Urticaria pigmentosa is a term that is applied to a disorder which is characterized by the appearance of tan to reddish brown areas of discoloration in the skin, which are due to a localized overgrowth of cells called mast cells. These may be flat or raised and exhibit some variation in size. Urticaria pigmentosa is a part of a broader disorder called mastocytosis. Mastocytosis is the term applied to disorders which result from the overgrowth of mast cells anywhere in the body. The important thing about mast cells is that they contain and liberate a substance called histamine.
Histamine when liberated into the blood skin can cause redness, swelling and itching.
When liberated into the blood stream, it can cause many of the symptoms commonly associated with allergic reactions, i.e., swelling and constriction of the bronchial tubes. The effects of histamine are widespread and numerous. The key concept is that urticaria pigmentosa is a skin disorder which is caused by the presence of localized overgrowths of mast cells. Urticaria pigmentosa may or may not be associated with a broader problem in which there is involvement of other organ systems with mast cells. Many but not all people with urticaria pigmentosa have involvement of the skin only.