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The leucocytes are also called white blood corpuscles (WBS) or simply white cells of the blood but they are not white but colourless. The WBCs defend the body against diseases by fighting infections (bacterial, viral, parasitic, etc.), antigens and also against malignancy. 
Types of Leucocytes

A. WBCs can be divided into:
  1. Granulocytes
    These WBCs have granules in their cytoplasm. The granulocyte WBCs are : neutrophils, eosinophils and basophils.
  2. Agranulocytes
    These WBCs do not have granules in their cytoplasm. The agranulocyte WBCs are : monocytes and lymphocytes
Normal values with range
Absolute value (Cells/cu.mm.)
Range (cells/cu.mm.)
Percentage of the total WBCs
Total WBC
4000 to 11000
3000 to 6000
50 to 70
150 to 300
1 to 4
0 to 100
1500 to 4000
20 to 40
300 to 600
2 to 8

Note : 1 cu mm = 1 microlitre

Morphology of the nuclei

  1. Granulocytes
    Young granulocytes : horseshoe-shaped nuclei
    Older granulocytes : multilobed nuclei
  2. Agranulocytes
    Lymphocytes : large round nuclei, scanty cytoplasm
    Monocytes : kidney-shaped nuclei, abun­dant cytoplasm
  1. Development of leucocytes (WBCs)
    1. Leucocytes develop from the same single type of pleuripotent hematopoietic stem cells (HSCs) from which all the blood cells develop. The HSCs give rise to committed stem cells (also called progenitor cells or colony-forming units or CFUs). The committed stem cells are separate for each type of leucocyte except for the neutrophil and monocyte; neutrophil and monocyte develop from a common committed stem cell.
    2. Stages in the development of neutrophil
    3. Pluripotent hematopoietic stem cell → give rise to committed stem cell ; the committed stem cells go through the following stages :
  1. Myeloblast:     
    1. Diameter : 12 to 18 µm;
    2. It is purple-blue, large and round with finely stippled chromatin with several nucleoli.
    3. This consists of a narrow blue rim without granules. Protein synthesis is very active, as shown by a highly developed endoplasmic reticulum. The cells show active mitosis.
  2. Promyelocytes
    These show primary granules in the cytoplasm (azurophilic granules). Nucleoli decrease in number. Nucleus is round, chromatin has started to condense. Mitosis is seen at this stage also.
  3. Myelocyte
    Diameter : 10 to 15  µm
This is more extensive and less basophilic than myeloblast. It has granules; the colour of the granules classifies them as neutrophil , eosinophil or basophil myelocytes. Primary granules are no longer visible; in this stage, the secondary or specific granules (for three different granulocytes i.e. neutrophilic, eosinophilic and basophilic granules) appear. The cytoplasm is still basophilic. Mitosis is observed in this stage also.
  1. Nucleus
    Smaller and more basophilic than myeloblast. There are no nucleoli; chromatin is coarser and it is further condensed.
    1. Metamyelocyte
      These cells have deep indented nuclei. The specific granules are plenty and they cytoplasm is yellow-pink.Mitosis is not seen; chromatin is highly condensed.
    2. Band form or juvenile neutrophil
      Nucleus becomes horse-shoe or crescent shaped. Specific granules are in plenty. In case of increased production of neutrophils, this form is also released into the circulation.
    3. Segmented form or the mature neutrophils
      It is a mature stage and the morphology is like a matured neutrophil. The cells have acquired all the properties of a neutrophil i.e. phagocytosis, chemotaxis etc. and also the surface receptors. The cells pass into the sinusoids to go into circulation.
  1. Duration
    From the stage of myeloblast to the stage of matured neutrophil it takes about 10 days, out of which half is required for development up to the stage of myelocyte (mitotic pool) and the other half is spent from metamyelocyte to matured neutrophil (maturation pool).
    This time period is decreased during an acute infection when more neutrophils are needed. The matured neutrophils stay in the circulation for a short time (half life = 6 hours). Then the cells enter the tissues and die after 3 to 4 days. The neutrophils are destroyed by the RE cells, eliminated via GIT and via the respiratory tract secretions. Bone marrow contains 3 days reserve of matured neutrophils.
  2. Eosinophils and Basophils
    These develop with their specific granules in the same way as the neutrophils.
    1. Monocytes
      These are produced from the same committed stem cells from which the neutrophils develop. The stages of development are:
      Pluripotent hematopoietic stem cell -> committed stem cell -> monoblast -> promonocyte -> monocyte. The monocyte after a short stay in the circulation pass to the tissues and are then called macrophages.
  1. Neutrophils
    1. These contain neutrophilic granules. They are the most numerous WBCs.
    2. Diameter: 10 to 15 microns
    3. Lobes in the nuclei
    4. Their nucleus shows variable number of lobes (2 to 7); hence, they are called as polymorphonuclear leucocytes. The number of lobes increases as the neutrophil gets older.
    5. Cytoplasm: pink
    6. Granules:
    7. fine violet (red-brown); amphophilic granules i.e. the granules take up both acidic and basic stains; thus although they are called neutrophils, their granules are not neutral.
    8. Nucleus : purple blue; chromatin is coarse and ropy
      1. Average half-life of a neutrophil in the circula­tion: 6 hours.
      2. (thus, in order to  maintain the normal circulating blood level, it is necessary to produce more than 100 billion neutrophils per day).
      3. The neutrophils are most numerous and are of shortest life span; therefore, their rate of formation is also high. The bone marrow contains the highest number of neutrophils in different stages of development and also the matured ones.
Neutrophil Pool

The neutrophils are divided into different pools or collections, depending on the site.
  1. The bone marrow pool
This represents the neutrophils present in bone marrow; it constitutes the largest pool viz. 90%
  1. circulation
This represents the neutrophils present in the circulation; it constitutes 3%; out of this 3%
a.  1.5% are in actual circulation and
b.  1.5% are attached to the endothelium; this is called the. marginal pool. 
  1. Tissue pool
This represents the neutrophils present in the tissues; it constitutes 7%;
  • Neutrophils are also called as microphages; this is because they engulf small-sized particles (the monocytes are called macrophages ; this is because they engulf large-sized particles).
  • Neutrophils are called the body’s first line of defence; this is because they are the first to move towards the invading the bacteria.
  1. Attraction towards the endothelium :
At first, the neutrophils get attracted to the en­dothelial surface by selectins and they roll along the endothelium.
  1. Binding to the endothelium:
Next, they bind to the endothelium with the help of neutrophil adhesion mole­cules of the integrin family. 
  1. Diapedesis:
The neutrophils have contractile proteins e.g. actin, myosin I etc. in their cytoskeleton. With the help of these contractile proteins, they come out of the walls of the capillaries by passing in between the endothelial cells. This process is called diapedesis.


  • Many of the neutrophils that come out of the circulation enter the gastrointestinal tract and are lost from the body.
  • Inflammatory response of the neutrophils to bacterial invasion
  • Invasion of the body by bacteria triggers the inflam­matory response.



Stimulation of the bone marrow:
The bone marrow is stimulated to produce and release large numbers of neutrophils.
Chemical agents move the neutrophils towards the infected area;such movement of neutrophils is called chemotaxis. The chemical agents responsible for chemotaxis are called as chemotactic agents.
Chemotactic Agents

Bac­terial products interact with plasma factors and cells to produce these agents The chemotactic agents are a part of a large family of chemokines.
The chemotactic agents include :
  1. A component of the complement system (C5a);
  2. Leukotrienes;
  3. Polypeptides (from lvm­phocytes, mast cells, and basophils).
G- globulin
This is a plasma protein that increases the ef­fect of C5a; the neutrophil membranes also contain this protein.
It also binds and transports vitamin D in the plasma.
  1. Opsonization
Coating of the bacteria by certain plasma factors helps the neutrophils in attacking the bacteria. The coating makes the bacteria “tasty” for the neutrophils. This process of coating of the bacteria is called opsonization. The factors used for coating are called opsonins. The principal opsonins are the IgG immunoglobulins and complement proteins.
2.   Phagocytosis
  • The opsonized bac­teria bind to the receptors on the neutrophil cell membrane
  • This binding to increases the motor activity of the neutrophil via a hetero trimeric G protein .
  • The in­creased motor activity leads to prompt ingestion of the bacteria by the neutrophil (phagocytosis) forming a phagocytic vacuole containing the bacteria
  1. Degranulation
    The neutrophil granules discharge their contents into the phagocytic vacuoles (and also into the interstitial space); this process is called as degranulation.
The granules release:
  1. various proteolytic enzymes and
  2. defensins : these are anti-microbial proteins; the defensins are of two types - alpha and beta.(Secreted by Paneth cells in GIT)
  1. Activation of NADPH oxidase
  • The neutrophil cell membrane-bound enzyme NADPH oxidase is also acti­vated; this leads to production of toxic oxygen metabolites.
  • The combination of proteolytic enzymes from the granules and the toxic oxygen metabolites help in killing and digestion of the bacteria.
  1. Respiratory burst
Activation of NADPH oxidase (present on the neutrophil cell membrane) results in :
  1. a sharp increase in O2 uptake and metabolism in the neu­trophil; this is called as respiratory burst)
  2. generation of O2 -­by the following reaction:
  3. NADPH + H +202   NADP+ + 2H+ +202-
  1. Killing and digestion of the bacteria by:
    1. Oxidants : These are
    2. 02-  (also called superoxide): 
      This is s a free radical formed by the addition of one elec­tron to O2.
2 02-  react with two H+ to form H2O2; this reaction catalyzed by the cytoplasmic form of superoxide dismutase (SOD-1):
02-  + 02-  + H+ + H+    -------→  H2O2 + O2
Both the oxidant 02- and H2O2 are effective bac­tericidal agents; however, H202  is converted to H20  and O2, by the enzyme catalase.
  1. The cytoplasmic form of SOD contains both Zn and Cu. It is found in many parts of the body.
  2. Defective SOD
  3. This can occur due a gene mutation; the defective SOD is the cause of a familial form of amyotrophic lateral sclerosis (ALS).
  4. Because of the deficiency of SOD, it is possible that 02-  accumulates in motor neurons and kills them in at least one form ALS.
  5. Two other forms of SOD encoded by at least one different gene are also found in humans.

These are also effective oxidants.How are they produced?

Neutrophils have an enzyme called myeloperox­idase; this enzyme catalyzes the conversion of CI-, Br, I-, and SCN- to the corresponding acids (HOCI, HOBr, etc). These acids are also potent oxidants. Since CI- is present in greatest abundance in body fluids, the princi­pal product is HOCI.
  1. Other agents in neutrophils that destroy bacteria :
Neutrophil granules have defensins, an elastase and two metallopro­teinases that attack collagen, and a variety of other pro­teases; all these help in destroying the invading organisms. These enzymes act in a cooperative fashion with the oxidants mentioned above to kill the bacteria.

In certain diseases, e.g. rheumatoid arthritis, the neutrophils may also cause local destruction of host tissue.

  1. Role of microtubules and microfilaments
These play a role in:
  • movement of the cell in phagocytosis
  • migration of the cell to the site of infection
Proper function of the microfilaments involves the interaction of the actin they contain with myosin-I on the inside of the cell membrane.


They are called so because they have eosinophilic granules; these get stained with acidic dyes.
  1. Characteristic features :
Eosinophils show many of the features of neutrophils. For example, they a short half-life in the circulation; they are attracted to the surface of en­dothelial cells by selectins; they bind to integrins which at­tach them to the vessel wall; they enter the tissues by di­apedesis. Like neutrophils, eosinophils also show chemotaxis.
  1. Sites
Eosinophils are especially abundant in:
1.  The mu­cosa of the gastrointestinal tract; here, they defend against parasites.
2.  In the mucosa of the respiratory and urinary tracts.
  1. Phagocytic function
Like neutrophil, eosinophils are also phagocytic; however, eosinophils are less motile than neutrophils. Like neutrophil granules, eosinophil granules are also lysosomal in nature and contain most of the enzymes found in neutrophil granules. Eosinophil granules have a very high peroxidase content which partly accounts for their parasiticidal action e.g. versus schistosomes.
  1. Allergic reactions
Eosinophils collect at the site of allergic reactions. It has been suggested that they limit the effects of mediators (e.g. histamine, bradykinin) of some types of antigen-antibody reaction. The aryl-sulphatase-B present in the eosinophil inactivates the slow reacting substance (SRS) released from mast cells and prevents anaphylaxis (anti-allergic reaction). Furthermore, histaminase etc. from eosinophils destroy the substances released from mast cells. Because of its phagocytic action, it takes up antigen-antibody complexes.
  1. Eosinophils contain a major basic protein (MBP) which damages the larvae of parasites.
  2. There is one eosinophilic cation protein which probably neutralizes heparin..
  3. Eosinophils also contains peroxidase.
Factors and conditions affecting eosinophil activation
  1. Their maturation and activation in tissues is particularly stimulated by IL-3,  IL-5, and GM-CSF. 
  2. The level of circulating eosinophils is reduced by adrenal corticosteroids and hence by secretion of ACTH. The eosinopenia is caused by sequestration of eosinophils in the lungs and spleen and by their destruction in the circulating blood.
  3. Circulating eosinophils are increased in:
    1. allergic diseases such as asthma and
    2. in various other respiratory and gastrointestinal diseases.
Basophils & Mast Cells

Basophils also enter tissues and release proteins and cytokines. They are also motile and phagocytic. They resemble but are not identical to mast cells. Like mast cells, basophils contain histamine and he­parin. They release histamine and other in­flammatory mediators when activated by a histamine ­releasing factor secreted by T lymphocytes; they are essential for immediate-type hypersensitivity re­actions. These reactions range from mild urticaria and rhinitis to severe anaphylactic shock.
Mast Cells


What are Mast Cells

Mast cells are heavily granulated wandering cells.


Sites where Found

They are found in areas rich in connective tissue, and they are abundant beneath epithelial surfaces.


Contents of their Granules

Their granules contain heparin, histamine, and many proteases. The heparin appears to play a role in granule formation.


 Role of WBCs
  1. In acquired immunity
Mast cells have IgE receptors on their cell membranes; like basophils, they degranulate when IgE-coated anti­gens bind to their surface. They are involved in inflam­matory responses initiated by immunoglobulins IgE and IgG. The inflammation fights invad­ing parasites.
  1. In natural immunity
Mast cells release TNF-alpha in response to bacterial products by an antibody-independent mecha­nism; thus they participate in the nonspecific natural im­munity that fights infections. Marked mast cell degranulation produces clinical manifestations ranging from allergy to anaphylaxis.
  1. Histamine released from the mast cells of the GIT(and from other gastrointestinal immune cells) stimulates GIT secretion of water and electrolytes
  2. Primed mast cells play a central role in the gastrointestinal response to antigen. A primed mast cell is one that carries antibody on its surface. When the antibody “recognizes” its particular antigen, the mast cells degranulate and release many different mediators. Several of these mediators induce hypersecretion of salts and water by the epithelial cells as well as hypermotility. Mast cells also release cytokines that recruit other mucosal immune cells to the response. These cells may then also release secretogogues.

Life span of monocytes:
  1. In the circulation
  2. Monocytes enter the blood from the bone marrow and circulate for about 72 hours.
  3. In the tissues
  4. After about 72 hours in the circulation, monocytes enter the tissues and become tissue macrophages
  5. Their life span in the tissues is unknown; studies suggest that they persist for about 3 months. It appears that they do not reenter the circulation.
  6. Some monocytes become the multinu­cleated giant cells seen in chronic inflammatory diseases such as tuberculosis.
  7. Monocyte- macrophage system
  8. As mentioned above, monocytes after a short stay in the circulation enter the tissues and becomes tissue macrophages. These tissue macrophages are found in many organs and are known by different names :
Tissue macrophage in
Known as
Kupffer cells
Connective tissue
Lymph nodes
Dendritic cells
Dendritic cells
Bone marrow
Dendritic cells
Adrenal glands
Endothelial cells
Endothelial cells
Pulmonary alveolar macrophages (PAMS)
Langerhans cells
The monocyte-macrophage system was previously known as reticuloendothelial system.

Functions of tissue macrophage system:
  • Phagocytic The macrophages become activated by lymphokines from T lymphocytes. The activated macrophages mi­grate in response to chemotactic stimuli phagocytose invading bacteria. The steps in phagocytosis is similar to that in neutrophils.
  • They play a key role in immu­nity.
  • They secrete many different substances e.g.
  • Substances that affect lymphocytes and other cells
  • Prostaglandins of the E series
  • Clot ­promoting factors etc
Factors stimulating/activating the cells of the bone marrow:

The hematopoietic stem cells (HSC) in the bone marrow are the pleuripotent uncommitted stem cells; they give rise to committed stem cells (also called progenitor cells). As the name suggests, committed stem cells produce one type of blood cell.
  1. Interleukins
Interleukins IL-1 and IL-6 followed by IL-3 act in sequence to convert pluripotential uncom­mitted stem cells to committed progenitor cells .IL-3 is also known as multi-CSF. (CSF = colony stimulating factor)
  1. Colony-stimu­lating factors (CSF)
CSFs are factors which stimulate/activate a particular committed stem cell. They are so called because these factors forms colonies of the committed stem cell in soft agar culture medium.

The various CSFs are:
  • Granulocyte-macrophage CSF (GM-CSF)
  • Granulo­cyte CSF (G-CSF)
  • Macrophage CSF (M-CSF).
Function of interleukins and CSFs
  1. Each of the CSFs stimulates mainly one type of stem cell
  2. In addition, each of the CSFs (as well as the interleukins) are capable of stimulating other stem cells as well.
  3. The interleukins and CSFs also they activate and sustain mature blood cells.
  4. The genes for many of these factors are located together on the long arm of chromosome 5. Basal hematopoiesis is normal in mice in which the GM-CSF gene is knocked out; this indicates that loss of one factor can be compensated for by others. However, the absence of GM-CSF causes accumulation of surfac­tant in the lungs.
Source of these factors
The factors are produced by macrophages, acti­vated T cells, fibroblasts, and endothelial cells. Mostly, the factors act locally in the bone marrow.


Note : The RBC stimulating hormone, erythropoietin,  is produced in part by kidney cells and is a circulating hormone


Applied Aspects
To fight invading bacteria, the phagocytic mechanism has to be intact.. If there is a defect in this mechanism, it can make the person prone to infections. The various defects in the phagocytic mechanism are as follows:
  1. 1.  Hypomotility of neutrophils.
    In this condition, actin in the neutrophils does not polymerize normally; as a result, the neutrophils move slowly.
  2. Congenital deficiency of integrins in the leukocyte
  3. Chronic granulomatous disease
    In this condition, there is a failure to generate O2 - in both the neutrophils and monocytes; thus, there is inability to kill many phagocytosed bacteria.
  4. Severe congenital glucose 6-phosphate dehydrogenase deficiency
    In this condition, there is failure to generate NADPH; as mentioned above, NADPH is required for producing O2 - ; consequently, the patient has multi­ple infections because of failure to generate O2 -
  5. Congenital myeloperoxidase deficiency,
    In this condition, hypohalite ions are not formed; thus, there is decrease in ability to attack microbes.

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