Urticaria pigmentosa is an uncommon skin disorder consisting of localized dermal infiltrates of tissue mast cells. Similar infiltrates at times occur in other organs, producing symptoms or signs of systemic mastocytosis. Since only occasional patients with cutaneous lesions also have visceral involvement, it is useful to recognize the major varieties of urticaria pigmentosa: (1) the solitary lesion, (2) multiple lesions of infancy or early childhood, and (3) multiple lesions appearing in later childhood, adolescence, or adulthood. Approximately one fifth of the patients in the third group may have systemic involvement.
Caplan RM. Urticaria Pigmentosa and Systemic Mastocytosis. JAMA. 1965;194(10):1077-1080. doi:10.1001/jama.1965.03090230045010