Faced with such daunting invaders how can our body fight back?

There are two approaches that the body uses, passive and active defences.

Passive defences are those that are set up to stop bacteria or viruses entering the body. They act as roadblocks. They are found in those places where the invaders are most likely to try to the body.

Invaders try to get inside us via five main areas:

1. The skin

The skin is a very good, tough barrier to infection so long as it does not get damaged.

When it is cut or punctured the flow of blood will carry out some invaders. Then when it clots it stops others getting in.

2. The eye

Contains very delicate tissues that can be easily crossed. To give protection our tears will wash out some invaders and releases an enzyme that kills bacteria.

3. The respiratory system

The airways all the way down into the alveoli in our lungs give a perfect way to invade.

Fortunately they are lined with cells that can produce mucus in which dust and microbes get trapped. This mucus is then brought up towards the mouth by the wave-like action of the cilia attached to the cells.

4. The reproductive system

Apart from the thin skin present around the penis and vagina, there aren't many defences to microbes. So quite a few microbes are able to cause diseases if they try this route of attack.

5. The digestive system

Many microbes enter through our mouths on our food.

However they face a difficult time! Our saliva contains an enzyme which attacks bacterial cells. Then our stomach contains hydrochloric acid and more enzymes.

Most microbes will be killed. So not a pleasant trip!

Now have a go at matching the correct part of the body to the description of the passive defences used there:

Other ways in!

Microbes also use another means of access. They hitch a ride on or inside another organism, called a vector.

For example, malaria gets into humans through the bite of a mosquito. As the mosquito feeds by boring a hole through the skin and sucking blood, the malarial parasite hitches a ride through the mosquito's saliva into our blood stream.

The bacteria that caused the Bubonic plague in the 1300's was even more crafty! The bacteria hitched a ride inside fleas. The fleas then were carried about on rats. The rats then hitched rides on sailing ships.

No wonder the disease spread so quickly, almost 20,000 miles in 16 years at one time, and killed at least a quarter of the European and Asian populations.

The body has no passive defence against vector-borne attack, it just does not expect microbes to enter this way.

Once the body's passive defences are breached there is another line of defence. The active system of cells go and deal with the invaders directly. This is the immune system.

The most important of the cells in this defence system are the white blood cells

White blood cells are obviously carried around in the blood, but they can also crawl out of blood vessels and get to any cell in the body. They can go anywhere!

The immune system cells act in 3 ways:

1. Consume the invaders

The white cells extend their cell membrane around the invader. Eventually it is taken inside the cell by endocytosis and enzymes are secreted to destroy it. The white cell then absorbs what is left.

2. Produce antibodies

Other white cells produce antibodies when they meet an invading microbe. These special protein molecules stick to the microbes and mark them as 'foreign' invaders.

The microbes are either killed or other white cells come along and consume them.

The beauty of antibodies are that once they have been made your body is always ready attack that same invader on another occasion. You have developed anatural immunity to it.

3. Produce antitoxins

Yet other white cells produce antitoxins. These are like an antidote to the toxin poisons made by some bacteria. The antitoxins stick to the toxins and make them harmless.