Urticaria occurs mostly due to allergens, which are usually formed in three main ways. 1. Ingested allergens: This is through food, such as peptonic urticaria, which is mainly formed by eating animal protein. 2. Inhalant allergens: Inhalant allergies are generally to particles in the air, such as pollen, which can produce hives all over the body. 3. The skin comes into contact with allergic substances, forming a metamorphic reaction on the skin surface, so forming hives. 4.Other: In addition to allergens forming hives, there are also a small number of hives caused by infections, such as upper respiratory tract infections, which can induce infective hives.