A collection of symptoms caused by an
response to substances that do not trigger an immune response in
most people. See also
mold, dander, dust;
Causes, incidence, and risk factors:
Allergy is caused by hypersensitivity of the immune system leading
to a misdirected
response. The immune system normally protects the body against
harmful substances such as bacteria, viruses, and
Allergy occurs when the immune system reacts to substances
(allergens) that are generally harmless and in most people do not
cause an immune response.
The first exposure to the
causes a mild immune response that sensitizes the immune system to
the substance (triggers the immune system to recognize the
substance). The second and subsequent exposure to the allergen
results in symptoms. The type of symptom that develops depends on
the specific allergen, the part of the body where exposure occurs,
and the way the immune system reacts to the allergen.
When an allergen enters the body of a person with a sensitized
immune system, it triggers
production. Histamine and other chemicals are released by body
tissues as part of the immune response. This causes
of affected tissues, mucus production,
and other symptoms. Symptoms vary in severity from person to person.
The part of the body contacted by the allergen will, in part, also
affect symptoms. For example, allergens that are inhaled often cause
nose/throat, mucus production,
or similar symptoms.
allergies often include
pain, cramping, or similar symptoms, although the whole body may
be affected when the food is absorbed. Allergies to plants often
Drug allergies usually involve the whole body.
Many disorders are associated with, triggered, or worsened by
allergies. These include
and many others.
Common allergens include environmental agents that contact the skin,
breathing passages, or the surface of the eye (such as pollen; see
allergy to mold, dander, dust). Food allergies and drug
allergies are common.
reactions can be caused by
bites, jewelry, cosmetics, and almost any substance that
contacts the body.
Some people have allergic-type reactions to hot or cold
temperatures, sunlight, or other physical
In some persons, friction (such as rubbing or vigorously stroking
the skin) will cause symptoms. The mechanism that causes this is not
well understood, but it is possible that minute changes in the
chemistry of the skin may occur in response to physical stimuli and
some component of this chemical change triggers the allergy. (See
atopic dermatitis and
Allergies are common. Heredity, environmental conditions, number and
type of exposures, emotional factors (stress
and emotional upset can increase the sensitivity of the immune
system), and many other factors can indicate a predisposition to
There is no known way to prevent allergies. Symptoms may be
prevented by avoiding known allergens.
Note: Allergies vary according to the type of
and the part of the body in which the
Signs and tests:
History is important in diagnosing allergies, including whether the
symptoms vary according to time of day or the season and possible
exposures that involve pets, diet changes, or other sources of
Testing may be required to determine if symptoms are an actual
allergy or caused by other problems. For example, contaminated food
poisoning") may resemble
allergies. Some medications (such as
ampicillin, and others) can produce non-allergic reactions,
rashes, that resemble
allergies but are not true allergies.
Antibody/immunoglobulin (particularly IgE) levels that are
Complement levels may be abnormal.
- Testing may reveal the specific
- Skin testing is the most common method of
allergy testing. This may include intradermal, scratch,
patch, or other tests.
- Occasionally, the suspected allergen is dissolved and
dropped onto the lining of the lower eyelid (conjunctiva)
as a means of testing for allergies.
- Other testing to determine the specific allergen may include
various types of "use" or "elimination" tests where suspected
items are eliminated and/or introduced while the person is
observed for response to the substance.
- Tests for reaction to physical
stimuli may include application of the stimuli (heat, cold,
and so on) and observation for an allergic response.
This disease may also alter the results of the following tests:
Treatment varies with the severity and type of symptom. Short-term
goals include relieving immediate symptoms. Long-term goals include
Corticosteroids such as dexamethasone or
immune response and may be prescribed to reduce symptoms.
Epinephrine is used to reduce
of the airways and other life-threatening symptoms.
Antihistamines such as
diphenhydramine often provide good relief of mild to moderate
Specific illnesses that are caused by allergies (such as
may require other treatments.
Avoidance of the
is the best long-term treatment, particularly with
reaction to foods or medications
Desensitization (immunotherapy, "allergy shots") is occasionally
recommended if the allergen cannot be avoided. It includes regular
injections of the allergen, given in increasing doses that may
"acclimatize" the body to the
of illness can often be helped by joining a
group where members share common experiences and problems. See
allergy - support group.
Most allergies are readily treated. However, treatment only affects
that exposure, so subsequent exposures cause another
Rarely, people may "outgrow" an allergy as the immune system becomes
less sensitive to the
However, as a general rule, once a substance has provoked an
allergic reaction, it continues to affect the person, and may cause
an increasingly severe response with repeated exposures.
Desensitization may cause uncomfortable side effects (such as
and may have dangerous outcomes (such as
anaphylaxis). It may require years of treatment and is effective
in about two-thirds of cases.