposure to a microbe, anti-
active immunity
gen recognition by B cells and T cells and
costimulation lead to the production of
antibody-secreting plasma cells, cyto-
toxic T cells, and B and T memory cells.
Naturally acquired
Transfer of IgG antibodies from mother
passive immunity
to fetus across the placenta, or of IgA
antibodies from mother to baby in milk
during breastfeeding.
Artificially acquired
Antigens introduced during a vaccination
active immunity
stimulate cell-mediated and antibody-
mediated immune responses, leading
to production of memory cells. The
antigens are pretreated to be immuno-
genic but not pathogenic; that is, they
will trigger an immune response but not
cause significant illness.
Artificially acquired
Intravenous injection of immunoglobu-
passive immunity
lins (antibodies).
antibodies. When you are exposed to a disease by coming
in contact with someone else who has the disease,
n a tu r a l-
ly acquired active im m u n ity
occurs: You react to that disease
organism by making antibodies through the process de-
scribed in the previous section. You can also be exposed
to that pathogenic organism through the use of a vaccine.
This type of exposure results in
a rtific ia lly acquired active
im m u n ity .
See
Table 12.2
for a summary of the types of
adaptive immunity.
While exposure to pathogens, getting sick, evoking
a full immune response, and recovering after producing
antibodies is the most effective process for developing
immunity, some pathogens are fatal. A
vaccination
(re-
ceipt of a vaccine) can provide you with active immunity
at greatly reduced risk. A
vaccine
consists of weakened or
dead pathogens—either a whole organism or parts of an
organism. When the vaccine is injected, your B and T cells
are activated in a full immune response to this weakened
opponent. Your body makes memory cells in response to
the pathogen, so that when you encounter the pathogen at
its full strength, the active secondary response should be
enough to fend it off. Some vaccines may require booster
doses to maintain adequate protection.
Most often, antibodies protect us from diseases, but
sometimes they can attack our own cells and cause dis-
ease; such diseases are called
a u to im m u n e diseases
(see
W h a t
a H e a lth P rovider S ees).
allergen
(AL-er-
jen) An antigen that
evokes a hypersensi-
tivity reaction.
histamine
(HISS-
ta-men) A substance
found in many cells
that is released when
the cells are injured;
results in vasodilation,
increased permeabil-
ity of blood vessels,
and constriction of
bronchioles.
360 CHAPTER 12
The Lymphatic System and Immunity
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