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CLINICAL
Fig 2. Eosinophil
Fig 3. Basophil
Fig 4. Monocyte
SPL, Mediscan
Fig 1. Neutrophil
Part three – Granulocytes and monocytes
Jonny Zygo
Blood cells
systems of life
Macrophages engulfing
Escherichia coli (left) and a
microphage with an extended
pseudopod, with which it
engulfs micro-organisms and
cellular debris
author Ken Campbell, MSc Clinical Oncology, is
clinical information officer, Leukaemia Research
Fund (written in a private capacity).
This article, the third in a series on blood cells,
describes two types of white blood cells: granulocytes
and monocytes. These along with lymphocytes,
which will be discussed in the next article in this
series, are responsible for the immune response.
Myeloid lineage white blood cells
As described in the first article in this series
(Campbell, 2005), all blood cells can be defined
as belonging to the lymphoid or myeloid lineage.
Apart from lymphocytes and their precursors and
derivatives, all white blood cells belong to the
myeloid lineage. These myeloid lineage white
cells are:
l Granulocytes – which encompass neutrophils,
eosinophils and basophils;
l Monocytes – which migrate to tissues and
transform into macrophages;
l Dendritic cells – these are tissue cells that are not
produced directly within the marrow, but are derived
from marrow cells and are key components of the
immune system.
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Granulocytes
Neutrophils
Neutrophils (Fig 1) are the most numerous type of
white blood cell, the normal neutrophil count is 2.5–
7.5 x 109 per litre of blood. Their lifespan in the
circulation is about 6–12 hours.
At any given time about two-thirds of the
neutrophils in the circulation are attached to the walls
of the blood vessels – these are known as the
marginal pool, which allows a rapid rise in
neutrophils in response to an infection or other
stimulus. Once they enter the tissues, neutrophils last
for 2–4 days. They cross the intact vascular
endothelium by an active process called diapedesis.
Neutrophils congregate at sites of tissue damage in
a reaction called chemotaxis. They respond to signals
released by bacteria, damaged white cells and dead
or dying tissue cells.
Neutrophils are phagocytes – their principal
function is to ingest and destroy bacteria and other
simple organisms. They are not active against
intracellular organisms such as viruses, and are not
potent enough to attack more complex organisms
such as Mycobacterium tuberculosis.
Neutrophils also ingest and break down cellular
debris both from necrosis and from apoptotic cells
NT 18 October 2005 Vol 101 No 42 www.nursingtimes.net
keywords n Blood cells n Monocytes n Granulocytes
(normal programmed cell death). Pus is a ‘soup’ of
degraded neutrophils with bacteria and damaged
cells that have been broken down by the neutrophils.
Neutrophils are:
l Increased in – bacterial infection, trauma, surgery,
burns, haemorrhage, inflammation, infarction,
polymalgia, myeloproliferative disorders and with
some drugs;
l Markedly increased in – leukaemia, disseminated
malignancy and severe childhood infections;
l Decreased in – viral infections, brucellosis, typhoid,
kala-azar, tuberculosis, hypersplenism, bone marrow
failure syndromes and also with some drugs,
especially cancer chemotherapy.
Eosinophils
Eosinophils (Fig 2) are less numerous than
neutrophils – the normal absolute eosinophil count is
0.04–0.44 x 109/L. They remain in the circulation for
longer than neutrophils.
The principal targets of eosinophils are persistent
antigens on objects too large to be engulfed;
whereas a neutrophil will ingest an invader,
eosinophils attack using cytolytic enzymes.
Eosinophils are enticed to enter the tissues by IgG
and IgE antibodies and complement components
bound to an invader, typically a multicellular parasite.
Their numbers are classically very high in parasitic
infestations and allergic responses. Persistent
eosinophilia should always trigger a search for
evidence of parasitic infection.
Eosinophils are:
l Increased in – conditions such as asthma and allergic
disorders, parasitic infestations, skin disease,
malignant disease, and during recovery from infection;
l Decreased in – acute stress or infection and steroid
therapy/Cushing’s syndrome.
Basophils
Basophils (Fig 3) are the least common, and probably
the least understood type of granulocytes. A normal
basophil count is 0.01–0.1 x 109/L. In a healthy blood
film one typically sees very few or no basophils.
The activity of basophils may facilitate access of
lymphocytes and other cellular players in the
inflammatory response to tissues. There is a
reduction in circulating basophil numbers during
flare-ups of chronic urticaria, which is thought to
reflect active recruitment into weals (Grattan et al,
2003). There is evidence that basophils may have an
immunoregulatory role in some situations (Galli, 2000).
Basophils are:
l Increased in – viral infections, urticaria,
myxoedema, post-splenectomy ulcerative colitis,
chronic myeloid leukaemia and other malignancies;
l Decreased in – acute stress/infection, steroid
therapy/Cushing’s syndrome and chronic urticaria
(during flare-ups).
NT 18 October 2005 Vol 101 No 42 www.nursingtimes.net
Monocytes
Monocytes (Fig 4), and their derivatives macrophages
and dendritic cells, are antigen-presenting cells
(APCs). Lymphocytes can only mount an effective
response to an antigen when it is presented on the
surface of an APC alongside appropriate costimulatory
molecules. This mechanism reduces the risk of
harmful autoimmune responses and allows tolerance
for commensal organisms (Banchereau, 2002).
As well as processing antigens, monocytes ingest
and eliminate micro-organisms such as
Mycobacterium tuberculosis, which have tough waxy
cell coatings that neutrophils cannot break down.
Despite their well-deserved reputation as the
‘heavies’ of the immune system, some organisms
hide from the immune system within the
macrophage population; these include certain
mycobacteria, protozoa, Histoplasma capsulatum and
HIV (Jandl, 1991).
Normal monocyte count is about 0.2–0.8 x 109/L.
They spend about 1.5 days in the circulation after
leaving the marrow then move into the tissues as
macrophages or dendritic cells, where they reside for
months, or possibly years (Hoffbrand et al, 2001).
Monocytes are:
l Increased in – acute and chronic infections (such as
tuberculosis, brucellosis, protozoa), malignant disease
and myelodysplasia;
l Decreased in – steroid therapy and hairy cell
leukaemia.
Macrophages and dendritic cells
On leaving the circulation monocytes are transformed
into either tissue macrophages or dendritic cells, both
of which migrate through their host tissue ingesting
and destroying micro-organisms and cellular debris
and presenting antigens (self and non-self) to cells of
the immune system.
Tissue macrophages have many names because
they were independently identified by early
anatomists. Modern techniques reveal they belong to
the same cell population.
Tissue macrophage populations include the following:
l Kidney – intraglomerular mesangial cells;
l Liver – Kupffer cells;
l Lung – alveolar macrophages;
l Spleen – sinus macrophages;
l Serosal membranes – serosal macrophages;
l Bone marrow – bone marrow macrophages;
l Lymph nodes – lymph node macrophages.
In some cases, monocytes are transformed by
developing cytoplasmic processes giving them a starlike appearance and are termed dendritic cells. These
principally derive from myeloid or monocytic cells; but
at least one type of lymphoid dendritic cell has been
reported (Shortman and Caux, 1997). Whether dendritic
cells of different origin have different functions is still
being debated (Szabolcs et al, 2003). n
References
Banchereau, J. (2002) The long arm
of the immune system: dendritic
cells. Scientific American; 287:
5, 34–41.
Campbell, K. (2005) Blood cells.
Part one – Bone marrow. Nursing
Times; 101: 40, 28–29.
Galli, S.J. (2000) Mast cells and
basophils. Current Opinion in
Hematology; 7: 1, 32–39.
Grattan, C.E.H. et al (2003) Blood
basophil numbers in chronic
ordinary urticaria and healthy
controls: diurnal variation, influence
of loratadine and prednisolone and
relationship to disease activity.
Clinical and Experimental Allergy;
33: 3, 337–341.
Hoffbrand, A.V. et al (2001) The
white cells 1: granulocytes,
monocytes and their benign
disorders. in: Hoffman, A.V. et al
(eds) Essential Haematology;
Oxford: Blackwell Science.
Jandl, J.H. (1991) Monocytes,
macrophages and the mononuclear
phagocytic system. In: Jandl, J.H.
(ed.) Blood: pathophysiology.
Oxford: Blackwell Scientific.
Shortman, K., Caux, C. (1997)
Dendritic cell development: multiple
pathways to nature’s adjuvants.
Stem Cells; 15: 409–419.
This article has been double-blind
peer-reviewed.
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