HISTAMINE
2
-RECEPTOR ANTAGONISTS
HOARSENESS
H
histamine2-receptor antagonists
See
H -receptor antagonists.
histiocytosis X
A rare childhood dis-
ease in which there is an overgrowth of
a type of tissue cell called a histiocyte.
The cause is unknown, but histiocytosis X
probably results from a disturbance of
the
immune system
.
In the mildest form,
rapid cell growth occurs in
1
bone only,
usually the skull, a
clavicle
, a rib, or a
vertebra
, causing swelling and pain. In
the most severe, and least common,
form, there is a rash and enlargement of
the
liver
,
spleen
,
and
lymph nodes
.
histocompatibility antigens
A group
of proteins that have a role in the
immune system
. Certain types of histo-
compatibility antigens are essential for
the immunological function of killer T
cells (see
lymphocytes
). The
antigens
act
as a guide for killer T cells to recognize
and kill abnormal or foreign cells.
The main group of histocompatibility
antigens is the human leukocyte antigen
(HLA) system, which consists of several
series of antigens. A person's tissue type
(the particular set of HLAs in the body
tissues) is unique, except for identical
twins, who have the same set.
HLA analysis has some useful appli-
cations. Comparison of HLA types may
show that 2 people are related, and it has
been used in
paternity testing.
The HLA
system is also used in
tissue-tysping
to
help match recipient and donor tissues
before
transplant surgery
Certain HLA
types occur more frequently in people
with particular diseases, such as
multiple
sclerosis, coeliac disease,
and
ankylosing
spondylitis.
HLA testing can help to con-
firm the presence of such diseases and
identify people at risk of developing them.
histology
The study of tissues, includ-
ing their cellular structure and function.
The main application of histology in
medicine is in the diagnosis of disease.
histopathology
A branch of
histology
concerned with the effects of disease on
the microscopic structure of tissues.
histoplasmosis
An infection caused by
inhaling the spores of the fungus
HISTO-
plasma capsulatum,
which is found in
soil contaminated with bird or bat
droppings. It occurs in parts of the
Americas, the Far East, and Africa.
history-taking
The process by which a
doctor learns from patients the symp-
toms of their illnesses and any previous
disorders. (See also
diagnosis
.)
HIV
The abbreviation for human immuno-
deficiency virus. HIV is a retrovirus (see
virus
) and is the cause of
AIDs
. There
are 2 closely related viruses: HIV-1, which
is the most common cause of AIDS
throughout the world; and HIV-2, which
is largely confined to West Africa.
HIV gains access to the body through
contaminated blood transfusions, non-
sterile needles, or sexual intercourse. A
fetus may be infected via the
placenta.
HIV has an affinity for the
T-lymphocytes
,
in which the virus multiplies and, in
some cases, destroys function. People
with HIV infection should have regular
monitoring in order to determine when
specific treatments, such as
antiretroviral
drugs
, are necessary.
HIV
hives
An alternative name for
urticaria.
HLA
The abbreviation for
human leuko-
cyte antigen.
HLA types
See
histocompatibility antigens.
hoarseness
A rough, husky, or croaking
voice. Short-lived hoarseness is often
due to overuse of the voice, which
strains the muscles in the
larynx
. It is
also commonly caused by inflammation
of the vocal cords in acute
laryngitis
.
Persistent hoarseness may be due to
chronic irritation of the larynx, which can
be caused by smoking, excessive con-
sumption of alcohol, chronic
bronchitis
,
284
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