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
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
Our Skin
Stops microbes entering the body unless it is cut or damaged
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
The blood contains
White blood cells: Help fight infection
Plasma: Liquid which carries the blood cells around the body

7. The blood flows around the body in one direction only
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

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