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White blood cells use the blood as a means of transport from their origination in the bone marrow to their major sites of activity. The majority of the functions of the white blood cells occur when they leave tthe blood circulation to enter other body tissues.
There are five types of white blood cell
The figures show the relative proportions of the different types of white blood cell. The reason for the range of figures shown is that the requirement for different types of white blood cell will vary from time to time.
Neutrophils, eosinophils and basophils are collectively known as granulocytes due to prominent granules in their cytoplasm.
Lymphocytes and monocytes are classed as white blood cells because they are a constituent of blood and ultimately originate from the bone marrow. However they are mainly found in structures such as the lymph nodes and the spleen
Neutrophils are the most common granulocyte. They have segmented nuclei, typically with 2 to 5 lobes connected together by thin strands of chromatin which can be difficult to see; the cell may thus appear to have multiple nuclei. The nuclear chromatin is condensed into coarse clumps. Small numbers of immature neutrophils or band form neutrophils may be seen in a blood smear. These are incompletely segmented and often have a 'C-shaped' nucleus.
The cytoplasm of neutrophils contains three types of granule.
Primary granules are non-specific and contain lysosomal enzymes, defensins, and some lysozyme. The granules are similar to lysosomes. They stain aviolet colour when prepared with Wright's stain which is commonly used in studying the blood. The enzymes produce hydrogen peroxide which is a powerful anti-bacterial agent.
Secondary granules are specific to neutrophils and stain light pink ('neutral stain'). They contain collagenase, to help the cell move through connective tissue, and lactoferrin, which is toxic to bacteria and fungi.
Tertiary granules have only recently been recognised. It is thought that they produce proteins which help the neutrophils to stick to other cells and hence aid the process of phagocytosis.
Once in the area of infection neutrophils respond to chemicals (called chemotaxins which are released by bacteria and dead tissue cells) and move towards the area of highest concentration. Here they begin the process of phagocytosis in which they engulf the offending cells and destroy them with their powerful enzymes. Because this process consumes so much energy the neutrophils glycogen reserves are soon depleted and they die soon after phagocytosis. When the cells die their contents are released and the remnants of their enzymes cause liquefaction of closely adjacent tissue. This results in an accumulation of dead neutrophils, tissue fluid and abnormal materials known as pus.
Eosinophils have a bi-lobed nucleus as shown above in a slide of a stained blood smear.
They increase greatly in many types of parasitic infection and defence against the larvae of parasitic worms and unicellular organisms seems to be one of their primary functions. The granules of eosinophils contain a substance called MBP (major basic protein) which is toxic to many parasitic larvae. Eosinophils also have surface receptors for the antibody immunoglobulin E (IgE). These receptors are not found in neutrophils and again this is thought to reflect their role in parasitic infection.
They also increase in number in some allergic states. For example their numbers increase in the nasal and bronchial mucosal linings in hay fever and asthma and in some adverse drug reactions. It is thought that they may neutralise the effect of histamine.
Eosinophils also have a marked diurnal variation with their numbers being highest in the morning and lowest in the afternoon although why this is the case is at present unclear.
Basophils are the least common of the white blood cells. They are characterised by their large cytoplasmic granules which obscure the nucleus in stained preparations as shown above. They have many similarities with mast cells and actually become mast cells on leaving the blood and entering surrounding tissues.
Mast cells are widely distributed throughout the body and are commonly found in close proximity to the walls of small blood vessels.
Both basophils and mast cells have highly specific receptors for IgE produced in response to various allergens.
Response to specific allergens is rapid and results in expulsion of the cells granular contents which contain histamines and other vasodilating agents. This results in the reaction known as immediate hypersensitivity. This can result in hay fever, some forms of asthma, urticaria (nettle rash) and most seriously anaphylactic shock.
Monocytes are the largest cell type seen in blood smears. Their nuclei are not multilobular like granulocytes, but may be deeply indented or U-shaped, with reticular-appearing chromatin. The cytoplasm of monocytes contains numerous lysosomal granules which give it a uniform grayish-blue "ground-glass" appearance. They form part of a cell network known as the monocyte-macrophage system. This comprises bone marrow precursor cells (monoblasts and promonocytes), circulating monocytes and both free and fixed tissue macrophages.
Monocytes eventually leave the bloodstream to become tissue macrophages which remove dead cell debris as well as attacking organisms such as Tubercule Bacilli (which causes TB) and some fungi. Neither of these can be dealt with effectively by the neutrophils. Unlike neutrophils monocytes are able to reolace their lysosomal contents and are thought to have a much longer active life.
Cells which derive from monocytes include the
These are the most numerous white blood cell in young children and the second most numerous in older children and adults.
Their numbers increase in response to viral infections.
Lymphocytes are distinguished by having a deeply staining nucleus which may be eccentric in location, and a relatively small amount of cytoplasm. The small ring of cytoplasm contains numerous ribosomes and stains blue. Small numbers of lysosomal granules may also be seen in the cytoplasm of some lymphocytes.
In this image, a lymphocyte and a dumbbell-shaped Red Blood Cell (RBC) can be seen in the lumen of a small blood vessel. Note the pseudopodia and the small amount of cytoplasm of the lymphocyte.
The two major types of lymphocyte found in the blood are B-lymphocytes and T-lymphocytes. Both have different but linked roles in the generation of specific immune response. The small mature lymphocytes circulating in the blood are constantly sampling their environment for foreign materials. Their role is discussed more fully in the sessions on the immune system.
Increased numbers of white blood cells appear in the peripheral blood in a variety of disorders.
The most important and most life threatening disorders are the leukaemias. Here there is a malignant proliferation of white cell precursors in the bone marrow. This produces vast numbers of white blood cells and their precursors which then spill over into the the blood stream. The various types of leukaemia are classified according to the cell line involved (granulocytic, monocytic, lymphocytic etc.) and also according to their degree of malignancy.
The slide on the right shows a blood sample from a patient suffering acute
monocytic leukaemia. Note the proliferation of the darker monocytes
In chronic leukaemias the proliferating cells are more differentiated (i.e. they have more closely reached maturity) whereas in acute leukaemias the proliferating cells are the virtually undifferentiated precursor cells such as lymphoblasts in acute lymphoblastic leukaemia.
The image above shows a slide of a blood smear from a patient suffering from acute lymphoblastic leukaemia. Note the prolifretaion of lymphoblastic cells as compared to other more normal blood slides in these pages.
The slide above shows a blood smear from apatient suffering from chronic lymphocytic leukaemia. Note the lymphocytes (x) and smudge cells (y).
The slide on the left shows a blood sample
taken from a patient suffering chronic myelogenous leukaemia. Note the proliferation of
various types of white blood cell.
This page last updated Wednesday, 28 April 1999 20:53 +0100
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