An allergy is the immune system treating something harmless as a threat. That single description stretches from a runny nose every April to a reaction needing adrenaline within minutes, which is why the useful questions are always the same three: what is causing it, how certain anyone can actually be, and how severe it is likely to become.
This section covers the common triggers, from pollen and dust mites to food and latex; how allergy testing works, and the considerable gap between what it can confirm and what it can rule out; the treatments, from antihistamines through to immunotherapy; and the symptoms that signal an emergency rather than an inconvenience.