Anaphylaxis is usually defined as a multi-system allergic reaction, but includes isolated shock or airway obstruction. Hives do not occur in 20-30% of cases. Gastrointestinal (GI) symptoms are more common with foods. Cardiac anaphylaxis can cause arrhythmias, bradycardia or chest pain with ECG changes. Plasma histamine level correlates with hives and GI symptoms. Serum tryptase is elevated less often in food allergy, and correlates poorly with plasma histamine level. Anaphylaxis occurs in 30/100 000 population/year (mortality 1-2%) and is caused by foods (35%), drugs/biologicals (25%), insect stings (15%), exercise (5%) or is idiopathic (20%). Onset of anaphylaxis to stings or allergen injections is usually rapid: 70% begin in <20 minutes and 90% in <40 minutes. Food/ingestant anaphylaxis may have slower onset or slow progression. Rapid onset is associated with greater severity. Prolonged anaphylaxis can be resistant to epinephrine and i.v. fluids. Biphasic allergic reactions which recur some hours after the early phase of the reaction were reported in 25% of cases of fatal and near-fatal food reactions, and in 23% of drug/biological reactions. But they occurred in only 6% of anaphylaxis of mixed causes and are uncommon with insect stings. Late phase (biphasic) reactions rarely occur without initial hypotension or airway obstruction.