Urticaria pigmentosa (UP) is the most common form of mastocytosis, which is a disorder characterized by mast cell proliferation (increases in cells that contain histamine and heparin) and accumulation within various organs, most commonly the skin. It is a disease that produces skin lesions, intense itching, and hive formation at the site of the lesion upon rubbing. UP is also known as systemic mastocytosis and mastocytoma.
Urticaria characterized by persistent, pigmented maculopapular lesions that urticate when stroked (Darier’s sign). It typically occurs in childhood. Biopsy reveals infiltration by mast cells.