The Basics of Human Defence System
ByThe body constantly protects itself against invasion by microorganisms (bacteria, viruses, fungi and protozoa) and damage by chemicals and other substances. The first line of defence is the physical barrier of the skin. Sweat and tears contain natural antiseptics, the skin itself has an oily barrier and wax protects the ear canals. Swallowed germs are attacked by the stomach’s acid.
The upper respiratory tract has especially good defences. The hairs and mucus in the nose discourage the entry of foreign organisms and substances, but the strongest defences are provided by the lymphoid tissue of the tonsils and adenoids which form a ring-shaped trap for germs at the back of the throat. Patches of pus on the tonsils or anywhere else in the body are evidence of an active fight against infection.
The lymphatic system is particularly important in the body’s fight against infection. It drains fluid called lymph from every part of the body (except the heart and the brain) via a series of progressively larger lymphatic channels (lymphatics and ducts) to one of the large veins. The lymph from each main area passes through a collection of lymph nodes (glands) which contain the white cells known as lymphocytes. Lymphocytes are also found in the lymphoid tissue of the tonsils and adenoids, gut lining, bone marrow, thymus, spleen and appendix, and they circulate around the lymphatic system and the blood system. When foreign matter such as micro-organisms comes into contact with lymphocytes, it stimulates the production of proteins known as antibodies. These are secreted into the lymph and carried round the body in the blood to fight the threatened invasion by foreign matter. Lymph nodes swell as they are producing antibodies and this is a sign that the defence system is working properly. Swollen lymph nodes can sometimes be felt in the neck, under the arms and in the groin. Other white cells and substances in the blood and in the tissues help destroy foreign matter without making antibodies.
A baby is born with some natural immunity and can be further protected by infection-fighting cells, antibodies and other substances in breast milk. The child’s own experiences of infection boost her immunity. For some infections, immunity is life-long while for others it is short-lived. Some illnesses, such as the common cold, are caused by one of several strains of virus, which is why having one cold doesn’t necessarily prevent another later.
Immunization renders a child immune without having the illness itself. Measles, whooping cough, tetanus, polio and German measles (rubella) immunizations are routinely offered.
Sometimes the immune system doesn’t work properly, as in AIDS and some other rare conditions, and sometimes it overworks and causes allergy. It can also be affected by emotional distress and physical exhaustion.
When things go wrong
The ring of lympnoid tissue in the throat is made up of the tonsils, the adenoids and patches of lymphoid tissue on the tongue. Together these trap viruses and bacteria as they are inhaled and prevent them passing down into the lungs. Enlarged red tender tonsils are a sign that the local lymphoid tissue is working overtime to produce lymphocytes with their antibodies against whichever germ is responsible for the infection. In some children, repeated infection eventually wears the tonsils out and they become either chronically enlarged or small and useless. If the tonsils are so large that they prevent a child from eating properly for a long time, they may have to be removed.
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