Question

Explain why the blood is sometimes called the second line of defense against infection. Include the various blood components and how they perform this function.

Answer 1

The white blood cells or leucocytes present in blood constitute the second line of defense against infection.

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- 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.

•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 .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 .

•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- 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.

*BASOPHILS- 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, serrotonin:-Theses substances produce the acute hypersensitivity reactions by causing vascular and tissue responses.

3).Preoteases and myeloperoxidase:- These enzymes destroy the microorganisms.

4).Cytokine: Cytokine such as interleukin-4 accelerates inflammatory responses and kill the invading organisms.

•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 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.

*MONOCYTES- 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.

*LYMPHOCYTES-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.