defending against infection

A microorganism (often called a microbe) is a tiny single-cell organism that you need a microscope to see.

There are billions of microorganisms inside us and on our skin. Over 95% of those inside us right now are helpful - or at least harmless. The rest are not nice to have around - they may multiply rapidly inside our bodies and cause infectious illness; these nasty ones are called pathogens.

Pathogens cause us problems by either:

• releasing toxins (poisons), or
• invading and damaging our cells (if they are viruses).

To a microbe, a human being is a bit like a luxury hotel on legs, so it tries to get inside our bodies. The body has two ways of defending itself.

• The first line of defence is to keep the pathogen out. The body is like a fortress: the outer defences include our thick skin, sticky mucus and cilia in the respiratory system, enzymes in tears - and acid in the stomach which kills most pathogens that enter our stomachs via our food.
  
  
• The second line of defence kicks in if a pathogen does manage to get through via the normal openings or via a cut or graze. What happens is that our white blood cells attack the microbes in various ways - and usually manage to destroy them. (In addition, if we have a wound, a protective scab forms.)

Two ways in which doctors help the body's fight against infection are:

• Vaccination, in which pathogens are introduced into the body in a weakened form. This process causes the body to produce enough white blood cells to protect itself against the pathogens, while not getting diseased.
  
  
• Antibiotics, which are effective against bacteria but viruses. Some strains of bacteria are resistant to antibiotics.

»  Pathogens: bacteria  and  viruses

                 »  White blood cells: part 1  and  part 2

                                   »  Vaccination

                                                     »  Antibiotics

[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]