The Body’s Natural Defences
The Body’s Natural Defences
• Our body fights harmful microbes every day
• Microbes must fight through three levels of our
body’s defences before they can cause us
harm
• The first line of defence
• The second line of defence
• The third line of defence
The First Line of Defence
The First Line of Defence
The Respiratory
System
Our Skin
The Eyes
Tears produce chemicals called
enzymes
which
can
killinsome
Mucus
and
tiny
hairs
the nose
Stops microbes entering
the
microbes
on
the
surface
of
the
eye
stop microbes entering our lungs
body unless it is cut or damaged
We sometimes sneeze or cough
these harmful microbes away
from our body
The Second Line of Defence
The Second Line of Defence
Non Specific White Blood Cells
Red blood cells:
The blood
contains
Carry iron and oxygen around
the body
White blood cells:
Help fight infection
Plasma:
Liquid which carries the blood
cells around the body
The Second Line of Defence
Non Specific White Blood Cells
The blood flows around the body in one direction only
The Second Line of Defence
Non Specific White Blood Cells
If the skin gets damaged,
microbes can enter the blood
stream and make us ill
The Second Line of Defence
Non Specific White Blood Cells
The White Blood Cells called ‘phagocytes’ notice anything
foreign or strange that gets through the first line of defence and
move towards that ‘foreign object’
The Second Line of Defence
Non Specific White Blood Cells
The phagocytes then surround and
destroy the foreign object or, in this
case, microbe
www.csulb.edu/depts/biology/media/cell751.gif
The Third Line of Defence
The Third Line of Defence
Specific White Blood Cells
All invading cells have
distinctive antigens on their
surface
When the specific white
blood cell comes across a
microbe, it will
a) Make an antibody to match
that microbe’s antigen
b) Tell all the other specific
white blood cells to make
lots of that antibody
The Third Line of Defence
Specific White Blood Cells
The body’s antibodies find
their matching antigen on the
harmful microbes and lock
on to them
The Third Line of Defence
Specific White Blood Cells
The antibodies will then
a) Mark the harmful microbes for destruction
b) Stay in the blood after the harmful microbes have been killed ready to
fight them if they should ever return