Anaphylaxis is a severe allergic reaction -- the extreme end of the allergic spectrum. The whole body is affected, usually within minutes of exposure to the allergen. Causes include food, insect stings and drugs.

An anaphylactic reaction is caused by the sudden release of chemical substances, including histamine, from cells in the blood and tissues. The release is triggered by the reaction of the antibodies (immunoglobulin E) with the offending substance (allergen). This mechanism is so sensitive that minute quantities of the allergen can cause a reaction.

In anaphylactic shock, bronchial tissues swell and blood pressure drops, causing choking and collapse. An adrenaline (epinephrine) injection should be given immediately. Adrenaline acts quickly to constrict blood vessels, relax smooth muscles in the lungs to improve breathing, stimulate the heartbeat and stop swelling around the face and lips (angioedema).