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| Internal Defence |
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| Second line of defence or body's internal defence is carried out by leucocytes, macrophages, inflammatory reactions, fever, interferons and natural killer cells. These operate together to check the damage to the body by the pathogens. |
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| The number of leucocytes increases due to an infection. They creep out of the capillaries by amoeboid movement into the intercellular spaces if there is an infection. This process is called diapedesis. The neutrophils engulf and digest the microorganisms infecting the body tissues. They are called phagocytes and the process is known as phagocytosis. The basophils release histamine that plays a role in inflammatory reaction. A macrophage is an irregular cell which can engulf about 100 bacteria and dispose them. |
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| It is initiated by chemical signals. The invading microbes release certain chemicals. Tissue injury causes the most cells to release histamine. Chemicals from microbes and histamines together cause dilution of capillaries and increases the permeability of capillary wall. As a result, more blood flows to that area, making it red and warm. Fluid leaks out into the tissue spaces causing its swelling. This reaction of the body is called inflammatory response and is a part of the internal defence. The plasma that accumulates at the injured site dilutes the toxins secreted by the bacteria and lessens their effect. The neutrophils move in and eat up the invading microorganisms. Macrophages not only phagocytise microbes but also clean up damaged cells and remains of neutrophils destroyed in phagocytic action. |
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| The dead phagocytes and bacteria damaged tissue cells, enzymes and the fluid and proteins leaked from the capillaries leave the body in the form of pus. Formation of pus is a sure sign of infection. Usually, the pus is absorbed by the body within a few days. |
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| The inflammatory response may be localized or systemic. The localised response is confined to the site of injury only. The systemic response affects the entire body in case of a severe infection or a serious injury. In this case, the WBC count of the blood increases. Body temperature rises causing fever. This may be brought about by toxins produced by the pathogen or by a protein called pyrogen or interleukin released by macrophages. |
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| When the pyrogens reach the brain, the body's thermostat is set at a higher temperature, allowing the temperature of the entire body to rise. Mild fever strengthens the defence mechanism by activating the phagocytes and by inhibiting the growth of microbes. Higher body temperature reduces blood iron level. Bacteria need more iron as temperature increases. Therefore, the bacterial growth is reduced in a fever ridden body. |
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| Interferon production and activity |
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| A very high temperature may prove dangerous. It must be brought down by giving antipyretics and by applying cold pack. |
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| This works specifically against viral infections. Cells invaded by a virus produce an antiviral protein called interferon, which is released from the infected and dying host cell. On reaching the nearby uninfected cell, it makes them resistant to the virus infection. Interferon induces the formation of certain proteins which prevent multiplication of the virus. Fever increases the production of interferons. |
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| Interferons have proved very effective in treating cold, influenza and hepatitis but have not succeeded against cancer. |
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| These cells are a kind of lymphocytes. They do not attack micro organisms directly, but lyse the viral infected body cells and abnormal cells which could form tumours. |
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