AQA- Defending against infectious disease

AQA- Defending against infectious disease

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  • Created on: 01-01-11 15:38
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Defending against infectious disease:
Pathogens are microorganisms that cause disease. Pathogens include bacteria and viruses.
Bacteria releases toxins and viruses damage our cells. White blood cells can destroy and ingest
pathogens. They can produce antibodies that destroy pathogens, and antitoxins that neutralise
In a vaccination, pathogens are introduced into the body in a dead or inactive form. This stimulates
the body to produce enough white blood cells to protect itself from the pathogens, while not getting
the disease Antibodies are only effective against bacteria, not viruses. Some strains of bacteria are
resistant to antibiotics.
Bacteria are microscopic organisms. They come in many shapes and
sizes, but even the largest are only 10 micrometres long 10 millionths of
a metre.
Bacteria are living cells and, in favourable conditions, can multiply rapidly.
Once inside the body, they release poisons or toxins that make us feel ill.
Diseases caused by bacteria include:
food poisoning
whooping cough
gonorrhoea a sexually transmitted disease
Viruses are much smaller than bacteria. They consist of a fragment of
genetic material inside a protective protein coat. Viruses can only
reproduce inside host cells, and they damage the cell when they do this.
A virus can get inside a cell and, once there, take over and make hundreds
of thousands of copies of itself. Eventually the virus copies fill the whole
host cell and burst it open. The viruses are then passed out in the
bloodstream. Diseases caused by viruses include:
influenza flu
mumps, measles and rubella
chicken pox
White blood cells:

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The body can protect itself against pathogens in many different ways. The first defence is passive
immunity. This aims to stop pathogens from entering the body. The body's passive immunity
includes, skin, mucus, cilia (respiratory system), acid in the stomach and enzymes (in tears).
Passive immunity is also maternal antibodies passed through the placenta to the fetus. If a
pathogen manages to get past this, the second defence takes over. This is called active
immunity. This involves white blood cells.…read more

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Antibodies neutralise pathogens in a number of ways:
they bind to pathogens and damage or destroy them
they coat pathogens, clumping them together so that they are easily ingested by phagocytes
they bind to the pathogens and release chemical signals to attract more phagocytes
Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging
the body.
People can be immunised against a pathogen through vaccination.
Different vaccines are needed for different pathogens.…read more

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Bacterial strains can develop resistance to antibiotics. This happens because of natural selection.
In a large population of bacteria, there may be some cells that are not affected by the antibiotic.
These cells survive and reproduce, producing even more bacteria that are not affected by the
antibiotic. MRSA is very dangerous because it is resistant to most antibiotics. It is important to
avoid overuse of antibiotics, so we can slow down, or stop the development of other strains of
resistant bacteria.…read more


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