Believe the person. Somebody who says they think they are reacting is the best-informed person in the room about their own body, and a child who says their mouth feels funny is reporting something rather than seeking attention.
Call your local emergency number early rather than late. Which number that is depends on where somebody is standing. An AllergyEZ card shows the number we hold for the country you picked, and we may be wrong or out of date about it — check it against a local source before you travel. Early is better: a call that turns out not to have been needed has cost nothing that matters.
Say the word allergy on the call, and say what was eaten if anybody knows. The people at the other end of that call triage on what they are told, and an allergic reaction described in those words gets a different response from one described as somebody feeling unwell.
Keep the person still and stay with them. Standing somebody up who has gone pale and floppy can make things worse, which is genuinely counterintuitive and worth knowing before it is ever needed. One person stays; another goes to meet the ambulance at the door.
Where a written plan from the person's own doctor exists, that plan is the authority in the room and it says what happens next. This page is not that plan and cannot stand in for one.
Afterwards still counts. A reaction that settles can return, so somebody who has had a serious one is watched, and a trip to hospital that ends in being sent home again is not a wasted trip.
Bystanders matter more than they think they do. The commonest failure in a serious reaction is not the wrong action, it is delay — waiting to be certain, waiting to avoid making a fuss, waiting for somebody more qualified to turn up.
Afterwards, tell somebody what happened, in order and in plain words: what was eaten, when it started, what was seen. Responders and doctors reconstruct an episode from exactly that, and a note typed into a phone in a waiting room is worth a great deal later.
This page says nothing about what to give anybody, and it never will. That is between an allergic person, their own doctor and whatever is written on their own plan. What a bystander can always do is believe them, call for help, and stay.
Questions this page raises
Ezzie will have a go at any of these.