hypersensitivityreactions and can have
any of four different mechanisms (termed
Types I to IV hypersensitivity reactions).
Most well known allergies are caused by
Type I (also known as anaphylactic or
immediate) hypersensitivity in which
allergens cause immediate symptoms
by provoking the immune system to
produce specific antibodies, belonging
to a type called immunoglobulin E
(IgE), which coat cells (called mast cells
or basophils). When the allergen is
encountered for the second time, it
binds to the IgE antibodies and causes
the granules in mast cells to release
various chemicals, which are responsi-
ble for the symptoms of the allergy.
Among the chemicals released is hist-
amine, which causes widened blood
vessels, leakage of fluid into tissues, and
muscle spasm. Symptoms can include
itching, swelling, sneezing, and wheez-
ing. Particular conditions associated
with Type I reactions include asthma,
hay fever, urticaria(nettle rash),angio-
edema, anaphylactic shock (a severe,
generalized allergic reaction), possibly
atopic eczema, and many food allergies.
Types II to IV hypersensitivity reac-
tions are less often implicated in
allergies. However, contact dermatitis,
in which the skin reacts to substances
such as nickel, is due to a Type IV hyper-
sensitivity reaction.
It is not known why certain individuals
and not others get allergies, but about
1 in 8 people seem to have an inherited
predisposition to them (see atopy).
Whenever possible, the most effective
treatment for allergy of any kind is
avoidance of the relevant allergen.
Drug treatment for allergic reactions
includes the use of antihistamine drugs,
which relieve the symptoms. Some anti-
histamine drugs have a sedative effect,
which is useful in treating itching at
night due to eczema. Many antihista-
mines do not cause drowsiness, making
them more suitable for daytime use.
Other drugs, such as sodium cromogli-
cateand corticosteroid drugs, can be
used regularly to prevent symptoms
from developing.
Hyposensitizationcan be valuable for a
minority of people who suffer allergicreactions to specific allergens such as
bee stings. Treatment involves gradu-
ally increasing doses of the allergen,
but it must be carried out under close
supervision because a severe allergic
reaction can result.
allopathyThe practice of conventional
medicine. (See also homeopathy.)
allopurinolA drug treatment for gout.
Taken long term, it reduces the frequ-
ency of attacks by decreasing production
of uric acid. Possible adverse reactions
include itching, rashes, and nausea.
alopeciaLoss or absence of hair, which
may occur at any hair-bearingsite on
the body but which is usually notice-
able only on the scalp.
Male-pattern baldness, the most com-
mon form of alopecia, is hereditary and
most often affects men. Normal hair is
lost initially from the temples and
crown and is replaced by fine, downy
hair; the affected area gradually widens.
Other hereditary forms are rare. They
may be due to an absence of hair roots
or abnormalities of the hair shaft.
In generalized alopecia, the hair falls
out in large amounts. Causes include
various forms of stress, such as surgery,
prolonged illness, or childbirth. Many
anticancer drugscause temporary alo-
pecia. The hair regrows when the
underlying cause is corrected.
Localized alopecia may be due to per-
manent skin damage (for example, by
burns or radiotherapy) or trauma to
the hair roots by styling or, rarely,
trichotillomania(a disorder in which suf-
ferers pull out their hair). The most
common type of localized hair loss is
alopecia areata, which is an autoim-
mune disorder. There is no specific
treatment, but the hair usually regrows
within a few months. Alopecia univer-
salis is a rare, permanent form of
alopecia areata that causes loss of all the
hair on the scalp and body, including
the eyelashes and eyebrows. Skin dis-
eases such as scalp ringworm (see
tinea), lichen planus, lupus erythemato-
sus, and skin tumoursmay also cause
localized hair loss.
Treatments for male-pattern baldness
include hair transplants or drug treat-
ments with minoxidilor finasteride.ALLOPATHY ALOPECIAA