I
Allergic reactions caused by H, can be
controlled by
a n t i h i s t a m i n e d r u g s .
Over-
production of stomach acid due to H 2
can
be
counteracted
by
H 2-receptor
antagonists (see
u l c e r - h e a l i n g d r u g s ) .
histamine1-receptor antagonists
See
H ^ r e c e p t o r a n t a g o n i s t s .
histamine2-
receptor antagonists
S e e H 2 - r e c e p t o r a n t a g o n i s t s .
histiocytosis X
A rare childhood disease, now more
commonly known as Langerhans cell
histiocytosis, in w hich there is an over-
growth of a type of tissue cell called a
histiocyte. The cause is unknown, but
the condition probably results from a
disturbance of the
i m m u n e s y s t e m .
In the
mildest form, rapid cell growth occurs
in one bone only, usually the skull, a
c l a v i c l e ,
a rib,
or a
v e r t e b r a ,
causing
swelling and pain. In the most severe,
and least common, form, there is a rash
and enlargement of the
l i v e r , s p l e e n ,
and
l y m p h n o d e s ,
as well as lung involve-
ment. In these cases, treatment is with
a n t i c a n c e r d r u g s ,
but the outlook is poor.
histocompatibility antigens
A group of proteins that have a role in
the
i m m u n e s y s t e m .
Certain types of his-
tocompatibility antigen are present on
every cell in the body; they are essential
for the immunological function of
k i l l e r
T c e l l s ,
w hich help to defend the body
against disease. The antigens act as a
guide enabling the killer T cells to dis-
tinguish between self and nonself and
to kill abnormal or foreign cells.
The main group of histocompatibility
antigens is the human leukocyte anti-
gen (HLA) system, w hich consists of
several series of antigens. There are six
different
g e n e s
known as HLA-A, HLA-B,
HLA-C, HLA-DP, HLA-DQ, and HLA-DR.
Each of these has many different forms,
or
a l l e l e s ;
for example, HLA-B has over
4 0
different numbered alleles. A per-
son’s tissue type (the particular set of
HLAs in the body tissues) is unique,
except in the case of identical twins,
who have identical sets of HLAs.
HLA analysis has some useful appli-
cations. Comparison of HLA types may
show that two people are related, and it
has been used in
p a t e r n i t y t e s t i n g .
The
HLA system is also used in
t i s s u e - t y p i n g
to help match recipient and donor tis-
sues before
t r a n s p l a n t s u r g e r y .
Certain
HIV
HLA types occur more frequently in
people
with
particular
diseases.
For
example, HLA-B
2
7
is associated with
several forms of arthritis, particularly
a n k y l o s i n g s p o n d y l i t i s ,
and HLA-DR
2
and
an HLA-DQ type w ith
n a r c o l e p s y .
HLA
testing can help to confirm the diagno-
sis in someone who may have one of
these conditions.
histology
The study of tissues, including their cel-
lular structure and function. The main
application of histology in medicine is
in the diagnosis of disease. This process
often involves obtaining a sample of tis-
sue (see
b i o p s y )
and examining it under
a microscope to detect any abnormali-
ties, such as cancerous cells or areas of
scar tissue. Examination of tissue sam-
ples
may
also
be
performed
to
determine the extent of a disease, such
as cancer, once it has been detected. If a
cancerous tumour is found, for exam-
ple, it may be removed, together with
an area of surrounding tissue; by exam-
ining this material,
doctors
can tell
whether the whole of the diseased area
has been removed or whether cancer-
ous cells have spread beyond the edges
of the tumour.
histopathology
A branch of
h i s t o l o g y
concerned with
the effects of disease on the microscop-
ic structure of tissues.
histoplasmosis
An infection caused by inhaling spores
of
the
fungus
H
lSTO PtA SM A
C A PSU tA TU M
,
w hich is found in soil contaminated
with bird or bat droppings. Histoplas-
mosis occurs in parts of the Americas,
the Far East, and Africa. Treatment is by
intravenous infusion of
a n t i f u n g a l d r u g s .
history-taking
The process by w hich a doctor gathers
inform ation from
patients regarding
the symptoms of their illnesses and
details of any previous disorders. (See
also
d i a g n o s i s . )
histrionic personality disorder
A psychiatric disorder characterized by
exaggerated
emotional
reactions
and
attention-seeking
behaviour.
Affected
people also constantly demand praise or
reassurance, and require immediate sat-
isfaction of their demands. The disorder
usually first appears in early adulthood,
and is more common in women.
HIV
The abbreviation for human im m uno-
deficiency virus. HIV is a
r e t r o v i r u s
that
infects and gradually destroys cells in
the immune system and may eventually
lead to
A I D S
(acquired immunodeficien-
cy syndrome). There are two closely
related viruses: H IV -
1
, w hich is the
most common cause of AIDS through-
out the world; and HIV-
2
, w hich is
largely confined to West Africa.
METHODS OF TRANSMISSION
H IV is transmitted in body fluids, such
as blood, semen, and vaginal secretions.
It most commonly gains access to the
body during sexual activity (either vagi-
nal, anal, or oral), particularly when
contaminated body fluids come into
contact with broken skin. Other sources
of infection are nonsterile needles (for
example,
among
people who
abuse
intravenous drugs and share needles
and syringes) and, in some parts of the
world,
contaminated
blood
transfu-
sions. In addition, if a pregnant women
is infected, the virus can pass to the
fetus via the
p l a c e n t a .
It is not transmit-
ted by everyday physical contact such as
shaking hands or hugging.
EFFECTS OF THE VIRUS
H IV infects cells that
have a special
structure, called a CD
4
receptor, on their
surface. This includes immune system
cells called CD
4
lymphocytes, which
defend the body against cancerous and
infected cells, as well as certain cells in
other tissues, such as the brain. The virus
multiplies within the cells, killing them
in
the
process;
the
dead
cells
then
release more virus particles into the
blood. If the virus is untreated, the num-
ber
of CD
4
lymphocytes
falls. This
results in a reduced ability to fight off
infections and certain types of cancer.
HIV is extremely successful at w ith-
standing any attempt by the body to
destroy it. Every time HIV replicates, it
changes its
a n t i g e n
makeup,
thereby
ensuring that it is extremely difficult for
the body to mount an effective immune
response to it.
SYMPTOMS
Initially, some people who are infected
w ith
HIV
may
have
no
symptoms.
However, between six and eight weeks
after exposure, some people develop a
flulike illness similar to glandular fever
(see
m o n o n u c l e o s i s ,
i n f e c t i o u s ) ,
with
fever, fatigue, sore throat, aching mus-
cles, and swollen
l y m p h
n o d e s .
These
symptoms usually clear up after a few
weeks. Some people may then develop
H
383
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