Tissue sample of the solitary mastocytoma type of Urticaria Pigmentosa. Urticaria pigmentosa is a rare disease that produces skin lesions and intense itching. If the lesions are rubbed, hives may form on the site. Urticaria pigmentosa is one of several forms of mastocytosis, which is caused by excessive numbers of inflammatory cells (mast cells) in the skin. Other forms include solitary mastocytoma (a single lesion) and systemic mastocytosis (involvement in organs other than the skin). The condition is most often seen in children, but it can occur in adults as well. Rubbing of a lesion produces a rapid wheal (a hive-like bump). Younger children may develop a fluid-filled blister over a lesion if it is scratched. A large histamine release from these extra mast cells may cause flushing, headache, diarrhea, a rapid heartbeat (tachycardia), and even fainting. This is uncommon with urticaria pigmentosa but may be seen in other forms of mastocytosis.