Human Defence Systems (AQA GCSE Biology: Combined Science)

• 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

The different mechanisms can be divided into biochemical and physical defences

• 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 antibody)

Phagocytosis

• Phagocytes engulf and digest pathogens, this can be non-specific or helped by antibodies which cause agglutination (clumping) of pathogens
• The phagocyte surrounds the pathogen and releases enzymes to digest and break it down to destroy it

Phagocytes engulf and digest all the pathogens that they come across, they can be enhanced by antibody production

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

The lymphocytes produce antibodies that are specific to the antigen on the pathogen

• It can take a few days to make the antibodies that are specific to a pathogen and this may give the pathogen causing the infection enough time to make you feel unwell (as its numbers in the body, and consequently the damage caused by them, increases)
• Memory cells are lymphocytes that remain in the body after an initial infection with a particular pathogen; they produce the specific antibodies against its antigens so that if you get infected by the same pathogen again in the future (and the antigens are the same) you can produce antibodies much quicker against it before its numbers increase and it can cause damage to the tissues of the body

Production of antitoxins

• Some pathogens (usually bacteria) can produce substances which act as toxins which make you feel unwell
• Lymphocytes can produce antibodies against these substances – in this case, they are called antitoxins
• The antitoxins neutralise the effects of the toxin

Lymphocytes produce specific antitoxins to specific pathogens