Human Defence Systems (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Non-specific defences

• The human body has a number of mechanisms that are the first line of defence against an infection

• The non-specific defence systems of the human body against pathogens include:

  • The skin

  • The nose

  • The trachea and bronchi

  • The stomach

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The immune system

• The immune system of the body is highly complex, with white blood cells being the main component

• Once a pathogen has entered the body, the role of the immune system is to prevent the infectious organism from reproducing and to destroy it

• White blood cells help to defend against pathogens by:

  • Phagocytosis

  • Production of antibodies

  • Production of antitoxins (these are special types of antibodies)

Phagocytosis

• Phagocytes destroy pathogens in a non-specific way through phagocytosis

• The phagocyte engulfs the pathogen and releases enzymes to digest and break it down to destroy it

• This can be enhanced by antibodies which cause agglutination (clumping) of pathogens

How antibodies enhance phagocytosis  

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Production of antibodies

• Lymphocytes produce antibodies

• Antibodies are Y-shaped proteins – each individual has the potential to make millions of different types of antibodies, each with a slightly different shape

• The aim of antibody production is to produce the antibody that is specific (complementary) to the antigens on the surface of the pathogen

  • This is a specific type of immune response, as the antibodies produced are specific to each pathogen's antigens

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• It can take a few days to make the antibodies that are specific to a pathogen, and this may give the pathogen enough time to increase in number, spread and cause illness

• Memory cells are lymphocytes that remain in the body after an initial infection with a particular pathogen and produce the antigen-specific antibodies

• If the same pathogen infects the body again, specific antibodies are produced much more quickly to effectively prevent illness

Production of antitoxins

• Some pathogens (usually bacteria) can produce substances that act as toxins and cause illness

• Lymphocytes can produce antibodies against these substances – in this case, they are called antitoxins

• The antitoxins neutralise the effects of the toxin 

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