Urticaria pigmentosa is an uncommon rash that usually affects the neck, arms, legs and trunk of children and young adults. The rash consists of reddish-brown spots that turn into hives when they are rubbed hard or scratched. Sometimes the spots will blister.
Mast cells make a substance called histamine.
Histamine causes hives, itching, and flushing. We do not known why people with urticaria pigmentosa have abnormal collections of mast cells in the skin. Most children who develop urticaria pigmentosa before the age of five will have resolution of the condition by adolescence or early adulthood. When urticaria pigmentosa starts after the age of five, the abnormal collections of mast cells will sometimes involve the internal organs and the disease does not always go away. These patients need blood tests and other studies.