KS4 Biology
Defence Against
Disease
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Contents
Defence Against Disease
Defence against microbes
Transmission
Attacking microbes
Why do humans fall ill?
Summary quiz
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How does the body defend itself from disease?
Imagine a country is invaded. If the army is
to defend that country, the best way is to
surround the enemy and keep it in one
place.
You can then concentrate all your efforts in
one place and your army is more powerful.
If the invading army quickly spreads, they
can attack more places at once and your
defence is thinned.
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How does the body defend itself from disease?
How do you stop an invading army from spreading out?
You stop it from reaching the transport networks.
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How does the body defend itself from disease?
If we consider the human body, the infection will
spread around the body if it reaches our transport system.
the blood stream
The body’s defence systems are trying to prevent
microbes access to the blood.
If they get into the blood, they can travel to every part of
the entire body!
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How do microbes enter the body?
eyes
ears
mouth
nose
skin
cuts
genitals
A number of these places where microbes can enter
the body are defended.
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Defence against microbes
Mouth
If microbes enter through the mouth (usually on food) they
will be swallowed and enter the stomach.
Within the stomach is hydrochloric acid, which is highly
concentrated and will kill them.
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Defence against microbes
Nose
Cells lining the trachea and bronchi have hair like folds in
their membrane. These are known as cilia and the cells are
termed ciliated. These cells also produce mucus, a sticky
liquid in which microbes can become stuck. The cilia then
waft the mucus (including the trapped microbes) towards the
mouth and nose where they are either swallowed or
coughed up.
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Defence against microbes
Skin
thick layer of skin
blood capillary
Microbes have to penetrate the layer of skin that covers
the entire surface of our body. Once through this, they can
then reach the blood system.
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Defence against microbes
Cuts
When you cut the skin, the body will bleed because the
blood vessels are ruptured. This leaves the blood system
exposed to the air. Microbes could then get into the blood.
To limit this problem, the body can
quickly heal wounds by firstly clotting
the blood and secondly forming a
scab over the wound to shield the
blood from the air.
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Defence against microbes
Once the scab has formed, the body repairs the damaged tissue
underneath and finally fresh skin tissue is formed.
If the cut is very deep, a scar will remain after the cut has healed.
cut
exposed
blood vessel
scab has
formed over
the wound
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microbes cannot
gain entry
new
tissue
being
formed
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???
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Different defence mechanisms
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Contents
Defence Against Disease
Defence against microbes
Transmission
Attacking microbes
Why do humans fall ill?
Summary quiz
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Spreading microbes
One easy way to defend the body against illness is by
keeping it clean and hygienic.
This is because microbes can spread very quickly in
unhygienic conditions.
The spreading of microbes is known as transmission.
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Methods of transmission
Transmission can occur in the following ways:
Droplet transmission
If an infected person sneezes or coughs, millions of
microbes are released into the air.
These microbes could then infect somebody else.
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Methods of transmission
Transmission in water
Microbes can feed off the pollution within water.
If water is not cleaned, the microbes can reproduce and
enter the body when the water is drunk.
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Methods of transmission
Transmission by vectors
A vector is an animal that carries the microbe from one
place to another.
Vectors can range from houseflies to pets. It is important
to be careful about the amount of contact you have with
vectors.
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Methods of transmission
Transmission by contact
Many microbes can be exchanged from one
person to another within bodily fluids e.g.
blood and semen.
This is how many microbes responsible for
sexually transmitted diseases are spread
from person to person.
The less hygienic people are, the greater the
chance of infection. Microbes will also
reproduce rapidly in unhygienic conditions.
This is the main reason why public places
such as restaurants have to abide by strict
hygiene regulations.
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Transmission of microbes
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Contents
Defence Against Disease
Defence against microbes
Transmission
Attacking microbes
Why do humans fall ill?
Summary quiz
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White blood cells
If viruses and bacteria do get into the body, it is
extremely likely that they will enter the blood stream.
If this happens, the previous defence systems are
useless and so the body brings in its next line of
defence…
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Methods of attacking microbes
As we saw when we covered the blood system, white blood
cells are found in blood. They have a nucleus, which can
vary in shape from one sort of white blood cell to the next.
They have 3 main methods of attacking microbes.
1. Producing antitoxins
Remember that microbes can cause an infected person to
feel ill by releasing toxins (poisons). The first way white
blood cells defend the body is by releasing antitoxins. These
chemicals are designed to neutralise the effects of the toxins
and render them harmless.
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Methods of attacking microbes
bacterium
white
blood cell
antitoxins
toxins
Antitoxin links to the toxin and neutralises its effect.
The toxin is now harmless. This happens on a large scale
and reduces the effects of the infection.
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Methods of attacking microbes
2. Ingesting Microbes
Some white blood cells actually ingest (eat) the microbes
and then break them down once they are inside the cell.
The microbe is destroyed by powerful digestive enzymes,
which are released within the cytoplasm of the white blood
cell.
white
blood cell
microbe
The white blood cell
surrounds the microbe.
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Methods of attacking microbes
Digestive enzymes attack
the ingested microbe and
begin to break it down.
The microbe is destroyed.
The chemicals that are
released from the digested
microbe are used in other
parts of the cell.
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Methods of attacking microbes
3. Releasing antibodies
As well as releasing
antitoxins, the white
blood cell can
release another
chemical known as an
antibody.
Antibodies - you can
think of them as target
specific bombs.
Their shape is important
because it fits exactly onto
a site on the microbe.
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Methods of attacking microbes
The antibody will only work if it is connected to this
specially shaped site. This design means that the
antibody does not harm normal body cells because
they do not carry the attachment site.
These are the areas where
the antibody will attach.
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Methods of attacking microbes
The antibody attaches to the specific site on the
microbe, which leads to its destruction.
attachment site
antibody
microbe
microbe is
destroyed
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Engulfing microbes - what’s the order?
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Engulfing microbes - what happens?
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Matching microbes and antibodies
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Contents
Defence Against Disease
Defence against microbes
Transmission
Attacking microbes
Why do humans fall ill?
Summary quiz
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Why do humans fall ill?
If white blood cells can produce antibodies against
microbes, why do humans fall ill at all?
Well, there are 3 problems…
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Why do humans fall ill?
1) The first is that different microbes require different
antibodies. This is due to the attachment sites on the
microbes being different shapes.
A microbe with these attachment sites …
will not be effected by
these antibodies.
The body must be able to produce a range of antibodies
against an enormous number of different microbes.
If it can’t produce the right antibody, the infected organism
will become ill.
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Why do humans fall ill?
2) Microbes are constantly changing and becoming
resistant to old antibodies.
The microbe has the ability to change its shape and
therefore the body may not recognise them. If this
happens, the microbe will release toxins until it is
recognised and an attack can be mounted.
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Why do humans fall ill?
3) When an organism becomes infected, it takes a
certain amount of time to mount an effective defensive
response.
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Antibodies and antitoxins
The microbes will start
At first,
the microbe
willAs
reproducing
rapidly.
enter
thedo,
body
release
they
theyand
produce
toxins.
more toxins. The white
blood cell number is
low because the body
was caught off guard.
microbe
microbe
number
number
toxins
toxins
white
white
blood
blood
cells
cells
antibody/
antibody/
antitoxin
antitoxin
This is the point where the levels of toxin in the body will
cause the infected organism to feel ill.
There are not enough antibodies /antitoxins to overpower
the microbes.
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Antibodies and antitoxins
Slowly
the starts
number
of white
The body
replicating
blood
cells
increases
to such
its white
blood
cells and
in
adoing
level so,
thatstarts
the concentration
making
of
antibodies/antitoxins
is
large
amounts of
high
enough to overpower
antibodies/antitoxins.
the microbes.
microbe
microbe
number
The illness is at its peak
number
The
feel
and person
there isbegins
a battletoraging
better.
in the body.
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toxins
toxins
white
white
blood
blood
cells
cells
antibody/
antibody/
antitoxin
antitoxin
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Antibodies and antitoxins
Not only does the body fight off the disease; it also readies
itself for another infection from the same disease by
storing high levels of that specific antibody/antitoxin.
In other words, if that specific microbe infects the same
organism, there is no need to spend time replicating the
specific white blood cells in order to make the relevant
antibody. They are already made and so they attack the
microbe immediately.
This process of preparation for re-infection is known as
gaining natural immunity.
This why humans tend to only suffer from certain
diseases once in their lifetimes.
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Constant attack
Those diseases that keep re-infecting and causing
illness are usually caused by microbes that keep
changing their structures.
The body does not recognise the altered form of the
microbe and its ready-made antibodies are useless.
Once again we see how remarkable the human body is.
Countless microbes are constantly bombarding it:
microscopic disease causing organisms with the ability to
hide, fight, replicate and adapt.
All this happens without any conscious effort on our parts!
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Disease defence - what’s the order?
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Contents
Defence Against Disease
Defence against microbes
Transmission
Attacking microbes
Why do humans fall ill?
Summary quiz
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Defence against disease quiz
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