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 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

The Second Line of Defence

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

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

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