ks4 defence against disease (boardworks)

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Defence Against Disease

KS4 Biology



Contents

Defence against microbes

Transmission

Why do humans fall ill?

Attacking microbes

Summary quiz


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.

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.



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!



How do microbes enter the body?

ears

mouth

nose

skin

cutsgenitals

A number of these places where microbes can enter the body are defended.

eyes



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.

Mouth



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.

Nose


Skin

blood capillary

thick layer of skin

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.



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.



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.

cutexposed blood vessel

scab has formed over the wound

new tissue being

formed

microbes cannot gain entry



???


Different defence mechanisms


Contents


Transmission


Attacking microbes


Summary quiz


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.

Spreading microbes


Methods of transmission

Transmission can occur in the following ways:

If an infected person sneezes or coughs, millions of microbes are released into the air.

These microbes could then infect somebody else.

Droplet transmission



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.

Transmission in water



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.

Transmission by vectors



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.


Transmission of microbes


Contents


Transmission


Attacking microbes


Summary quiz


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…

White blood cells


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.

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.

1. Producing antitoxins

Methods of attacking microbes


toxins

bacteriumwhite

blood cell

antitoxins

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.



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.

microbewhite blood cell

The white blood cell surrounds the microbe.

2. Ingesting 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.



As well as releasing antitoxins, the white blood cell canrelease 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.

3. Releasing antibodies




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.


The antibody attaches to the specific site on the microbe, which leads to its destruction.

microbe is destroyed

attachment site antibody microbe



Engulfing microbes - what’s the order?


Engulfing microbes - what happens?


Matching microbes and antibodies


Contents


Transmission


Attacking microbes


Summary quiz


Well, there are 3 problems…

If white blood cells can produce antibodies against microbes, why do humans fall ill at all?



1) The first is that different microbes require different antibodies. This is due to the attachment sites on the microbes being different shapes.

will not be effected by these antibodies.

A microbe with these attachment sites …

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.



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.



3) When an organism becomes infected, it takes a certain amount of time to mount an effective defensive response.



At first, the microbe will enter the body and release toxins.

microbe number

toxins white blood cells

antibody/antitoxin

The microbes will start reproducing rapidly. As they do, they produce more toxins. The white blood cell number is low because the body was caught off guard.

microbe number


antibody/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.

Antibodies and antitoxins


The body starts replicating its white blood cells and in doing so, starts making large amounts of antibodies/antitoxins.

The illness is at its peak and there is a battle raging in the body.

microbe number


antibody/antitoxin

Slowly the number of white blood cells increases to such a level that the concentration of antibodies/antitoxins is high enough to overpower the microbes.

The person begins to feel better.

microbe number


antibody/antitoxin




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.


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!

Constant attack


Disease defence - what’s the order?


Contents


Transmission


Attacking microbes


Summary quiz


Defence against disease quiz

ks4 defence against disease (boardworks)

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