I
ALLERGY
may occur during a panic attack or at
high altitudes due to lack of oxygen.
(See also
acidosis.)
Alka-Seltzer
A brand-named analgesic and antacid
containing
aspirin, sodium bicarbonate,
and citric acid. Alka-Seltzer is used to
treat headaches and stomach upset.
alkylating agents
A class of
anticancer drugs.
allele
One of two or more different forms of a
gene
that occupies a specific position on
a
chromosome.
(See also
inheritance
.)
allergen
A normally
harmless substance
that
causes an allergic reaction (see
allergy
)
in people who have become sensitized
to it. Allergens can include foods (for
example,
nuts,
eggs,
and shellfish);
inhaled substances
(such
as
pollen,
house dust, and fur); and some drugs.
Electron micrograph of various pollen grains
Pollen is a common example of an allergen. The
airborne pollen grains from plants (such as grasses
and trees) can trigger an allergic reaction, the most
common of which is allergic rhinitis (hay fever).
allergic alveolitis, extrinsic
See
extrinsic allergic alveolitis.
allergic rhinitis
See
rhinitis, allergic
.
allergy
Various conditions caused by inappro-
priate or exaggerated reactions of the
immune system
to a wide variety of sub-
stances
known
as
allergens.
Many
common illnesses, such as
asthma
and
hay
fever
(see
rhinitis, allergic),
are
caused by allergic reactions to sub-
stances that, in the majority of people,
cause no symptoms.
Allergic reactions occur only on sec-
ond or subsequent exposure to the
allergen, after first contact has sensitized
the body. It is unclear why only certain
ALLERGY
AND THE BODY
An allergy is an inappropriate immune
system response (causing symptoms)
to substances that, in most people,
cause no response. The response is
mainly to harmless substances that
come in contact with the respiratory
airways, skin, or eye surface. Common
allergens
are pollen, spores, house-
dust mites, and animal
dander
.
Certain
drugs, and some foods, most commonly
dairy products, seafood, strawberries,
and cereals, can also cause allergies. In
diagnosing an allergy, the individual’s
medical history is important. The doctor
needs to know if the symptoms vary
according to the time of the day or the
season, and ifthere are pets or other
likely sources of allergens in the home.
THE ORIGIN OF AN ALLERGY
The immune system is sensitized once it has been exposed to an allergen
(steps 1 to 3). Symptoms occur when the allergen is met again (step 4).
1
An allergen enters the body and is
recognized by lymphocytes (blood cells
1
lymphocytes produce antibodies that
that form part of the body’s immune system).
are specific to the allergens.
ies called mast cells, which contain
granules of histamine.
4
Binding ofallergensto antibodies on the
surface of mast cells leads to the release
of histamine and to the symptoms ofallergy.
One type ofskin allergy, also known as allergic contact dermatitis, develop
slowly. Tests are performed to identify specific reactions to allergens. Small
amounts ofvarious substances are applied to the skin to see whether or not
a reaction occurs.
I
Tinysamples
of allergens
are placed on
small discs and
stuckto the skin
with inert tape.
The discs are
removed after
two days.
2
A reddened
area ofskin
where a disc had
been in contact
denotes a
positive reaction.
Some reactions
maytake longer
to appear.
A
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