Defending ourselves against disease

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Pathogens and disease

  • Pathogens cause infectious diseases
  • Pathogens are tiny microorganisms - usually bacteria or viruses
  • When bacteria or viruses enter the body they reproduce rapidly. They can make you feel ill by producing toxins
  • Viruses are much smaller than bacteria and reproduce inside cells. The damage to the cells also makes you ill
  • Before bacteria and viruses had been discovered, a doctor called Semmelweis realised that infection could be transferred from person to person in a hospital - infected people are said to be infectious
  • Semmelweis told his staff to wash their hands between treating patients. However, other doctors did not take him seriously. We now know that he was right
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Defence Mechanisms

  • Pathogens can be transmitted from one person to another by droplet infection or direct contact. When you cough or sneeze pathogens are sprayed into the air in drops of water
  • The skin prevents pathogens getting into the body
  • Pathogens are also trapped by mucus and killed by stomach acid

White blood cells are part of the immune system. They do three things to defend the body:

  • They can ingest pathogens. This means they digest and destroy them
  • They produce antibodies to help destroy particular pathogens
  • They produce antitoxins to counteract the toxins that pathogens produce
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Using drugs to treat disease

  • Antibiotics kill infective bacteria in the body
  • Penicillin is an antibiotic, but there are many others. It was first discovered by Alexander Fleming in 1928.
  • Viruses are difficult to kill because they reproduce inside the body cells so any treatment could also damage the body cells.
  • Painkillers and other drugs relieve the symptoms of a disease but do not kill the pathogen.
  • Your immune system will usually overcome the viral pathogens
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Growing and Investigating Bacteria

  • Pure cultures on fon-pathogenic (safe) bacteria can be used for laboratory investigations
  • A culture of microorganisms can be used to find the effect of antibiotics on bacteria
  • Investigations need uncontaminated cultures of microorganisms. Strict health and safety procedures are used to protect yourself and others.
  • Contamination might come from your skin, the air, the soil or the water around you.
  • If the culture is contaminated other bacteria could grow, including pathogens.
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Growing Cultures

To culture (grow) microorganisms in a laboratory you must:

  • Give them a liquid or gel containing nutrients - a culture medium. It contains carbohydrate as an enrgy source, various minerals and sometimes other chemicals. A culture medium called agar jelly is used.
  • Provide warmth and oxygen.
  • Keep them incubated at 25°C in school laboratories and at 35°C in industry.

To keep the culture pure you must:

  • Kill all the bacteria on the equipment - pass metal loops through a flame; boil solutions and agar
  • Prevent microorganisms from the air getting into the equipment.
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Changing Pathogens


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  • Dead or inactive forms of a pathogen are used to make a vaccine. Vaccines can be injected into the body
  • The white blood cells react by produce antibodies
  • This makes the person immune. It prevents further infection because the body responds quickly by producing more antibodies
  • The antibodies recognise the antigen (protein shape) on the pathogen
  • The MMR vaccination (immunisation) is one of several vaccines. MMR is given to prevent measles, mumps and rubella
  • Most people in a population need to be vaccinated to protect society from very serious diseases. This is known as herd immunity
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How do we deal with disease?

  • Most people in a population need to be vaccinated to protect society from very serious diseases
  • Diseases such as measles can lead to long term damage to the boyd, such as deafness and occasionally death.
  • Some vaccines cause side effects which may be mild or serious. So there are advantages and disadvantages of vaccination (immunisation).
  • Overuse of antibiotics can lead to the development of new strains of bacteria
  • Doctors do not prescribe antibiotics for mild infections such as minor sore throats
  • Scientists are always trying to find new ways of treating disease
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