The way in which the human body protects itself from infection is extremely complex, involving several different organs and systems.

Infection arises when the body is invaded by bacteria, viruses or fungi: collectively called micro-organisms, but many such organisms enter the body continuously, and some live permanently in it without causing harm. Infection is only said to be present when an invading organism begins to reproduce and multiply within the body.

The first line of defence is the skin, and the mucous membrane which lines the mouth, nose, lungs, etc. Bacteria cannot penetrate the skin unless it is broken, and both sweat and sebum are mildly antiseptic. The mucous membranes are a somewhat less effective barrier, being able to exclude certain bacteria but not others.

Once any threatening micro-organism enters the body, a chain of events is set in motion, involving various specialised cells in the blood, lymphatic system, spleen, thymus and tissue fluids. This is known as the immune response. Although the cells most actively involved in this response are collectively known as white blood cells, they are found in large numbers in the lymph nodes, tissue fluids, etc., and it is possible that their major activity takes place outside the bloodstream which merely serves to transport them to where they are most needed.

Phagocytes, formed in the bone marrow, are large white cells which literally wrap themselves round foreign particles, including bacteria, and kill them, although often the cell itself dies in the process. The pus which gathers at an infected wound, for example, contains vast numbers of these cells together with the dead bacteria. Phagocytes are often referred to as 'scavenger' cells.

Lymphocytes, which are formed both in the bone marrow, and in lymph tissue (lymph nodes, spleen and thymus) have a different function: they are able to manufacture antibodies from complex proteins (gamma globulins). These are produced in response to threat from a specific micro-organism, and when the same organism is encountered in future, it is opposed by the antibodies already present in the blood, which help to suppress the growth and activity of bacteria and viruses. When sufficient antibodies to any organism are present to prevent disease symptoms arising, we are said to have immunity to that particular organism.

Co-ordinating the activity of these cells are the T-cells. T-helper cells stimulate the production and activity of phagocytes and lymphocytes, while T-suppressor cells initiate the 'winding-down' process when danger from the infection has passed. When the immune system is functioning normally, T-helper cells out­number T-suppressor cells by about 2 to 1, but if the system is damaged or depleted, the number of T-helper cells declines.

The lymphatic system plays an important role in the immune response. Large numbers of lymphocytes are formed in the lymph-nodes in response to infection. A larger than usual number of bacteria circulating in the lymphatic fluid causes production of lymphocytes to increase dramatically. Also in the lymph nodes are large 'scavenger' cells called macrophages, which filter and engulf bacteria and other unwanted particles, During infection, all activities of the lymph nodes are heightened and the accumulation of active and dead cells and bacteria may cause the nodes to become enlarged. This can often be felt and seen in the neck, armpits and groin, and this swelling is characteristic of some illnesses, such as glandular fever.

The adrenal glands also play a part in the immune response, secreting hormones that trigger some of the processes. Stress lowers the body's resistance to infection partly because periods of stress exhaust the adrenals.

Although an orthodox definition of the immune system does not include the colon, it is now thought that a healthy colon is an important part of the protective mechanism. Millions of 'friendly' bacteria in the gut (the intestinal flora) help to control and suppress micro-organisms that could otherwise endanger the body.

Essential oils can support and strengthen the immune response in two ways: by directly opposing the threatening micro­organisms or by stimulating and increasing the activity of the organs and cells involved. A number of essential oils combine both these actions, for example, Lavender, Bergamot, Eucalyptus and Rosemary all act against a wide variety of bacteria and viruses while at the same time increasing the immune response. Rosemary and Geranium support the adrenal glands in their action and are also stimulants of the lymphatic system. Black Pepper and Lavender have a beneficial action on the spleen.

A very important group of oils in this context is the Melaleuca family: Cajeput, Niaouli and, most important, Tea tree, for they combine bactericidal, antiviral and fungicidal properties with a powerfully stimulant action on the immune response. Ti-Tree is active against all three categories of micro-organism and is also one of the most powerful immuno-stimulants we know. The use of these oils, as well as those mentioned in the previous paragraph, therefore assist the body very effectively in resisting and combatting infection, and this action is especially marked if the oils are used at the first onset of symptoms.

However, virtually every essential oil in therapeutic use is active against one or more bacteria, and almost all of them stimulate production of white blood cells, though lavender, Bergamot and Tea tree do this most markedly. People who use essential oils all the time, as part of their daily bathing, skincare and household routines, mostly have a high level of resistance to illness, 'catching' fewer colds, etc., than average and recovering quickly if they do.

Where infection is recurrent or prolonged, and the immune system is depleted, treatment with a variety of essential oils should be continued for at least a month, so that not only are micro-organisms in the body brought under control, but the immune system repaired and strengthened to resist future attack.

Nutrition is also important, because a variety of essential nutrients are needed for the manufacture of the white blood cells. A diet with adequate protein and a high proportion of raw, fresh fruits and vegetable, seeds and grains with a little unsaturated vegetable oil, will usually provide all that is needed, but if the system is very depleted, supplementation is vital until a healthy balance has been restored.