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

Our Skin

Stops microbes entering the

body unless it is cut or damaged

The Respiratory System

Mucas 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

The Second Line of Defence

The Second Line of DefenceNon 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

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

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The Third Line of Defence

The Third Line of Defence

Specific White Blood CellsAll 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 microbes 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 CellsThe 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