The immune system is a complex network within the body and its main function is to eradicate invading infectious agents, viruses, bacteria, protozoa and parasites. The cells making up the immune system are produced in the bone marrow and circulate in the blood and lymphatic vessels. Both blood and lymph pass through organs such as the spleen and liver where foreign material to which the immune system has reacted is destroyed.

A distinctive feature of the system is the ability of immune cells to leave the blood and travel into the tissues of the body. About half of one of the most important types of cells, called lymphocytes, are associated with the skin or lining of the lungs and bowel where it is most likely that micro-organisms might invade the body. In addition to the protective effect of the immune system, the skin and linings of the lungs and bowel are good barriers against micro-organisms. Sebum, tears, saliva and mucus have a good but non-specific protective effect against infection, while helpful 'symbiotic' bacteria in the gut prevent the growth of disease-causing micro-organisms, probably by competing more successfully for nutrients.

Apart from cells the immune system uses molecules usually proteins which function to help destroy micro-organisms and to co-ordinate the cells by passing messages between them. Co-ordination can occur for cells in close proximity or over long distances.

The immune system has two major components:

• Non-specific or innate immunity reacts to any type of micro-organisms, but these reactions do not change with experience.

• Specific or acquired immunity allows the body to react to particular micro-organisms more powerfully and more quickly if they invade the body more than once. Acquired immunity involves the immune system developing a memory, and during our lifetime a vast number of highly specific memories accumulate to help the immune system become more efficient. This ability is exploited in vaccination where a non-harmful vaccine induces specific immunity to subsequent infection with a particular micro-organism. However, because these reactions are specific, a polio vaccine, for example, will not give protection against chickenpox.