An allergy is an abnormal reaction of the body’s immune system to a foreign substance. In most people, the substance produces no symptoms, but in a susceptible person it triggers an allergic reaction. Most allergies are mild and merely unpleasant, and they are easily treated with drugs and self-help remedies. However, sometimes allergies can be life-threatening.

In this section, allergic reactions due to foreign substances such as pollen, food, and certain drugs are discussed first. The trigger substance (allergen) causes no symptoms on initial contact. However, the immune system begins to form antibodies against the allergen, and certain types of white blood cells become sensitive to it. Later contact with the allergen may stimulate specialized cells called mast cells to release histamine, the chemical that triggers the allergic response. Allergic reactions may lead to urticaria or angioedema, which are dealt with next. The last article covers anaphylaxis, a potentially life-threatening reaction to an allergen. Allergies often develop in childhood and may either persist or disappear in adulthood. Conditions such as asthma and eczema, which may have an allergic basis, are dealt with in the sections that cover the relevant body systems.

Key anatomy

For more information on the allergic response, see Preventing Indigestion.

Allergic response

Allergic Rhinitis

Food Allergy

Drug Allergy




From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

Back to top