the reading ability of a person who was
previously literate. (See also dyslexia.)
alienationFeeling like a stranger, even
when among familiar people or places,
and being unable to identify with a cul-
ture, family, or peer group. Alienation is
common in adolescents and also occurs
in people who are isolated by cultural
or language differences. In some people,
it may be an early symptom of schizo-
phreniaor a personality disorder.
alignment, dentalThe movement of
teeth by using either fixed or removable
orthodontic appliances(braces) to cor-
rect malocclusion(incorrect bite).
alimemazineAn antihistamine drug, also
known as trimeprazine, that is used
mainly to relieve itching in allergic con-
ditions such as urticariaand atopic
eczema. Alimemazine often causes
drowsiness.
alimentary tractThe tube-like struc-
ture that extends from the mouth to the
anus (see digestive system).
alkaliAlso known as a base, an alkali is
chemically defined as a donor of hydroxyl
ions (each of which comprises an atom
of hydrogen linked to an atom of oxygen
and has an overall negative electrical
charge). Antacid drugs, such as sodium
bicarbonate, are alkalis. Some alkalis,
such as sodium hydroxide, are corro-
sive. (See also acid; acid–base balance.)
alkaloidsA group of nitrogen-containing
substances obtained from plants. Mor-
phine, codeine, nicotine, and strychnine
(see strychnine poisoning) are examples.
alkalosisA disturbance of the body’s
acid–base balancein which there is an
accumulation of alkali or a loss of acid.
There are 2 types: metabolic and res-
piratory. In metabolic alkalosis, the
increase in alkalinity may be caused by
taking too much of an antacid drugor
by losing a large amount of stomach
acid as a result of severe vomiting. In
respiratory alkalosis, there is a reduc-
tion in the blood level of carbonic acid
(derived from carbon dioxide). This
reduction is a consequence of hyper-
ventilation, which may occur during a
panic attack or at high altitudes due to
lack of oxygen. (See also acidosis.)
alkylating agentsA class of anticancer
drugs.
alleleOne of 2 or more different forms of
a gene that occupies a specific position
on a chromosome(see gene; inheritance).
allergenA 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 shell-
fish); inhaled substances such as pollen,
house dust, and fur; and some drugs.
allergyVarious conditions caused by
inappropriate or exaggerated reactions
of the immune system (known as hyper-
sensitivity reactions) to a variety of
substances. Many common illnesses,
such as asthmaand allergic rhinitis(hay
fever), are caused by allergic reactions
to substances that in the majority of
people cause no symptoms.
Allergic reactions occur only on 2nd or
subsequent exposure to the allergen,
once 1st contact has sensitized the body.
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 the same anti-
gens, the antibodies and sensitized
lymphocytes interact with them, leading
to 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 allergensare misidenti-
fied as potentially harmful antigens.
The inappropriate or exaggerated re-
actions seen in allergies are termedALIENATION ALLERGY
A
ALLERGYHistamine
releasedAllergen
binds to
antibodyMast cellNucleusAntibody
attaches to
mast cellAllergenTYPE I HYPERSENSITIVITYHistamine