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.


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 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. 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

Granules of neutrophils contain enzymes like proteases, myeloperoxidases, elastases and metalloproteinases. These enzymes destroy the microorganisms. The granules also contain antibody like peptides called cathelicidins and defensins, which are antimicrobial peptides and are active against bacteria and fungi. 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. 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. 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

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. All the neutrophils move by diapedesis towards the site of infection due to chemotaxis. Chemotaxis occurs due to the attraction by some chemical substances called chemoattractants, which are released from the infected area. After reaching the area,

the neutrophils surround the area and get adhered to the infected tissues. Chemoattractants increase the adhesive nature of neutrophils so that all the neutrophils become sticky and get attached firmly to the infected area. Each neutrophil can hold about 15 to 20 microorganisms at a time. Now, the neutrophils start destroying the invaders. First, these cells engulf the bacteria and then destroy them by means of phagocytosis.

Respiratory Burst

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. During respiratory burst, the free radical O2 – is formed. 2O2 – combine

with 2H+ to form H2O2 (hydrogen peroxide). Both O2 – and H2O2 are the oxidants having potent bactericidal


Pus and Pus Cells

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. The dead WBCs are called pus cells. During the battle against the bacteria, many WBCs are killed by the toxins released from the bacteria. The dead cells are collected in the center of infected area. 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 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. 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

Eosinophils are neither markedly motile nor phagocytic like the neutrophils. Some of the parasites are larger in size. Still eosinophils attack them by some special type of cytotoxic substances present in their granules. When released over the invading parasites from the granules, these substances become lethal and destroy the parasites. 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


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


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 play an important role in healing processes. So their number increases during healing process.

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


Mast Cell

Mast cell is a large tissue cell resembling the basophil. 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.


Mast cells are developed in the bone marrow, but their precursor cells are different. After differentiation, the immature mast cells enter the tissues. Maturation of mast cells takes place only after entering the tissue.


Mast cell plays an important role in producing the hypersensitivity reactions like allergy and anaphylaxis. When activated, the mast cell immediately releases various chemical mediators from its granules

into the interstitium. 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


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 are the largest cells among the leukocytes. Like neutrophils, monocytes also are motile and

phagocytic in nature. These cells wander freely through all tissues of the body. Monocytes play an important role in defense of the body. Along with neutrophils, these leukocytes provide the first line of defense.

Monocytes secrete:

1. Interleukin-1 (IL-1).

2. Colony stimulating factor (M-CSF).

3. Platelet-activating factor (PAF).

Monocytes are the precursors of the tissue macrophages. Matured monocytes stay in the blood only for

few hours. Afterwards, these cells enter the tissues from the blood and become tissue macrophages. Examples of tissue macrophages are Kupffer cells in liver, alveolar macrophages in lungs and macrophages in spleen. Functions of macrophages are discussed in Chapter 24.


Lymphocytes play an important role in immunity. Functionally, the lymphocytes are classified into two

categories, namely T lymphocytes and B lymphocytes. T lymphocytes are responsible for the development of cellular immunity and B lymphocytes are responsible for the development of humoral immunity.

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