How medicines work

Allergies are an excessive response of the body to substances in the environment. Any substance that triggers an allergic reaction is known as an allergen. Allergens trigger the release of chemicals called histamines from the mast cells of the immune system.

Allergic responses are based around a type of antibody found in the blood plasma known as immunoglobulin E (IgE). IgE appears to have a role in protection against gut parasites, but scientists are not completely clear about what it does. What they do know is that in susceptible people, IgE antibodies bind to the antigens on the surface of common environmental factors such as dust mites, cat skin flakes and pollen grains. Once the antibody is bound to the allergen, it also binds to a mast cell.

Mast cells are produced in the bone marrow and are carried all around the body. Each mast cell contains granules full of a chemical called histamine. When the mast cell binds to an allergen through IgE, the histamine is released.

Histamine brings about a number of different responses in the cells around it.

• It sets up an inflammatory response. This includes an increase in the permeability of blood vessel walls that can result in fluid accumulation. This can result in swelling, redness and heat as seen in eczema and hives. It can also cause the 'stuffed up' feeling in the nose in allergic rhinitis, along with excess mucus production, and the rashes and swelling that result from insect bites and stings, or nettle stings.

• It causes constriction of the smooth muscle. This is the response that causes so many problems in asthma and severe allergic responses such as anaphylaxis. The constriction of the smooth muscle closes up the airways and, combined with the excess mucus production, can make it impossible to breathe.

There are a number of medicines that can be used to treat allergies.

Antihistamines are the best known drugs used to treat allergic reactions affecting the skin and the nose. There are antihistamines that are applied topically to the skin in response to stings and bites, and there are antihistamines that are taken orally and are carried through the body systems in the blood.

Antihistamines work by targeting the histamine receptors on cells. By blocking the receptors, they block the response to the histamines. This damps down, and gets rid of, the allergic response.