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The Body's Defence System
· The first line of defence stops pathogens entering the body
· The first line of defence consists mostly of physical barriers
· The skin ­ undamaged skin is a very effective barrier
against microorganisms and if it gets damaged, blood clots
quickly seal cuts to keep microorganisms out
· The respiratory system ­ the whole respiratory tract (nasal
passage, trachea and lungs) is lined with mucus and cilia.
The mucus catches dust and bacteria before they reach the
lungs and the cilia push the gunk-filled mucus away from
the lungs.
· The eyes ­ eyes produce (in tears) a chemical called
lysozyme which kills bacteria on the surface of the eye. This
is a chemical barrier…read more

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The Body's Defence System
· The second line of defence is non-specific white blood cells
· Anything that gets through the first line of defence and into
the body should be picked up by white blood cells called
· Phagocytes detect things that are `foreign' to the body, e.g.
microbes. They engulf microbes and digest them.
· Phagocytes are non-specific ­ they attack anything that's
not meant to be there…read more

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The Body's Defence System
· The third line of defence is specific white blood cells
· White blood cells called lymphocytes attack specific
· Some lymphocytes have receptors that recognise particular
antigens (an antigen is a substance that causes an immune
response ­ usually protein molecules on the surface of
pathogen cells.)
· Some of the lymphocytes attach to an antigen and then
destroy the cell carrying it. Others coordinate the rest of
the immune response
· A different kind of lymphocytes secretes antibodies ­ these
are specific to a particular antigen. These antibodies latch
onto the invading cells and mark them out for destruction
by other…read more

Slide 5

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Microbes: our defence against
Our bodies have four major defence mechanisms against
invading microbes: The breathing
organs produce
The skin acts mucus to cover the
as a barrier lining of these
organs and trap the
If our skin is
cut platelets Our blood contains
seal the wound white blood cells
by clotting…read more

Slide 6

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How do our bodies resist infection?
Micro-organisms ...... pathogens
Skin Stomach acid Cilia
White blood cells…read more

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