Systemic mastocytosis is classified as Indolent mastocytosis, with no organ dysfunction and a good prognosis Mastocytosis associated with other hematologic disorders (eg, myeloproliferative disorders, myelodysplasia, lymphoma) Aggressive mastocytosis, characterized by impaired organ function Mast cell leukemia, with > 20% mast cells in bone marrow, no skin lesions, multiorgan failure, and a poor prognosis Mast cell activation syndrome Mast cell activation syndrome is characterized by increased and inappropriate activation of mast cells with mediator release but without clonal proliferation or organ infiltration by mast cells (1).