How you respond depends on how severe the reaction is. Mild reactions, such as a few hives, an itchy mouth, or sneezing, can often be managed by removing or stopping the trigger and taking an antihistamine. Keep a close eye on the person, because mild symptoms can sometimes progress.
Treat it as an emergency if there are signs of a severe reaction: difficulty breathing or noisy breathing, swelling of the throat, tongue, or face, a hoarse voice, widespread hives with feeling faint, persistent vomiting, or collapse. In that situation, give an adrenaline (epinephrine) auto-injector into the outer thigh if one is prescribed, then call your local emergency number and say anaphylaxis. Lie the person flat with legs raised, or let them sit up if breathing is the main problem, and do not let them stand suddenly.
After any significant reaction, the person should be assessed by a doctor even if they feel better, because symptoms can return. Anyone who has had anaphylaxis should see a doctor to confirm the trigger, get an emergency action plan, and be prescribed auto-injectors to carry.