The world is full of life - much of it unseen and a threat to the human body. Countless viruses, bacteria, fungi and parasites have the capacity to invade the body. If they multiply, and produce poisonous wastes, they can damage or even kill. Yet most invasions by microbes and parasites come to nothing or cause nothing more than temporary illness. In a healthy individual, infection is almost always checked by the immune system before serious damage is done.

Inflammation
An important part of the immune system that is involved in the body’s response to injury or infection is inflammation. Although it may provide an uncomfortable experience, inflammation is very useful. It helps send the immune system's defences to the site of injury or invasion. Inflammation increases blood supply so more of the immune system's cells in the blood are available and this helps white cells move out of blood vessels to where they are needed.

There are two parts to the immune system:
The innate immune system>
The adaptive immune system>

You might not think of the skin as part of the immune system, but it is. It’s one of a number of physical defences, such as the acid in the stomach and the cilia (hair-like structures) that line part of the respiratory tract, that keep out harmful bacteria, destroy them or expel them when they enter the body. There are other parts of the system too. An enzyme called lysozyme (found in many of the body’s fluids) can break down the walls of many bacteria. Also part of this system are the so-called “good bacteria”, such as those that normally inhabit the digestive system.

If these external barriers are breached, certain types of white blood cells, phagocytes and natural killer cells - go to work. Phagocytes (phago is Greek for eating, cyte relates to cell) engulf bacteria and other foreign particles and destroy them. Natural killer cells are able to recognise some cells that have been invaded by viruses or have become cancerous and kill them. Various proteins in the body help these processes.

As the name innate suggests, this is a defence mechanism that we are born with and it is incapable of learning or improving as a consequence of repeated infection.

If the innate immune system fails to deal completely with an invader, the adaptive immune system comes into play, producing substances called antibodies. These are produced by different white blood cells called lymphocytes. Antibodies are tailor-made to match the invader precisely, so they only bind onto a part of the particular germ they were made for. Phagocytes are then able to recognise the combination of microbe and antibody and engulf the invader.

The body can make millions of different antibodies. Each variety is specific for a particular invading organism. Once antibodies have been made against a particular microbe, they can stay in the system for years or even for life. Next time the same invader is detected, the antibodies deal with it before it has a chance to cause a problem. This is the mechanism that stops you catching some diseases more than once in a lifetime and it is how vaccination works.

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