Essay on White Blood Corpuscles (WBC) | Blood Cells | Biology

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1. Essay on the Introduction to White Blood Corpuscles
2. Essay on the Development of White Blood Corpuscles
3. Essay on the Functions of White Blood Corpuscles
4. Essay on the Count of White Blood Corpuscles
5. Essay on the Life and Fate of White Blood Corpuscles

Essay # 1. Introduction to White Blood Corpuscles:

White blood corpuscles (W.B.C.) are an important variety of cells in the blood. These cells differ from the red cells in many respects.

For instance:

i. They do not contain any haemoglobin.

ii. They are bigger in size.

iii. They are nucleated, living cells.

iv. They are actively amoeboid.

v. They are much less in number.

vi. Their span of life is shorter.

vii. Their origin is purely from extravascular tissue.

viii. Their functions are absolutely different from those of red cells.

ix. There are several varieties of leucocytes, whereas red cells are only of one variety.

Chemistry:

White blood corpuscles are rich in nucleoprotein and also contain lipids, glycogen, cholesterol, ascorbic acid and a variety of enzymes, specially proteolytic.

Total Number:

The average total number of white cells is 6,000 to 8,000 per cu. mm, the normal range being 4,000-11,000 per cu. mm. The average ratio of the total white cell count with the total red cell count is about 1: 700; i.e., for, one White blood corpuscles there are 700 R.B.C.

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Essay # 2. Development of White Blood Corpuscles:

Monophyletic and Polyphyletic Theories:

Broadly, there are two views regarding the development of all blood cells in postnatal life:

(a) The monophyletic theory, and

(b) The polyphyletic theory.

The monophyletic theory holds that all the blood cells are derived from a common primitive ancestor, which is called the ‘stem cell’ or haemocytoblast. The polyphyletic school, in its complete form, holds that for every variety of blood cell, there is a distinct type of ‘blast cell; viz., erythroblast, myeloblast, lymphoblast, monoblast, etc. The dualistic school believes that there are two distinct types of primitive cells.

One of them remains in the bone marrow and gives rise to the red cells, granulocytes and megakaryocytes, from which the platelets develop. The other cell remains in the lymphoid tissue from which the lymphocytes are derived. Similarly, there is a trialistic school believing in three primitive cells. All school agrees that in the early embryo all the blood cells are derived from a single primitive reticulo-endothelial cell.

It is generally conceded that, in postnatal life, the development of leucocytes is completely extravascular. The gran­ulocytes are derived exclusively from the red marrow; while the lymphocytes and the monocytes come mainly from the spleen and lymphatic glands and to some extent, from the bone marrow.

Development of Granular Leucocytes or Granulocytes:

1. Reticulum Cell of the Bone Marrow:

According to the monophyletic theory this stage starts with haemocytoblast.

2. Primitive White Blood Cell:

Cells are large with a large number of round nucleus and a thin rim of basophilic, non-granular cytoplasm.

Myeloblast Smaller in size (12 to 18 µ), non-granular cytoplasm and a round or bean-shaped nucleus with several nucleoli. There are numerous mitochondria. They are non-motile or less motile. They di­vide and form the next stage. It should be noted that up to this stage the cytoplasm has no gran­ules when stained with the ordinary Romanowsky methods.

By the peroxidase method granules may be found in the more mature of the myeloblast. The number of nuclei is an important differ­entiating point from lymphoblasts. The latter have definite and distinct nucleoli whereas in the, former, the nucleoli look like irregular gaps

3. Myelocyte:

This stage is characterised by certain remarkable peculiarities which are quite distinct from the previous stages.

The peculiarities are as follows:

(a) Multiplicity although all the cells before this stage are multiplying, yet the multiplicity of the myelocytes is maximum. It is by the multiplica­tion of the myelocytes that the normal supply of the white cells is kept up.

(b) Granules appear in the cytoplasm.

The granules may take neutral, acid or basic stains and accordingly the myeloytes are of three varieties:

(1) Neutrophilic myelocyte,

(2) Eosinophilic myelocyte,

(3) Basophilic myelocyte.

Recent observations show that each type of myelocyte passes through three stages of develop­ment—A, B, and C. Type A contains few granules and many mitochondria. Type C contains many granules but few mitochondria.

Type B is intermediate:

(c) Cytoplasm is less basophilic,

(d) Nucleoli disappear and chromatin is coarser.

4. Metamyelocyte:

At this stage, again certain characteristic changes appear:

(a) The nucleus becomes deeply indented and is almost bilobed. This shows that maturation is advancing,

(b) The cells no more multiply but only mature henceforward,

(c) Amoeboid movement appears and due to this, a few of these cells may burrow into the blood vessels, and

(d) Appear in the peripheral circulation.

5. Leucocyte:

The nucleus contains many lobes which are produced by maturation of the previous stage. The maturation takes place in the bone marrow. Because once in the peripheral circulation, no more maturation can take place. It is believed that the degree of maturation is proportional to the number of lobes of the nucleus.

Development of Lymphocytes:

In the central part of lymph node, there is a small area about 1 mm in diameter and pierced by a small blood vessel. This is called the germinal centre or secondary lymphoid follicle. It takes a lighter stain than the surrounding area. Some of the reticulum cells in this area proliferate and give rise to the lymphoblasts.

In recent years, evidence has been accrued and shows that the thymus is an important and probably the main source of production of lymphocytes. Apparently, there seem to be two types of lymphocytes in the peripheral blood, a longer-lived type derived from the thymus, and a shorter-lived type, probably arising from the lymphoid tissue.

The stages of proliferation are the same. The lymphoblast is 15 to 20 µ in diameter, with a round or oval nucleus and a non-granular cytoplasm. These cells divide and give rise to the large lymphocyte. The normal number of lymphocytes in the blood is maintained chiefly by the division of the lymphoblast cells.

The large lymphocytes are fairly mature cells and do not multiply any further. These cells are found in the circulation. The small lymphocytes are derived by further maturation of the large lymphocytes. These cells leave the gland through the efferent lymphatics, thoracic duct and right lymphatic duct, and reach the circulation.

Development of Monocytes:

The monocytes are developed mainly from the reticulum cells of spleen and lymph nodes and to a lesser extent of the bone marrow.

The stages are:

Reticulum cells → Monoblasts → Monocytes.

Monocytes can be differentiated from the myeloblast by positive peroxidase reaction in the latter. The so-called ‘transitional leucocytes’ are one variety of monocytes.

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Essay # 3. Functions of White Blood Corpuscles:

i. Phagocytosis:

The neutrophil polymorphonuclear leucocytes and the monocytes engulf foreign particles and bacteria, and generally digest them. This process is called phagocytosis. When the bacteria invade the body, the leucocytes pass out of the blood vessels and surround the threatened area and through their pseudopodial processes engulf the bacteria and destroy them. The eosinophils and lymphocytes have got slight phagocytic action. Migrations of lymphocytes occur in chronic inflammation.

The neutrophil polymorphonuclear leucocytes manufacture a trypsin-like enzyme with which they digest the bacteria and the dead tissue. Due to this, the dead tissue in an inflammatory area becomes liquified and the so-called pus is formed. Lymphocytes and monocytes are supposed to elaborate pepsin like enzyme by which they partially liquefy the solid dead tissue, thus converting them suitable for phagocytosis and removal.

ii. Antibody Formation:

Lymphocytes manufacture β-and γ-fractions of serum globulin. Immune bodies are associated with γ-globulin fraction. So the lymphocytes play in important role in the defensive mechanism of the body of an immunological nature. Adrenal cortical steroids are responsible for dissolution of lymphocytes and so increase in antibody concentration in blood.

iii. Formation of Fibroblasts:

It is believed that the lymphocytes may be converted into fibroblasts in an area of inflammation and thus help the process of repair.

iv. Manufacture of Trephones:

Leucocytes manufacture certain substances from plasma proteins which exert great influence on the nutrition growth and repair of tissues. These substances are called trephones.

v. Secretion of Heparin:

The basophil leucocytes are supposed to secrete ‘heparin’; which prevents intravascular clotting.

vi. Antihistamine Function:

The granulocytes, specially the eosinophil cells, are very rich in histamine. They are believed to defend against allergic conditions, in which histamine like bodies are produced in excess.

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Essay # 4. Count of White Blood Corpuscles:

Normal Count of White Blood Corpuscles:

i. Diurnal Variation:

The total count varies from day to day and even from hour to hour. Sometimes, such variations are found to be random and without any obvious cause. In the morning or after rest, the count is lowest. The count rises after midday and usually becomes highest in the evening.

ii. Muscular Exercise:

Muscular exercise and any condition leading to asphyxia (O₂lack and CO₂ excess) increase the total count.

iii. Relation with Meal:

Formerly it was believed that there is leucocytosis after meals, which is now thought to be only a diurnal variation.

iv. Injection of Adrenaline:

Injection of adrenaline increases the count.

v. Emotional Stress:

Fear, pain, etc., due to liberation of adrenaline.

vi. Age:

In the new-born, the count is very high, about 20,000 per cu. mm. After second week, the count starts falling, but throughout infancy and childhood, the count remains proportionally higher. During infancy and childhood, the lymphocytes constitute about 40-50%.

vii. Relation with Pregnancy and Labour:

In pregnancy at full term the count is higher, being highest (17,000) during labour. There is also an increase during menstruation.

viii. Increased Cellular Destruction:

Increased cellular destruction from infections, surgical operations, etc., produces derivatives of nucleic acid and causes a rise in neutrophil leucocytes.

ix. Asthma, Hay Fever, Skin Disease:

Asthma, hay fever, skin disease etc., and also after hostile invasion with parasites there is an increase in eosinoophils (eosinophilia).

x. Adrenal Cortical Steroids and ACTH:

Adrenal cortical steroids and ACTH (adrenocorticotrophic hormone) cause an increase in neutrophill and decrease in lymphocytes and eosinophils. The fall in eosinophil is very specific for this hormone, and is used as an assay method.

xi. Starvation and Administration:

Starvation and administration of certain chemicals like benzene, sulphonamide, etc., produce leucopenia (fall in leucocytes).

xii. Leucopenia:

Also occurs after bacterial, viral or protozoal infection and also in aplastic bone marrow disease or after exposure to ionising radiation.

It is generally agreed that stores of white blood corpuscles remain in various parts of the body and under different conditions they may rapidly appear and disappear from the blood.

Classification and Differential Count of White Blood Corpuscles:

There are several varieties of white blood corpuscles, each type possessing characteristic morphology and staining property. Determination of the percentage of different varieties of leucocytes is known as the dif­ferential count of white blood corpuscles.

The classification and the differential count, as generally accepted, are as follows:

Both lymphocytes and monocytes are derived from the reticulo-endothelial system although a few lym­phocytes may be formed in the bone marrow.

1. Granular Leucocytes or Granulocytes:

Cells with granular cytoplasm they are formed in the bone marrow from the time of birth onwards. The gran­ules may take either of the three different stains, neutral or acid or basic. Consequently this group includes the three varieties as listed in Table 4.3.

(a) Neutrophil:

It is about 10-12 µ in diameter. Most numerous in the adult blood, about 60-70%. The nucleus is many lobed; the number of lobes varies from 2 to 7 or more. The youngest cell has a single nucleus. The number of lobes increases with the degree of maturity of the cells. Due to presence of lobes in the nuclei these cells are called polymorphs.

In the peripheral blood, the cells with three-to four-lobed nuclei are the most numerous. The cyto­plasm contains fine neutrophilic granules which appear pale-violet with Leishman’s or Giemsa’s stain. They are actively amoeboid and phagocytic. The neutrophil leucocytes contain a variety of enzymes like phosphatase, nucleotidase, protease, amylase, lipase, etc.

They also contain ascorbic acid, glutathione and glycogen. Metabolically these cells are much more active than erythrocytes. Some physiologists have described certain sex differences in the nuclei of neutrophil (polymorphonuclear).

The sex chromatin body is a small mass, usually adjacent to the nuclear membrane which stains deeply with haematoxylin, and is attached to the nucleus by a slender thread, giving a drumstick-like appearance. This sex difference is found in polymorphs of about 17% of normal females and is absent in the males.

Arneth count or Arneth index (modified by Von Bonsdorff and later by Cooke):

Following the idea that the age of the neutrophils is proportional to the number of lobes in their nuclei, Arneth (1904) divided them into five groups. The Arneth index can be determined by counting the number of nuclear lobes in each of hundred neutrophils.

Of 100 neutrophils counted the proportion of different groups are as follows; Group I (One-lobed nucleus-round, oval, indented or horseshoe-shaped) -5-10%, Group II (two -lobed nucleus) -25-30%, Group III (Three-lobed nucleus)—45-47% Group IV (Four lobed nucleus) —16-18%, Group V (Nucleus with five or more lobes) -2%. Cells with three-lobed nuclei are maximum in number, fully mature and functionally most efficient (Fig. 4.12).

Cells with Less nuclear lobes are immature, while cells with more lobes are increasingly senile. A shift to the left ([Group 1 + Group II] > 45%) means a relative or absolute increase of immature cells and consequently a less efficient leucocyte defensive system. A shift to the right means diminished leucopoiesis with the increase of hypermature and senile cells.

Schilling Index:

It is simpler classification of neutrophils into four groups such as:

(a) Myelocyte having only one lobe,

(b) Juvenile metamyelocyte in which the nucleus is indented,

(c) Staff cell or band cell of Schilling having unsegmented but T-, V- or U-shaped nuclei. It is an older metamyelocyte and corresponds to the first stage of Arneth, and

(d) Older or segmented neutrophils.

In response to acute need certain younger forms of neutrophils such as myleocyte and juvenile type are used to come out from the bone marrow. This has been indicated by Schilling as regenerative shift.

Another concept the degenerative shift to the left is referred to the failure of the neutrophils to mature as the result of the depression of bone marrow function. In this type of shifting, an increased number of immature forms is found in the blood. It is assumed that these cells can never mature.

(b) Eosinophil:

It is about 10-12µin diameter. The nucleus commonly possesses two lobes and stains less deeply than those in the polymorphonuclear cells. The cytoplasm contains coarse granules, oval or round in shape, taking acid stain and showing red colour with eosin.

The cell membrane seems to be very delicate and is often found to be broken in a blood film. The eosinophils are amoeboid, but not phagocytic. They contain variable amount of histamine. By some unknown mechanism the cells are drawn towards the tissue concentration of histamine and inactivate it. They also counteract with 5-hydroxytryptamine (5-HT).

(c) Basophil:

Their size is somewhat smaller, with a diameter of about 8-10 µ. The nucleus is usually kidney-shaped or only slightly lobulated. The cytoplasm contains a large number of round granules which take deep basophilic stain. The granules are less numerous than in the eosinophils and very much in size.

Unlike the eosinophils, the granules are often found to overlap the nucleus and obscure the details of its morphology. They are actively amoeboid but are less so than the other varieties of granulocytes. They are believed to belong to cells of the reticulo-endothelial system.

The bosophils are similar to connective tissue mast cells and like them exhibit metachromasia, the granules containing both histamine and heparin and also 5-hydroxytryptamine (5-HT). They possibly have some role in local anticoagulation and formation of ground substance. The increase of granulocytes in the blood is called as granulocytosis. Diminution of granulocytes in the blood is known as granulocytopenia. Complete disappearance of granulocytes is known as agranulocytosis.

2. Lymphocytes:

(a) Small Lymphocyte:

It is slightly larger than a red cell; the diameter being about 7.5 µ. The nucleus is relatively large and occupies the major part of the cell. The cytoplasm is basophilic, showing no distinct granules and makes a thin rim around the nucleus. In early childhood they make up about 50% of the total white cell count. Their number diminishes with age. At the age of 10 years it makes about 35%.

(b) Large Lymphocyte:

About 12 µ in diameter, the nucleus may be rounding, oval or kidney-shaped. The cytoplasm is proportionally more and forms a wider zone around the nucleus. It is basophilic and shows no distinct granules. They are considered to be the younger forms of small lymphocytes. In the adults they are very few in numbers (4-8%) but are more frequent in children. In well-stained specimens a few reddish- purple granules (metachromatic granules) are found in the cytoplasm, generally collected at one pole of the cell.

3. Monocytes (Large Monouclear cell, Transitional cell etc.):

About 16 to 18 µ in diameter, the nucleus is round or oval when the cells are young. But as they grow older, the nucleus becomes convoluted kidney -shaped or horseshoe-shaped. [Following the original view of Ehrlich, this last variety was formerly called transitional cells. Because this type was supposed to represent the transitional stage in the formation of many-lobed polymorphonuclear cells from its mononuclear precursor. But this view is no more accepted and the transitional cells are now considered as the more mature and active forms of monocytes.]

The nucleus is eccentric with a large amount of clear non-granular cytoplasm, may be, with vacuoles in it. Fine reddish-blue metachromatic granules may be found throughout the cytoplasm in well- stained specimens. When the nucleus is round it is difficult to distinguish them from the large lymphocytes.

But the following points may be helpful. In the monocytes, the nucleus may be eccentric and the cytoplasm resembles ‘frosted glass’; whereas in the large lymphocytes the nucleus is central and the cytoplasm has a ‘clear glass’ appearance. In the warm stage the monocytes are found to be actively motile whereas the large lymphocytes show very little movement.

Total Count of Red Blood Corpuscles and White Blood Corpuscles:

Enumeration of Red Corpuscles:

Instrument used for this purpose is Improved Neubauer Haemocytometer (Fig. 4.13).

The instrument consists of:

(a) A special glass slide (counting chamber),

(b) One graduated pipette for R.B.C. count, and

(c) Another pipette for White blood corpuscles count.

In addition to this, diluting solutions and cover-slip are also required:

i. R.B.C:

The R.B.C. pipette has got 3 graduations. Two graduations—0.5 and 1, are present on the stem of the pipette and the third mark—101, is placed just above the bulb. Blood is drawn, say, up to mark 1 and the rest of the bulb is filled by sucking up diluting solution up to the mark 101. The bulb of the pipette is so constructed that it holds exactly 100 times the volume of fluid contained in the stem of the pipette up to mark 1.

Although fluid is drawn up to 101, the dilution of the blood will be 100, because the last part of the fluid remains locked up in the stem and is not available for dilution. If blood be drawn up to mark 0.5, the dilution will be 1: 200.

ii. W.B.C:

The White blood corpuscles pipette is also constructed on similar lines but the third mark above the bulb is 11. This will give a dilution of 1 in 10.

iii. The Counting Chamber:

The ruling area consists of 9 square millimeters. The central square millimeter is ruled into 25 groups of 16 small squares, each group separated by triple lines as indicated in Fig. 4.13. The side of each small square is 1/20^th mm.

Method of Counting R.B.C:

1 drop of diluted blood from the red cell-counting pipette is introduced in the counting chamber under the cover-slip. Since the under surface of the cover-slip is 1/10^th mm high from the surface of the counting chamber, the volume of each small square is 1/4000^th cu. mm. Red cells are counted in the five groups of 16 small squares. To avoid counting a corpuscle twice, those on a line are counted only when on the top and left lines or on the bottom and right lines.

The number of red cells per cu. Mm:

When the dilution is 1: 200 the formula reduces to:

= 10,000 × number of cells counted in 80 small squares.

Method of Counting W.B.C:

The white cells are counted in the four corners of 1 square millimeter and in the central ruled area on both sides of the counting chambers of the haemocytometer (10 square millimeters in all). The white cells are recognized by the refractile appearance and by the slight colour given to them by the stain contained in the diluting fluid. The cells touching the boundary lines are not counted. The number is multiplied by 10 or 20 (the dilution).

The number of leucocytes per cu, mm:

Diluting Fluid for R.B.C:

The following solutions are commonly used:

(a) Sodium chloride—0.6 gm, Sodium citrate—1 gm, Formalin – 1 gm, Distilled water up to 100 ml.

(b) Hayem’s solution—Sodium chloride—1 gm, Sodium sulphate—5 gm, Corrosive sublimate—0.5 gm, Distilled water—200 ml.

Diluting Fluid for W.B.C:

Commonly the fluid is made up as follows: Glacial acetic acid—1.5 ml; 1% solution of gentian violet in water—1 ml; Distilled water—98 ml. The glacial acetic acid haemolyses the red cells, while the gentian violet slightly stains the nuclei of leucocytes. The leucocytes may, therefore, be easily recognized. A small quantity of thymol may be added to the diluting fluid to prevent the growth of moulds.

Abnormal Variation in White Blood Corpuscles Count:

i. Leucopenia:

Leucopenia is a condition in which there is a decrease in the W.B.C. count below 4,000 per cu. Mm. Commonly this is due to a fall in the neutrophil cells.

ii. Leucocytosis:

A rise in the W.B.C. count above 11,000 per cu. mm is called leucocytosis. It might be due to an increase in neutrophil, eosinophil or basophil. There might also be lymphocytosis or monocytosis.

Number of inflammatory conditions like Pneumonia, etc., because neutrophil leucocytosis in which this par­ticular variety of the cell rises up to 90 percent of total W.B.C. in allergic conditions and also during infections there is eosinophilic leucocytosis.

iii. Agranulocytosis:

In such a condition there is a great fall in the number of circulating granulocytes, which usually result from the harmful effect of certain drugs.

iv. Leukaemia:

This is a malignant disease of one or other variety of the W.B.C., the number of which are greatly increased. Immature forms of the W.B.C. make their appearance in the circulating blood in such condition.

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Essay # 5. Life and Fate of White Blood Corpuscles:

The life of the different varieties of leucocytes differs. Sabin holds that granulocytes live from 1—2 days only. Osgood believes that neutrophils live from 2—4 days; eosinophils, 8-12 days; basophils, 12—15 days. The average life of lymphocyte seems to be more than a day, and may be 2 or 3 days.

Regarding the fate of leucocytes, it is known that all the varieties die, disintegrate and disappear. The neu­trophils and other granular leucocytes undergo fragmentation in the blood stream and are also broken down by the reticulo-endothelial cells. The monocytes have the same fate.

Regarding the fate of the lymphocytes, it is believed that most of the lymphocytes leave the body (and are destroyed) by passing out through the intestinal and other mucosa. But some are certainly destroyed by the phagocytic cells of reticulo-endothelial system like the other varieties.

Control of Leucopoiesis:

The factors which control the formation of leucocytes under physiological conditions is rather complex. From the fact that these cells are constantly dying away, and in spite of it, a fairly constant number is maintained in the circulation, it is evident that some stimulus must be constantly acting upon the leucopoietic tissues, so that the dead cells are replaced by new formation and a balance is kept up between production and loss. One thing is almost certain that for each variety of cells a specific stimulus is necessary and the stimulus is largely chemotactic.

It has been shown that injection of nucleic acid or its derivatives increases the neutrophil cells in the blood. From such observations it has been suggested that the nucleic acid, derived from the dead neutrophils, may act as the normal stimulus for the formation of fresh neutrophils. In this way the normal neutrophil count is maintained. Regarding the normal stimulus, for the other varieties of leucocytes, nothing definite is yet known.

Some suggest that proteins are responsible for lymphocytic proliferation and lipids for monocytic prolifer­ation.

Experimental and clinical evidences suggest that some control regulating the release of cells into the circulation is exercised by the actual number of circulating leucocytes. Menkin (1955) isolated two simple polypeptides, one thermolabile and the other thermostable as the specific leucocytosis- promoting factors in the pseudoglobulin fraction of exudates and two thermostable polypeptides as specific leucopenic factors.

Others in 1961 obtained evidence of the presence of a neutropoietin and still others in 1965 isolated from serum fraction which stimulates production and another which depresses production and maturation of cells.