Allergic reactions are common complaints in the ED. Often they are mild (e.g., urticaria without any other symptoms) and can be easily managed by antihistamines alone.  They can be caused by many things; but in pediatrics, allergic reactions are usually due to foods (in all ages) and insect stings (in older kids). 

Anaphylaxis, by definition, is a life-threatening allergic reaction with severe respiratory or cardiovascular compromise or involving at least 2 organ systems (e.g., skin, pulmonary, cardiovascular, neurologic, gastrointestinal). Typically, patients present during the “acute” phase when symptoms first occur.  In some cases, there is a “late” phase to the reaction that can occur 12-24 hours later, sometimes even later (up to 72 hours) in some case series. It may take several hours after an exposure to an allergen before symptoms develop. The risk of a biphasic response is greater in those patients who have a severe initial presentation or require more than one dose of epinephrine.