The classification of urticaria-angioedema focuses on mechanisms that elicit clinical disease and can be useful for differential diagnosis, but most cases of chronic urticaria are idiopathic (Table 156-1).

Urticaria-angioedema can occur secondary to inhalation, physical contact, or more commonly ingestion (fruits, shellfish, fish, milk products, chocolate, legumes including peanuts, drugs) that may elicit urticaria alone or the anaphylactic syndrome (Chap. 26. Anaphylaxis).