Mastocytosis is caused by a clonal neoplastic proliferation of mast cells and are no longer classified as part of the myeloproliferative neoplasm (MPN) according to the WHO 2016-classification. Mastocytosis is now considered to be a disease of its own. Mastocytosis can affect the skin only (cutaneous mastocytosis) or impair different organ systems (systemic mastocytosis). While increased mast cell counts in the blood are very rare (mast cell leukemia), the bone marrow is almost always affected. There, dense clusters of atypical, often spindle-shaped mast cells are found. Mastocytosis must be differentiated from reactive mast cell hyperplasia in which the typical mast cell clusters are absent.
In cutaneous mastocytosis, patients suffer from urticaria and pruritus. In systemic mastocytosis the bone marrow and various organs are affected. Patients suffer from general symptoms (fatigue, weight loss, fever), skin symptoms (urticaria, pruritus), symptoms caused by mast cell-degranulation (abdominal pain, nausea, diarrhea, headache, hypotension, tachycardia, dyspnea) and bone symptoms (bone pain, osteopenia).
With the exception of "systemic mastocytosis with associated clonal hematological non-mast cell lineage disease (SM-AHNMD)" and the very rare mast cell leukemia, there are no abnormalities in the blood.
In most cases of systemic mastocytosis, multiple, clearly delimited infiltrations of mast cells are found in the bone marrow. Often, these infiltrations lay paratrabecular or perivascular. Beside mast cells, the infiltrations consist to different degrees of lymphocytes, eosinophils and fibroblasts.
In "systemic mastocytosis with associated clonal hematological non-mast cell lineage disease (SM-AHNMD)", clues for an additional clonal, hematological disease exist. It can be e.g. a myelodysplastic syndrome, a myeloproliferative neoplasm, an acute myelocytic leukemia or a mature cell lymphocytic neoplasm.