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Protection Against Disease
Natural Defence Mechanisms
Learning Objectives
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How the natural defence mechanisms
prevent the entry of pathogens
Starter
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List all the adaptations that the body
has to prevent entry of pathogens
Preventing Entry
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The most effective way for humans to avoid infection
is to prevent them from gaining access
Skin is an effective barrier because it is impermeable
Only a few pathogens (mainly viruses) are able to
penetrate undamaged skin
The skin secretes sweat and sebum which contain
substances that destroy bacteria
We also have our own flora of harmless bacteria
called COMMENSALS
They compete more successfully than pathogens for
the small amount of nutrients on the skin
They are also found in the mouth, respiratory tract,
vagina and digestive system
Preventing Infection
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Most pathogenic bacteria however
cannot survive in these places-partly
because of the commensals and partly
because of the lactic acid and fatty
acids secreted from sweat and
sebaceous gland which provide a pH
too low for them
Candida and Thrush
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The normal bacterial flora can help to prevent
infection by other microorganisms e.g. the
bacteria that normally live in the vagina keep
the pH low by secreting lactic acid
If a person takes antibiotics these normal
bacteria may be killed and the pH of the
vagina then rises and other organisms like
Candida can multiply too much greater
population densities causing thrush
Methods of prevention
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Moist body surfaces e.g. the surface of the
eyes and mouth are bathed with fluids which
have some BACTERICIDAL ACTION
An enzyme called LYSOZYME is present in
saliva and tears and this enzyme can damage
and destroy many bacteria
Semen contains a bactericide called
spermine, milk contains a bacteriacidal
enzyme called lactoperoxidase
Other methods
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The HYDROCHOLORIC ACID
secreted by
the stomach is very effective in destroying
bacteria in ingested food
MUCUS helps to protect the digestive and
respiratory tracts from infection
Mucus contains LYSOZYME an enzyme which
destroys bacteria and it also acts as a barrier
so that bacteria cannot make contact with the
epithelia cells lining the walls of the tubes
Mucus
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In the trachea and bronchi the mucus is swept
upwards to the back of the throat by cilia and then
swallowed
Coughing and sneezing help to expel the mucus
containing microorganisms from the trachea and
bronchi
Once the mucus is swallowed the acid and enzymes
in the stomach destroy any bacteria trapped in it.
Any bacteria which manages to get throught he
defence mechanims may then enter the blood stream
and by ingested by phagocytic white blood cells
Tasks
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Complete the summary table
Answer ppq
Plenary
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