B1a4 Infectious Diseases - Defence against Pathogens

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 Natural Defences against diseases.

The body has different ways of protecting itself agains pathogens.  White blood cells help to defend us against pathogens which may have entered the body.  They protect us in the following ways:

  • by ingesting ('eating')  the pathogens
  • by producing antibodies which destroy particular bacteria or viruses
  • by producing antitoxins which act against the toxins produced by pathogens.


People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body.  This is the process of vaccination.  The vaccine stimulates the white blood cells to produce antibodies that bind to and destroy the pathogen.  The person becomes immune to future infections of the microorganism as the body can rapidly make the correct antibody (as it has been previously exposed to an inactive form of the disease).  The MMR vaccine is used to protect children against three diseases caused by viruses:  measles, mumps and rubella.  This vaccine had been incorrectly linked to an increased risk of autism, a claim that has now been proved to be unfounded.  How vaccines work is shown in the diagrams below:

Girls are now routinely vaccinated against the human papilloma virus (HPV) which can cause cervical cancer.  This virus is spread by sexual contact.  Vaccinating girls before they become sexually active means they are immune to the disease caused by the virus before they can become exposed to it.  The risk of contracting HPV increases with the number of sexual partners a girl has.  If a girl has few (or no) sexual partners there is a very low risk of contracting the disease and the vaccination is not likely to be of benefit.  However, mass vaccination against smallpox and polio has led to smallpox being eradicated from the world and polio is now found in a few countries.  These diseases used to kill or maim (polio causes limbs to become deformed) thousands of people each year.