Defence Against Disease
The body has several methods of preventing the entry of micro-organisms:
- The skin acts as a barrier
- Mucus produced by cells lining the trachea, trap dirt and germs, and tiny hairs called cilia move the mucus up and out of the lungs
- The blood clots and forms scabs to seal cuts in the skin
- Hydrochloric acid produced in the stomach kills microbes
If microbes do enter the body, the white blood cells defend the body against disease in several ways:
- Some white cells engulf and digest micro-organisms
- Others produce antibodies which destroy a particular virus or bacterium
- Others produce antitoxins which counteract the toxins (poisons) released by some microbes
Diseases are more likely to occur if large numbers of organisms enter the body due to contact with infected people or unhygienic conditions. Disease can be spread in the air, water, food, through contact with an infected person or through animals.
The symptoms of a disease are usually caused by the toxins that the micro-organisms produce.
To prevent people becoming infected, they may be vaccinated against a disease.
They are given a weakened, or dead, form of the infecting organism.
The white blood cells respond by producing antibodies.
If the organism enters the body at a later time, the white blood cells recognise it and quickly switch on an attack producing antibodies to destroy it. The person is said to be immune to the disease.
In South Africa, people are routinely immunised against TB, polio, measles, hepatitis B and tetanus.
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