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When an organism gets into your body and causes an infection, your body gathers its defences and fights against them. This is the basic principle of how vaccines work.

Certain cells in your blood make what are called 'antibodies', molecules which are designed to attack specific germs and viruses. These attach to the invaders in your bloodstream and prevent them from invading other cells. Each virus or bacterium has an individual shape, and the antibodies are designed to fit exactly to that shape.

This is how vaccines work to convince your body that the vaccination is a 'full-blooded' attack by the offending viruses or bacteria, and stimulate them into action to develop the 'memory' or 'blueprint' for the antibody in the event of future invasion.

This is all done by your white blood cells. You have two types called B cells and T cells. The B cells manufacture the antibodies while the T cells have two functions. The 'helper' T cells help the B cells to make the antibodies while the 'killer' T cells kill any cells which have been invaded by the viruses or bacteria, and prevent them from reproducing. . How vaccines work to stimulate this action is to mislead the white cells into believing that your body has been infected.

Your body reacts to kill the invaders in two ways: directly through the antibodies, and indirectly through the T cells destroying any infected cells and preventing reproduction.

Viruses cannot reproduce by themselves: they have to use the host's cells for this. If the T cells continually kill off any invaded cells, the invaders themselves must eventually be killed off by the antibodies If the virus or bacterium is strong and reproduce too quickly, the host can be overcome before it can produce enough antibodies to kill them off. The pus which occurs during an infection is the mix of dead white blood cells and bacteria/virus cells destroyed by them.

If your body survives the attack, your B cells retain a memory of the structure of the invaders and should the same viruses or bacteria ever return, antibodies can rapidly be produced and the infection killed off before it starts. The stimulation of this memory is exactly how vaccines work.

Vaccines produce the same memory effect without the patient having to suffer the disease. The organisms that cause the disease are either killed or weakened, then introduced into your body. The strength is calculated to be just enough to enable your white cells to manufacture the antibodies. This is how vaccines work to give you protection against future infection without actually making you ill. The strength of the vaccine is designed to allow this. The dead vaccine can also work, but less efficiently, and the effect is not generally as long lasting.

The 'live' vaccines produce life-long immunity after only one or two doses, but the 'dead', or 'inactivated', ones need multiple doses to get the correct effect. Some dead vaccines even need booster doses throughout your life. Examples of these are tetanus and diphtheria vaccines, normally given together every 10 years as the Td vaccine. The measles vaccine is an example of a 'live' vaccine'.

Vaccinations do not affect your ability to fight off other infections you have not been immunized against