Mastocytosis and Mast Cell Activation Syndrome (Mast Cell Disease) By James Fernandez, MD, PhD, Cleveland Clinic Lerner College of Medicine at Case Western Reserve University Reviewed / Revised Oct 2022 View Patient Education Classification Symptoms and Signs Diagnosis Treatment Key Points Topic Resources Cutaneous Mastocytosis (Back) Cutaneous Mastocytosis (Legs) Urticaria Pigmentosa Urticaria Pigmentosa (Infant) Urticaria Pigmentosa (Child) Urticaria Pigmentosa (Legs) Urticaria Pigmentosa (Thorax) Darier Sign Mastocytosis is mast cell proliferation with infiltration of skin or other tissues and organs.