The Three Lines of the Body’s Defence Mechanisms
There are thousands of microbial spores and parasites in the environment. Some organisms are pathogenic and can cause diseases when they enter the human body. These pathogens can be transmitted by air, contaminated food or drinking water, as well as by animal vectors such as mosquitoes and houseflies. Some skin diseases can even be transmitted by contact. The human body have three lines of defense to protect against these pathogens.
First Line of Defence
The skin and mucous membranes act as the first line of defence. The skin acts as a physical barrier. It is made up of a dead keratinised layer which is difficult to penetrate. If there is a cut, the blood clots quickly to seal the wound. Tears secreted by tear glands and acidic sebum secreted by sebaceous glands contains lysozymes which destroy bacteria. Mucus secreted by mucous membranes in the nasal cavity and trachea trap dust particles and bacterial spores. The cilia in the respiratory tract sweep the trapped particles to the pharynx. When microorganisms enter the stomach, they are killed by the hydrochloric acid in the gastric juices.
Second Line of Defence
If pathogens get through the first line of defence, they will meet the second line of defence. Some white blood cells, such as neutrophils act as phagocytes. They are attracted by chemicals produced at the sites of infection. The phagocytes move towards the pathogens, for example, bacteria, and engulf them by phagocytosis. Digestive enzymes are secreted into the phagocytic vacuoles to destroy and digest the bacteria. Useful soluble products are absorbed and assimilated by the phagocytes. Sometimes the phagocytes are destroyed by toxins produced by the pathogens. When there is an infection, the number of white blood cells increases in the body to try to destroy the pathogens.
Third Line of Defence
The third line of defence in the body is the lymphocytes. Lymphocytes are white blood cells found in the lymph nodes and in the blood circulatory system. There are two main types of lymphocytes. The T-lymphocytes attack cells infected by pathogens or produce certain chemicals to coordinate immune response. The B-lymphocytes produce antibodies. An antibody is a protein produced by lymphocytes in response to the presence of an antigen. An antigen is a foreign substance which stimulates the body to produce an immune response. It takes some days to produce an immune response. It takes some days to produce the antibodies. The antibodies are specific in action and promote the destruction of antigens in different ways. After an infection, some lymphocytes remain in the body as memory cells which may last for several months or years. The memory lymphocytes help to defend the body against further infection by the same antigen. The body is then said to be immune against the particular diseases.
There are thousands of microbial spores and parasites in the environment. Some organisms are pathogenic and can cause diseases when they enter the human body. These pathogens can be transmitted by air, contaminated food or drinking water, as well as by animal vectors such as mosquitoes and houseflies. Some skin diseases can even be transmitted by contact. The human body have three lines of defense to protect against these pathogens.
First Line of Defence
The skin and mucous membranes act as the first line of defence. The skin acts as a physical barrier. It is made up of a dead keratinised layer which is difficult to penetrate. If there is a cut, the blood clots quickly to seal the wound. Tears secreted by tear glands and acidic sebum secreted by sebaceous glands contains lysozymes which destroy bacteria. Mucus secreted by mucous membranes in the nasal cavity and trachea trap dust particles and bacterial spores. The cilia in the respiratory tract sweep the trapped particles to the pharynx. When microorganisms enter the stomach, they are killed by the hydrochloric acid in the gastric juices.
Second Line of Defence
If pathogens get through the first line of defence, they will meet the second line of defence. Some white blood cells, such as neutrophils act as phagocytes. They are attracted by chemicals produced at the sites of infection. The phagocytes move towards the pathogens, for example, bacteria, and engulf them by phagocytosis. Digestive enzymes are secreted into the phagocytic vacuoles to destroy and digest the bacteria. Useful soluble products are absorbed and assimilated by the phagocytes. Sometimes the phagocytes are destroyed by toxins produced by the pathogens. When there is an infection, the number of white blood cells increases in the body to try to destroy the pathogens.
Third Line of Defence
The third line of defence in the body is the lymphocytes. Lymphocytes are white blood cells found in the lymph nodes and in the blood circulatory system. There are two main types of lymphocytes. The T-lymphocytes attack cells infected by pathogens or produce certain chemicals to coordinate immune response. The B-lymphocytes produce antibodies. An antibody is a protein produced by lymphocytes in response to the presence of an antigen. An antigen is a foreign substance which stimulates the body to produce an immune response. It takes some days to produce an immune response. It takes some days to produce the antibodies. The antibodies are specific in action and promote the destruction of antigens in different ways. After an infection, some lymphocytes remain in the body as memory cells which may last for several months or years. The memory lymphocytes help to defend the body against further infection by the same antigen. The body is then said to be immune against the particular diseases.
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