< NOT lymphatic system, now we are talking about circulatory sys. >
2. There are 3 lines of defence mechanisms in our body:
- the first line of defence
- the second line of defence
- the third line of defence
The first line of defence
1. It consists of physical and chemical barriers that prevent pathogens from entering the body.
2. Pathogens are disease-causing microorganisms. e.g.: bacteria, viruses and parasites.
3. The barriers are non-specific defences, that is, they do not differentiate one pathogen from another.
4. The first line of defence : the skin, mucous membrane and others.
The Skin
1. The tough outer layer : provides physical barrier that is impenetrable (不能穿透的） to bacteria and viruses.
2. The continues shedding of dead skin cells makes it difficult for bacteria to grow on skin.
3. The skin is also a chemical barrier. [WHY?] --> secretes sebum that forms a protective film over the skin. The acids and oils in sebum prevent the growth of many microorganisms.
4. The sweat excreted contains lysozyme (F4 C2: :from lysosome), an enzyme capable of breaking down the cell walls of certain bacteria.
Mucous Membrane
1. They line the trachea, respiratory passageways, digestive and urinary tracts to stop the entry of potentially harmful microorganisms.
2. It secretes a sticky fluid called mucus that contains lysozyme which traps and destroys bacteria.
e.g.: nose -> mucus-coated hairs -> trap & filter microorganisms, dust and pollutants from inhaled air
Other first line of defences
1. Tears and saliva contain lysozyme -> protect eyes and mouth from bacterial invasion.
2. Hydrochloric acid (stomach) -> destroys most pathogens from food and drinks consumed.
3. Blood clotting mechanism -> prevents entry of pathogens by sealing the wound. (revise back)
** P/S : Most secretion contain lysozyme in first line of defence **
The second line of defence
1. also non-specific and functions to prevent or overcome any mocrobial invasion.
2. Phagocytocic white blood cells or phagocytes can perform phagocytosis.
3. Main phagocytes are neutrophils and monocytes.
4. Process of phagocytosis on infection part:
(a). When an infection occurs, they migrate to the infected area, attracted by chemicals released by damaged cells.
(b). They enter interstitial fluid by squeezing through capillary walls.
(c). During migration, monocytes enlarge and develop into macrophages. Macrophages are found mainly in interstitial fluid.
(d).When a phagocyte encounters an invading pathogen, it engulfs and ingests the pathogen in a process known as phagocytosis.
(e). Inside phagocyte, pathogen is destroyed by lysozyme.
(f). Figure.
The Third Line of Defence
1. It involves the immune system.
2. Specific or targeted defence = recognises specific pathogens, cancer cells and certain chemicals, and defends the body against them.
3. Immunity means the body resistance to the pathogen which causes a specific disease.
4. The external surface of an invading microorganisms contains specific molecules called antigens.
5. Antigens.
(a). They are substances, usually proteins, that the immune system recognises as foreign or not part of the body. Normally found on the outer surface of an invading microorganism.
(b). Include snake venom and bacterial toxins.
6. When immune system identifies the antigens invading body, it is stimulated to defend body against those antigens. This type of defence is known as an immune response.
7. Each antigen induces certain lymphocytes to secrete specific antibodies that only react specifically with that antigen.
8. Antibodies are proteins found on surface of lymphocytes, or proteins released by lymphocytes into the blood plasma.
9. Interaction between antibody and antigen -> elimination of antigen from body -> immune response
10. An antibody molecule has an antigen-binding site or antigen receptor.
(a). The antigen-binding site is highly specific.
(b). 像enzyme, each binding site has particular shape that fits the one found on the antigen.
11. After it is bound to the antibody molecule, the antigen can be destroyed in several ways.
12. Figure : agglutination, neutralisation, organisation and lysis.
... to be continued ...
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