Minutes after a peanut, a child's lips swell and breathing wheezes. This is type I hypersensitivity — peanut-specific IgE on mast cells cross-linked by the allergen, degranulating histamine: vasodilation, bronchospasm, laryngeal edema — anaphylaxis. Intramuscular adrenaline is the treatment (reverses all three: vasoconstricts, bronchodilates, reduces edema); antihistamines are too slow for the airway. The child needs an EpiPen and an action plan — type I kills in minutes, and adrenaline is measured in minutes.