What are allergies?

Allergic diseases result from the immune system reacting to a substance that it considers foreign. This system plays a role in defence against microorganisms and potentially disease-causing substances.

When there is an allergy, the immune system acts against a normally harmless substance, considers it foreign and mounts an inflammatory reaction against it with certain special characteristics.

The substance that triggers the reaction is normally a protein and is called an allergen. The body manufactures another protein, called an antibody, against it. Antibodies belong to a special group of proteins called immunoglobulins.

The antibodies responsible for neutralising allergens belong to a class of immunoglobulins, specifically Immunoglobulin E, also called IgE.

When the body first comes into contact with the allergen, it manufactures IgE against it. This is called sensitisation. From this point on, any subsequent exposure to the allergen is generally going to cause symptoms characteristic of allergic reactions.

The IgE formed in the process of sensitisation travels throughout the body and eventually binds to certain cells: mast cellsand basophils. When there is further contact with the allergen, these cells burst and release a number of inflammation-mediating substances. The best known of these is histamine, responsible for the inflammatory allergic reaction, whose symptoms may be rhinitis, conjunctivitis, urticaria, asthma and others.

Each type of IgE targets a specific allergen, i.e. is specific, such that a person who only manufactures IgE against cat fur is not going to be allergic to other substances. These patients are said to be monosensitised, since they are only sensitised to a single allergen.

By contrast, some people manufacture IgE against multiple allergens; they are called polysensitised..

The process by which one person becomes allergic and another does not is unknown. However, it is known that people with a family history of allergic diseases are more prone to suffering from them.

In an allergic disease, the immune system mounts an exaggerated inflammatory reaction when it comes into contact with a substance that the body considers foreign.