BLOOD CELLS
DISORDERS OF THE BLOOD
Abnormalities can occur in any of the
components of blood: in red blood
cells, white blood cells, platelets, and
the numerous constituents of plasma.
There are various types of
anaemia
(a reduced level of the oxygen-carrying
pigment haemoglobin in the blood),
which is by far the most common
blood disorder. Some abnormalities of
the blood are inherited; others may be
the result of various diseases, such as
cancer, or be caused by poisoning by
infective organisms, toxins, or drugs.
Genetic disorders
Some blood disorders are inherited
(genetic) and are present from birth
(congenital). Such disorders include
sickle cell anaemia
and
thalassaemia,
in which the red blood cells are
abnormally fragile, and
haemophilia,
in which there is a deficiency of one
of the blood clotting factors.
Sickle-cell anaemia
This electron micrograph shows a red blood cell
deformed by sickle-cell anaemia (left). On the
right is another red cell that has started to sickle.
Nutritional disorders
Heavy or persistent blood loss, most
commonly as a result of menstruation,
may mean that iron (an essential
component of the red cell pigment
haemoglobin) is lost faster than it can
be replaced in the diet (see
anaemia, iron-
deficiency).
Deficiencies of the vitamins
B
12
or folic acid interfere with the
production of red blood cells in bone
marrow and give rise to abnormally
large, deformed red blood cells (see
blood,
anaemia, megaloblastic).
Cancer
There are various types of bone marrow
cancer, all of which affect the blood.
Leukaemia
causes an overgrowth of
abnormal white blood cells and destroys
healthy bone marrow. In
polycythaemia,
too many red blood cells are produced.
Another bone marrow cancer,
multiple
myeloma,
can cause an excess of certain
proteins in the blood plasma. Secondary
deposits that have spread from cancers
elsewhere in the body may also involve
the bone marrow.
Clotting disorders
Defects in the blood platelets and in blood
clotting mechanisms may lead to
bleeding
disorders,
such as haemophilia and
disseminatedintravascularcoagulation (DIC).
Liver disease may cause deficiencies of
some clotting factors. Unwanted clot
formation (see
thrombosis
) may have any
of a variety of causes, such as a mutation
in the gene that controls production
of a clotting factor (see
factor V,
for
example) or use of oral contraceptives.
People with
Hughes' syndrome
are also at
increased risk of thrombosis.
Other disorders
Blood poisoning may be caused by the
multiplication of bacteria in the blood
(see
septicaemia
) or by the toxins
released by bacteria (see
toxaemia).
Poisoning can also be caused by toxins
such as carbon monoxide and lead.
Some drugs can cause blood
abnormalities. For example,
thiazide
diuretic drugs
may depress
the production of white blood cells
and/or platelets;
methotrexate
may
interfere with red cell production;
and too high a dose of
anticoagulant
drugs
can cause abnormal bleeding.
Albumin, which is an important
protein in blood plasma, may become
deficient as a result of either liver or
kidney disease.
INVESTIGATION
Blood disorders are investigated
principally by various blood tests,
such as
blood count, blood film,
and
blood-clotting tests.
Levels o f vitamins
and minerals, such as iron, may
also be measured. In some cases, a
bone marrow
biopsy
may also
be required.
B
They circulate in the bloodstream for
about one to three days.
Lymphocytes Lymphocytes are usually
formed in the lymph nodes, rather than
in the bone marrow. They play an
important role in the immune system,
roving throughout the body between the
bloodstream, the lymph nodes, and the
channels between lymph nodes. Lym-
phocyte cells may survive for anywhere
between three months and ten years.
There are two principal types
of
lymphocyte: T-lymphocytes (or T-cells)
and
B-lymphocytes
(or
B-cells).
T-
lymphocytes are responsible for delayed
hypersensitivity reactions (see
allergy)
and are also involved in protection
against cancer. T-lymphocytes manufac-
ture
chemicals,
called
lymphokines,
which affect the functioning of other
cells. T-lymphocytes can be classified
according to their surface marker pro-
teins. For example, T-lymphocytes with
CD4 surface marker proteins are partic-
ularly important in monitoring HIV
infection. In addition, T-lymphocytes
moderate the activity of B-lymphocytes,
which form the antibodies that can act
to prevent a second attack of certain
infectious diseases.
Disorders The
leukaemias
are
blood disor-
ders
in which there is uncontrolled
overproduction of white blood cells in
the bone marrow. Other disorders arise
when white blood cells are not pro-
duced in sufficient numbers.
PLATELETS
Platelets, which are also called thrombo-
cytes, are the smallest type of blood cell
(0.002 mm to 0.003 mm in diameter).
There are about 250,000 of them per
cubic mm of blood. Like other blood
cells, they originate in the bone mar-
row. Platelets survive in the blood for
about nine days.
Function Platelets circulate in the blood
in an inactive state until brought into
action by certain circumstances, when
they begin to stick to blood-vessel walls
and to each other. These activities play
an
important part
in
blood clotting,
which helps wounds to heal. However,
the accumulation of platelets can, occa-
sionally, lead to the formation of clots
in blood vessels (see
thrombosis).
BLOOD CELLS IN
DIAGNOSIS
The numbers, shapes, and appearance
of the various types of blood cell are of
great value in the diagnosis of disease
(see
blood count; blood film).
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