To better understand urticaria, it helps to understand the anatomy of the skin.
The skin contains three main layers:
- The superficial layer that makes up the surface of the skin
- It is composed of skin cells and can be divided into 5 layers based on cell type.
- The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.
- The thickness of the epidermis varies, according to location: it is very thick over the soles of the feet, and very thin over the ears.
- Lies beneath the epidermis
- Also varies in thickness depending on the location of the skin. It is .3 mm on the eyelid and 3.0 mm on the back.
- The dermis is composed of 2 layers that contain a connective tissue called collagen
- The dermis contains blood vessels, nerves, sweat glands, and hair follicles.
- The number of structures in the dermis varies, according to location.
- The dermis under the arms contains more sweat glands and hair follicles than the dermis on the back.
- Subcutaneous layer:
- Mainly fat and connective tissue.
- Contains blood vessels and nerves.
- Baxi S, Dinakar C. Urticaria and angioedema. Immunol Allergy Clin North Am. 2005 May;25(2):353-67, vii. 
- Kozel MM, Bossuyt PM, Mekkes JR, Bos JD. Laboratory tests and identified diagnoses in patients with physical and chronic urticaria and angioedema: A systematic review. J Am Acad Dermatol. 2003 Mar;48(3):409-16. 
- Muller BA. A comprehensive review of physical urticaria. Compr Ther. 2002 Winter;28(4):214-21. 
- Varadarajulu S. Urticaria and angioedema. Controlling acute episodes, coping with chronic cases. Postgrad Med. 2005 May;117(5):25-31.