A systemic disorder defined by a clonal expansion of mast cells that in most instances is indolent and nonmalignant. Generally involves bone marrow, skin, gastrointestinal (GI) mucosa, liver, and spleen. Classified as cutaneous mastocytosis with variants, five systemic forms (indolent, smoldering, associated with concomitant hematologic non–mast cell lineage disease, aggressive, mast cell leukemia), and rare mast cell sarcoma.

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