Mast Cell Disorders
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Direct answer
An activating KIT D816V mutation in a clone of mast cells underlies most adult systemic mastocytosis, diagnosed by one major plus one minor, or three minor, WHO criteria. The major criterion is multifocal dense aggregates of at least 15 mast cells in marrow or extracutaneous tissue; the minor criteria are atypical (spindled) morphology in at least a quarter of the infiltrate, aberrant CD25 or CD2 expression, a KIT codon 816 mutation, and serum total tryptase persistently above 20 ng/mL. Disease categories then run from cutaneous mastocytosis (urticaria pigmentosa with a positive Darier sign in children) through indolent and smouldering systemic forms, systemic mastocytosis with an associated haematologic neoplasm — the commonest pattern in older adults — to aggressive mastocytosis and the rare mast cell leukaemia. Management layers trigger avoidance and antihistamines over cytoreductive therapy with midostaurin, avapritinib or cladribine for advanced disease.
What you must remember
- Diagnostic arithmetic: one major plus one minor criterion, or three minor criteria — the counting rule examiners quote directly.
- Major criterion: multifocal, dense infiltrates of 15 or more mast cells in bone marrow or another extracutaneous organ, confirmed by tryptase or CD117 staining.
- Minor criteria quartet: 25 per cent or more atypical or spindled mast cells in the infiltrate; aberrant CD25 (with CD2 or CD30) expression; KIT D816V; serum tryptase persistently above 20 ng/mL.
- Morphology clue: mast cells in clonal disease are spindled and hypogranulated with elongated nuclei, forming perivascular and peritrabecular aggregates — not the round, densely granulated normal cell.
- Clinical chemistry: histamine and other mediators cause flushing, syncope, wheeze, abdominal cramps and osteopenia or osteolysis; serum tryptase is the tracking marker.
- Darier sign: urticaria and dermographism on stroking urticaria pigmentosa lesions — the paediatric bedside favourite.
- Treatment ladder: H1 and H2 antihistamines with adrenaline autoinjector for mediator symptoms; midostaurin, avapritinib (avoided in severe thrombocytopenia because of intracranial haemorrhage risk per current labelling), cladribine or interferon for cytoreduction; allogeneic transplant for selected aggressive disease.
From an anaphylaxis presentation to a marrow diagnosis
A 55-year-old presents with recurrent unexplained anaphylaxis, flushing and a faint urticarial eruption; insect-sting allergy is the classical trigger, but the workup must first exclude a bland clone. Serum tryptase drawn at baseline weeks after the event returns at 65 ng/mL, and bone marrow biopsy shows multifocal aggregates of spindled CD117-positive cells with aberrant CD25, fulfilling the major criterion and two minor ones. A concurrent myelodysplastic clone on the same aspirate reclassifies the disease as systemic mastocytosis with an associated haematologic neoplasm, the commonest category in this age group, and KIT D816V sequencing confirms clonality. Because D816V distorts the activation loop of KIT, imatinib is useless here; symptom control with antihistamines plus vigilance for progression is the plan, with midostaurin held in reserve. The tryptase lesson generalises: it rises acutely in any anaphylaxis, so the diagnostic baseline sample waits until at least 24 hours after the event.
Where candidates slip
Three slips recur. Tryptase is treated as specific for mastocytosis — it is a marker of mast cell burden and activation, raised transiently in ordinary anaphylaxis and in some myeloid neoplasms; timing of the sample is everything. CD25 is overlooked as the single most useful aberrant marker, without which a sparse spindled infiltrate escapes diagnosis. And imatinib is offered as therapy because it inhibits KIT — true for wild-type and juxtamembrane mutants, but D816V sits in the activation loop and is resistant, which is precisely why midostaurin and avapritinib exist. Paediatric cutaneous disease, by contrast, frequently regresses by puberty and needs no marrow evaluation unless symptoms suggest systemic spread.
Frequently asked questions
What constitutes the WHO major criterion for systemic mastocytosis?
Multifocal dense aggregates of at least 15 mast cells in bone marrow or another extracutaneous organ.
Name the minor criteria.
Atypical spindled morphology in a quarter or more of mast cells, aberrant CD25 or CD2 expression, KIT codon 816 mutation, and serum tryptase persistently above 20 ng/mL.
Why does imatinib fail in systemic mastocytosis?
The D816V mutation affects the activation loop of KIT, a conformation imatinib cannot inhibit; multikinase and newer agents such as midostaurin and avapritinib overcome it.
When should tryptase be sampled after anaphylaxis?
Acute levels rise transiently in any anaphylaxis, so the baseline diagnostic sample should be drawn at least 24 hours after the reaction settles.
What is Darier sign?
Wheal-and-flare urtication produced by stroking lesional skin of urticaria pigmentosa, reflecting the mechanical degranulation of dermal mast cells.
What is the commonest systemic category in older adults?
Systemic mastocytosis with an associated haematologic neoplasm, typically a myeloid disorder sharing the same KIT-mutated clone.