"A food allergy can be a terrifying thing. When someone eats even a small amount of a food to which they are allergic, a minor reaction can involve itching, swelling, and stomach ache. But one in four people unlucky enough to have a food allergy, even a mild one, will at some point experience a severe reaction : anaphylaxis, a state of shock defined as a reaction involving two of the body's organs, is characterised by symptoms like wheezing, dizziness and vomiting. The pulse can slow, blood...