Mycosis fungoides, Sézary syndrome and mastocytosis
- Lesion
Some blood-borne cells settle in the skin and can grow there as a tumour: T lymphocytes in mycosis fungoides and mast cells in mastocytosis.
What it is
Mycosis fungoides is the commonest primary cutaneous lymphoma, a neoplasm of skin-homing CD4 T cells that evolves slowly through patch, plaque and tumour stages; Sézary syndrome is its leukaemic, erythrodermic relative. Mastocytosis is a clonal proliferation of mast cells, usually with an activating KIT mutation, that is limited to skin in children (urticaria pigmentosa) and often systemic in adults.
Structure
- Mycosis fungoides: band-like infiltrate in the upper dermis, atypical lymphocytes in the epidermis without much spongiosis, clusters in the epidermis (Pautrier microabscesses).
- Tumour stage: dermal sheets of large cells; large cell transformation worsens prognosis.
- Mastocytosis: dermal mast cells with oval nuclei and granular cytoplasm, highlighted by tryptase, CD117 (KIT) and Giemsa or toluidine blue (metachromatic).
- Aberrant CD25 (and CD2) expression on neoplastic mast cells.
Markers
- CD4
- CD3
- Loss of CD7
- TCR gene rearrangement
- Tryptase
- CD117 (KIT)
- CD25
Clinical notes
- Early mycosis fungoides resembles eczema or psoriasis for years before diagnosis.
- Children's urticaria pigmentosa often regresses by adolescence; adult disease is usually systemic and lasting.
Test yourself
Think of the answer, then tap the question to see it.
What are Pautrier microabscesses?
Intraepidermal clusters of atypical CD4 T cells, characteristic of mycosis fungoides.
Which mutation is found in most systemic mastocytosis?
KIT D816V, an activating mutation of the KIT receptor tyrosine kinase.
What is the Darier sign?
Urticarial swelling and redness of a mastocytosis lesion after rubbing, from mast cell degranulation.
Elsewhere in DurgiPedia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Mycosis Fungoides and Other Cutaneous T-Cell Lymphomas Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2025); Systemic mastocytosis (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.