Allergies occur when the immune system reacts to environmental substances that are harmless to most people.
Common allergens are food, pollen, dust mites, animals, insect stings, or medicines.
Most allergic reactions are only mild to moderate and can effectively be treated with antihistamines. But other reactions can be life-threatening and require emergency medical treatment.
The most severe allergic reactions are known as anaphylaxis, and people who suffer from it should have an emergency management plan that includes an adrenaline auto-injector.
Allergies are becoming more frequent in the western world. One in twenty will develop a food allergy in their life, and one in 100 will have a life-threatening allergic reaction.
From 1994 to 2004, hospital admissions for anaphylaxis doubled. In children under five years old, it was five times higher over the same period, suggesting allergy in early life is increasing faster than in adults.
Theories for why the number of allergies is rising: