What is an allergy?

At least a quarter of the Swiss population is affected by allergies. The trend is upwards. The reasons are many and varied. Experts distinguish between allergy types.

Doctor doing prick tests on a child's back. The child sits on his mother's lap.

In an allergy, the body has a hypersensitive reaction to substances that are harmless in themselves, such as proteins of pollen. The incidence of allergies in industrialised countries is partly linked to our modern lifestyle.

Hypersensitivity reaction of the body

An allergy or an allergic disease means a hypersensitivity reaction of the body to harmless foreign substances. These substances, nearly always proteins (e.g. of pollen, house dust mites, animals, foods or medicines), are referred to as allergens. If sufferers come into contact with these allergens, for instance through their diet or by inhaling them, the body reacts with a defence that is inappropriate and is referred to as an allergy. This reaction to the foreign-body substance leads to a variety of allergic symptoms.

In recent years and decades, allergies have generally increased sharply. The reasons for this have still not been fully explained. It is obvious, however, that allergies are on the increase in countries with a high standard of living and improved hygiene. There is an assumption that the high hygiene standards are partly responsible. The immune system is put under far less stress from natural enemies (but also harmless bacteria, viruses), so that it has forgotten how to distinguish between harmful and harmless substances and overreacts to harmless proteins.

Four allergy types

Different allergens trigger four mechanisms and symptoms. The reactions can occur immediately or with a delay. Experts distinguish between allergy types.

Immediate-type allergy (Type I)

In this type, the symptoms occur immediately or within a few minutes. They usually affect the skin or mucous membranes. The immediate type is the most common form of allergy, as in pollen allergy, food allergy, allergy to animals or to insect venom.

Type II allergy

This is a very rare form of allergy. Blood cells may become damaged. Medicines, for example, are possible triggers of Type II allergy.

Type III allergy

This often plays a key role in drug allergy. Antibodies are directed against foreign substances (antigens) that are dissolved in the blood. This gives rise to larger complexes which are deposited on the walls of the small blood vessels, e.g. in the skin or the kidneys, where they trigger an inflammatory reaction.

Delayed-type allergies (Type IV)

This is a late allergic reaction. It is the only form of allergy in which the sensitised defence cells (T-lymphocytes) act directly against allergens. In most cases, Type IV allergens are not large molecules, such as proteins, but metals or simpler chemicals. Type IV allergies appear, for instance, in the form of contact eczema, e.g. to nickel in fashion jewellery or fragrances in perfumes.

Facts and figures

Around 35 per cent of the Swiss population are sensitised to allergens and hence have a predisposition to allergies. Approximately 20 to 25 per cent have already displayed allergic symptoms. These figures are in line with experiences in other industrialised countries. The risk of allergy is unequally distributed. Children whose parents or siblings already suffer from allergies have a higher risk of developing allergic reactions as well.

Editors: aha! Swiss Allergy Centre in co-operation with the Scientific Advisory Board.