Anaphylactic shock occurs because your body's immune system overreacts in response to a particular substance (allergen) that you have been exposed to before (see the box, right).
When the allergen gets into your bloodstream, it causes the body to release massive amounts of the protein histamine and other chemicals.
Histamine causes:
- blood vessels to widen, leading to a sudden severe lowering of blood pressure and narrowing of the airways in the lungs, and
- blood vessels to leak, causing swelling (oedema) in the surrounding tissues and a further drop in blood pressure.
For detailed information on common food allergens, see External links.
