An allergy is an adverse reaction that the body has to a particular food or substance in the environment.
They are very common and are thought to affect more than one in four people in the UK at some point in their lives.
Some of the most common allergens include:

Symptoms of a mild allergy can include:
Anaphylaxis (or anaphylactic shock) is a severe allergic reaction
The reaction can be very fast, happening within seconds or minutes of contact with the thing a person is allergic to.
When anaphylaxis occurs, a chemical called histamine is over-produced, causing one or more life-threatening Airway, Breathing, or Circulation problems.