The document summarizes the body's three lines of defense against harmful microbes. The first line of defense includes the skin, respiratory system, and eyes acting as barriers. The second line of defense is the blood, which contains white blood cells that fight infections. The third and most specific line of defense involves specialized white blood cells that produce antibodies to target unique antigens on microbes they encounter. Together, these three levels of defense protect the body from infection.

1. The Body’s Natural Defences

2. 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

3. The First Line of Defence

4. The First Line of Defence
Our
Skin
Stops microbes entering
the body unless it is cut
or damaged
The Respiratory System
Mucus and tiny hairs in the nose
stop microbes entering our
lungs
We sometimes sneeze or cough
these harmful microbes away
from our body
The
Eyes
Tears produce chemicals
called enzymes which can
kill some microbes on the
surface of the eye

5. The Second Line of Defence

6. The Second Line of Defence
Non Specific White Blood Cells
Red blood cells: Carry iron and oxygen around
the body
White blood cells: Help fight infection
Plasma: Liquid which carries the blood
cells around the body
The blood
contains

7. The Second Line of Defence
Non Specific White Blood Cells
The blood flows around the body in one direction only

8. 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

9. 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’

10. The Second Line of Defence
Non Specific White Blood Cells
The phagocytes then surround and
destroy the foreign object or, in this
case, microbe
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11. The Third Line of Defence

12. 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

13. 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

14. 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