White blood cells (WBCs) Introduction .
[1]. White blood cells (WBCs) or leukocytes are the colorless and nucleated formed elements of blood (leuko is derived from Greek word leukos = white).
[2]. Alternate spelling for leukocytes is leucocytes. Compared to RBCs, the WBCs are larger in size and lesser in number.
[3]. Yet functionally, these cells are important like RBCs because of their role in defense mechanism of body and protect the body from invading organisms by acting like soldiers.
WBCs Vs RBCs .
WBCs differ from RBCs in many aspects.
1. Larger in size.
2. Irregular in shape.
3. Nucleated.
4. Many types.
5. Granules are present in some type of WBCs.
6. Lifespan is shorter.
Classification of WBCs .
Some of the WBCs have granules in the cytoplasm. Based on the presence or absence of granules in the cytoplasm, the leukocytes are classified into two groups:
1. Granulocytes which have granules.
2. Agranulocytes which do not have granules.
1. Granulocytes .
Depending upon the staining property of granules, the granulocytes are classified into three types:
i. Neutrophils with granules taking both acidic and basic stains.
ii. Eosinophils with granules taking acidic stain.
iii. Basophils with granules taking basic stain.
2. Agranulocytes .
Agranulocytes have plain cytoplasm without granules. Agranulocytes are of two types:
i. Monocytes.
ii. Lymphocytes.
Morphology of White Blood Cells .
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| Different white blood cells . |
Neutrophils .
[1]. Neutrophils which are also known as polymorphs have fine or small granules in the cytoplasm.
[2]. The granules take acidic and basic stains. When stained with Leishman’s stain (which contains acidic eosin and basic methylene blue) the granules appear violet in color. Nucleus is multilobed .
[3]. The number of lobes in the nucleus depends upon the age of cell. In younger cells, the nucleus is not lobed. And in older neutrophils, the nucleus has 2 to 5 lobes.
[4]. The diameter of cell is 10 to 12 µ . The neutrophils are ameboid in nature.
Eosinophils .
[1]. Eosinophils have coarse (larger) granules in the cytoplasm, which stain pink or red with eosin.
[2]. Nucleus is bilobed and spectacle-shaped. Diameter of the cell varies between 10 and 14 µ.
Basophils.
[1]. Basophils also have coarse granules in the cytoplasm. The granules stain purple blue with methylene blue.
[2]. Nucleus is bilobed. Diameter of the cell is 8 to 10 µ.
Monocytes .
[1]. Monocytes are the largest leukocytes with diameter of 14 to 18 µ. The cytoplasm is clear without granules. Nucleus is round, oval and horseshoe shaped, bean shaped or kidney shaped.
[2]. Nucleus is placed either in the center of the cell or pushed to one side and a large amount of cytoplasm is seen.
Lymphocytes .
Like monocytes, the lymphocytes also do not have granules in the cytoplasm. Nucleus is oval, bean-shaped or kidney-shaped. Nucleus occupies the whole of the cytoplasm. A rim of cytoplasm may or may not be seen.
Types of Lymphocytes .
[1]. Depending upon the size, lymphocytes are divided into two groups:
1. Large lymphocytes: Younger cells with a diameter of 10 to 12 µ.
2. Small lymphocytes: Older cells with a diameter of 7 to 10 µ.
[2]. Depending upon the function, lymphocytes are divided into two types:
1. T lymphocytes: Cells concerned with cellular immunity.
2. B lymphocytes: Cells concerned with humoral immunity.
Normal White Blood Cell Count .
1. Total WBC count (TC): 4,000 to 11,000/cu mm of blood.
2. Differential WBC count (DC) .
Lifespan of White Blood Cells .
Lifespan of WBCs is not constant. It depends upon the demand in the body and their function. Lifespan of these cells may be as short as half a day or it may be as long as 3 to 6 months.
Properties of White Blood Cells .
1. Diapedesis .
Diapedesis is the process by which the leukocytes squeeze through the narrow blood vessels.
2. Ameboid Movement .
Neutrophils, monocytes and lymphocytes show amebic movement, characterized by protrusion of the cytoplasm and change in the shape.
3. Chemotaxis .
Chemotaxis is the attraction of WBCs towards the injured tissues by the chemical substances released at the site of injury.
4. Phagocytosis .
Neutrophils and monocytes engulf the foreign bodies by means of phagocytosis .
Functions of White Blood Cells .
Generally, WBCs play an important role in defense mechanism. These cells protect the body from invading organisms or foreign bodies, either by destroying or inactivating them. However, in defense mechanism, each type of WBCs acts in a different way.
Neutrophils .
[1]. Neutrophils play an important role in the defense mechanism of the body. Along with monocytes, the neutrophils provide the first line of defense against the invading microorganisms.
[2]. The neutrophils are the free cells in the body and wander freely through the tissue and practically, no part of the body is spared by these leukocytes.
Substances Present in Granules and Cytoplasm of Neutrophils .
[1]. Granules of neutrophils contain enzymes like proteases, myeloperoxidases, elastases and metalloproteinases . These enzymes destroy the microorganisms.
[2]. The granules also contain antibody like peptides called cathelicidins and defensins, which are antimicrobial peptides and are active against bacteria and fungi.
[3]. Membrane of neutrophils contains an enzyme called NADPH oxidase (dihydronicotinamide adenine dinucleotide phosphate oxidase). It is activated by the toxic metabolites released from infected tissues.
[4]. The activated NADPH oxidase is responsible for bactericidal action of neutrophils (see below). All these substances present in the granules and cell membrane make the neutrophil a powerful and effective killer machine.
[5]. Neutrophils also secrete platelet-activating factor (PAF), which is a cytokine. It accelerates the aggregation of platelets during injury to the blood vessel, resulting in prevention of excess loss of blood.
Mechanism of Action of Neutrophils .
[1]. Neutrophils are released in large number at the site of infection from the blood. At the same time, new neutrophils are produced from the progenitor cells.
[2]. All the neutrophils move by diapedesis towards the site of infection due to chemotaxis.
[3]. Chemotaxis occurs due to the attraction by some chemical substances called chemoattractant, which are released from the infected area. After reaching the area, the neutrophils surround the area and get adhered to the infected tissues.
[4]. Chemoattractant increase the adhesive nature of neutrophils so that all the neutrophils become sticky and get attached firmly to the infected area.
[5]. Each neutrophil can hold about 15 to 20 microorganisms at a time. Now, the neutrophils start destroying the invaders.
[6]. First, these cells engulf the bacteria and then destroy them by means of phagocytosis .
Respiratory Burst .
[1]. Respiratory burst is a rapid increase in oxygen consumption during the process of phagocytosis by neutrophils and other phagocytic cells. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is responsible for this phenomenon.
[2]. During respiratory burst, the free radical O2 – is formed. 2O2 – combine with 2H+ to form H2 O2 (hydrogen peroxide). Both O2 – and H2 O2 are the oxidants having potent bactericidal action.
Pus and Pus Cells .
[1]. Pus is the whitish yellow fluid formed in the infected tissue by the dead WBCs, bacteria or foreign bodies and cellular debris. It consists of white blood cells, bacteria or other foreign bodies and cellular debris.
[2].The dead WBCs are called pus cells. During the battle against the bacteria, many WBCs are killed by the toxins released from the bacteria.
[3]. The dead cells are collected in the center of infected area.
[4]. The dead cells together with plasma leaked from the blood vessel, liquefied tissue cells and RBCs escaped from damaged blood vessel (capillaries) constitute the pus.
Eosinophils .
[1]. Eosinophils play an important role in the defense mechanism of the body against the parasites. During parasitic infections, there is a production of a large number of eosinophils which move towards the tissues affected by parasites.
[2]. Eosinophil count increases also during allergic diseases like asthma. Eosinophils are responsible for detoxification, disintegration and removal of foreign proteins.
Mechanism of Action of Eosinophils .
[1]. Eosinophils are neither markedly motile nor phagocytic like the neutrophils. Some of the parasites are larger in size.
[2]. Still eosinophils attack them by some special type of cytotoxic substances present in their granules.
[3]. When released over the invading parasites from the granules, these substances become lethal and destroy the parasites.
Lethal substances .
The lethal substances present in the granules of eosinophils and released at the time of exposure to parasites or foreign proteins are:
1. Eosinophil peroxidase:
This enzyme is capable of destroying helminths (parasitic worms), bacteria and tumor cells.
2. Major basic protein (MBP):
It is very active against helminths. It destroys the parasitic worms by causing distension (ballooning) and detachment of the tegumental sheath (skin-like covering) of these organisms.
3. Eosinophil cationic protein (ECP):
This substance is the major destroyer of helminths and it is about 10 times more toxic than MBP. It destroys the parasites by means of complete disintegration. It is also a neurotoxin.
4. Eosinophil-derived neurotoxin:
It destroys the nerve fibers particularly, the myelinated nerve fibers.
5. Cytokines:
Cytokines such as interleukin-4 and interleukin-5 accelerate inflammatory responses by activating eosinophils. These cytokines also kill the invading organisms.
Basophils .
[1]. Basophils play an important role in healing processes. So their number increases during healing process.
[2]. Basophils also play an important role in allergy or acute hypersensitivity reactions (allergy). This is because of the presence of receptors for IgE in basophil membrane.
Mechanism of Action of Basophils .
Functions of basophils are executed by the release of some important substances from their granules such as:
1. Heparin:
Heparin is essential to prevent the intravascular blood clotting.
2. Histamine, slow-reacting substances of anaphylaxis, bradykinin and serotonin:
Theses substances produce the acute hypersensitivity reactions by causing vascular and tissue responses.
3. Proteases and myeloperoxidase:
These enzymes destroy the microorganisms.
4. Cytokine:
Cytokine such as interleukin-4 accelerates inflammatory responses and kill the invading organisms.
Mast Cell .
[1]. Mast cell is a large tissue cell resembling the basophil.
[2]. Generally, mast cells are found along with the blood vessels and are prominently seen in the areas such as skin, mucosa of the lungs and digestive tract, mouth, conjunctiva and nose. These cells usually do not enter the bloodstream.
[3]. Mast cells are developed in the bone marrow, but their precursor cells are different. After differentiation, the immature mast cells enter the tissues.
[4]. Maturation of mast cells takes place only after entering the tissue.
Functions of Mast Cell .
[1]. Mast cell plays an important role in producing the hypersensitivity reactions like allergy and anaphylaxis .
[2]. When activated, the mast cell immediately releases various chemical mediators from its granules into the interstitium.
[3]. Two types of substances are secreted by mast cell:
1. Preformed mediators:
These substances are already formed and stored in secretory granules. These substances are histamine, heparin, serotonin, hydrolytic enzymes, proteoglycans and chondroitin sulfates.
2. Newly generated mediators:
These substances are absent in the mast cell during resting conditions and are produced only during activation. These substances are arachidonic acid derivatives such as leukotriene C (LTC), prostaglandin and cytokines.
Monocytes .
[1]. Monocytes are the largest cells among the leukocytes. Like neutrophils, monocytes also are motile and phagocytic in nature.
[2]. These cells wander freely through all tissues of the body Monocytes play an important role in defense of the body.
[3]. Along with neutrophils, these leukocytes provide the first line of defense.
[4]. Monocytes secrete:
1. Interleukin-1 (IL-1).
2. Colony stimulating factor (M-CSF).
3. Platelet-activating factor (PAF).
[5]. Monocytes are the precursors of the tissue macrophages. Matured monocytes stay in the blood only for few hours.
[6]. Afterwards, these cells enter the tissues from the blood and become tissue macrophages.
[7]. Examples of tissue macrophages are Kupffer cells in liver, alveolar macrophages in lungs and macrophages in spleen.
Lymphocytes .
[1]. Lymphocytes play an important role in immunity. Functionally, the lymphocytes are classified into two categories, namely T lymphocytes and B lymphocytes.
[2]. T lymphocytes are responsible for the development of cellular immunity and B lymphocytes are responsible for the development of humoral immunity.
Leukopoiesis .
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| Leukopoiesis . |
Leukopoiesis is the development and maturation of leukocytes .
Factors Necessary for Leukopoiesis .
Leukopoiesis is influenced by hemopoietic growth factors and colony stimulating factors .
Colony stimulating Factors .
Colony stimulating factors (CSF) are proteins which cause the formation of colony forming blastocytes. Colony stimulating factors are of three types:
1. Granulocyte-CSF (G-CSF) secreted by monocytes and endothelial cells.
2. Granulocyte-monocyte-CSF (GM-CSF) secreted by monocytes, endothelial cells and T lymphocytes.
3. Monocyte-CSF (M-CSF) secreted by monocytes and endothelial cells.
