ALLOGRAFT
A
people develop allergies, but about one
person in eight seems to have an inher-
ited predisposition to them (see
atopy).
TYPES AND CAUSES
The function of the immune system is
to recognize
antigens
(foreign proteins)
on the surfaces of microorganisms and
to form
antibodies
(also called immuno-
globulins)
and sensitized
lymphocytes
(white blood cells). When the immune
system next encounters these antigens,
the antibodies and sensitized lympho-
cytes interact with them, leading to the
destruction of the microorganisms.
A similar immune response occurs in
allergies, except that the immune system
forms antibodies or sensitized lymph-
ocytes
against
harmless
substances
because these allergens are misidenti-
fied as potentially harmful antigens.
The
inappropriate
or
exaggerated
reactions that are seen in allergies are
known as
hypersensitivity
reactions and
can have any of four different mecha-
nisms (which are termed Types I to IV
hypersensitivity reactions).
TYPE I 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 into
producing specific antibodies, belong-
ing to a type called immunoglobulin E
(IgE),
which
coat
cells
(known
as
mast cells or basophils) that are present
in the skin and the lining of the stom-
ach,
lungs,
and
upper
respiratory
airways. When the allergen is encoun-
tered for the second time, it binds to
the IgE antibodies and causes the gran-
ules in mast cells to release various
chemicals, which are responsible for
the symptoms of the allergy.
Among the chemicals released is
hist-
amine,
which causes widening of blood
vessels, leakage of fluid into tissues, and
contraction of muscles, especially in the
airways of the lung. Symptoms can
include itching, swelling, sneezing, and
wheezing. Particular conditions asso-
ciated with Type I reactions include
asthma, hay fever,
urticaria
(nettle rash),
angioedema
,
anaphylactic shock
(a severe,
generalized allergic reaction), possibly
atopic
eczema,
and some food allergies.
TYPES II TO IV HYPERSENSITIVITY REACTIONS
Because Types II to IV hypersensitivity
reactions have different mechanisms to
Type I reactions, they are less often
implicated in allergies. However, contact
allergic dermatitis, in which the skin
reacts to prolonged contact with sub-
stances such as nickel, is the result of a
Type IV hypersensitivity reaction.
TREATMENT
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-
histamines have a sedative effect, which
is useful, for example, in treating itch-
ing at night due to eczema; many do
not cause drowsiness, making them
more suitable for daytime use.
Drugs such as
sodium cromoglicate
and
corticosteroids
can be used regularly to
prevent symptoms from developing.
Hyposensitization
can of valuable for a
minority of people who suffer allergic
reactions to specific allergens such as
bee stings. Treatment involves gradually
increasing doses of the allergen to pro-
mote formation of antibodies that will
block future reactions. Hyposensitiza-
tion must be carried out under close
supervision because a severe allergic reac-
tion can result. (See also
delayed allergy.)
allograft
Sometimes referred to as a homograft,
tissue or an organ transplanted from one
person to another. (See also
grafting.)
allopathy
The practice of conventional medicine.
(See also
homeopathy.)
allopurinol
A drug used in the long-term preven-
tion of
gout
attacks. Allopurinol works
by decreasing production of
uric acid
in
the body, thereby preventing the forma-
tion of uric acid crystals in the joints.
Possible adverse effects of allopurinol
include itching, rashes, and nausea. The
drug cannot relieve the pain of an acute
gout attack and may even precipitate one
at the start of treatment. Such attacks can
be prevented by taking a combination
of allopurinol and a
nonsteroidal anti-
inflammatory drug
(NSAID) or
colchicine.
almond oil
An oil prepared from the seeds of the
bitter almond tree (PRUNUS AMYGDALUS).
Almond oil is a common ingredient of
emollient
skin preparations.
almotriptan
A
serotonin agonist
drug that is used in
the treatment of acute migraine attacks.
aloe
The juice of leaves from various plants
of the ALOE genus. Aloe may be added to
compound benzoin tincture, an aromat-
ic
inhalation
for relieving
sinusitis
and
nasal congestion
.
alopecia
Loss or absence of
hair,
which may
occur at any hair-bearing site on the
body but which is usually noticeable
only on the scalp.
TYPES
Male-pattern baldness, the most com-
mon form of alopecia, is hereditary and
most commonly 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 of alo-
pecia are rare and may be due to
absence of hair roots or abnormalities
of the hair shaft.
Stages in male pattern baldness
In this common form of alopecia, the man first
loses hair from the temples and the crown, then the
bald area gradually widens.
In generalized alopecia, the hair falls
out in large amounts. Such hair loss
occurs when all the hairs simultaneous-
ly enter the resting phase; they then fall
out about three months later. Causes
include surgery, prolonged illness, or
childbirth. In such cases, the hair will
regrow without treatment. Many
anti-
cancer drugs
cause temporary alopecia.
The hair usually regrows when the
treatment is completed.
Localized alopecia may be the result
of permanent skin damage (for exam-
ple, following burns or
radiotherapy
) or
of trauma to the hair roots by styling
or, rarely,
trichotillomania
(a disorder in
which sufferers pull out their hair). The
most common type of localized hair
loss is alopecia areata, an
autoimmune
disorder
(in which the immune system
attacks the body’s own tissues). There is
no specific treatment for alopecia area-
ta, but the hair usually regrows within a
few months. Alopecia universalis is a
rare, permanent form of alopecia areata
that causes loss of all hair on the scalp
and body, including the eyelashes and
3 2
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