[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Skin](https://durgical.com/en/general-pathology/areas/skin/) › Mycosis fungoides, Sézary syndrome and mastocytosis

# 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.

Updated Nov 12, 2026

## Key points

- Mycosis fungoides: CD4 T cells expressing CLA (skin homing); epidermotropism and Pautrier microabscesses; cerebriform nuclei.
- Stages: patch, plaque, tumour; can spread to nodes and blood.
- Sézary syndrome: erythroderma, lymphadenopathy, circulating Sézary cells.
- Mastocytosis: KIT D816V activating mutation (most systemic cases); tryptase raised.
- Darier sign: rubbing a lesion releases histamine and causes a wheal.
- Systemic mastocytosis: marrow, liver, spleen; flushing, anaphylaxis, peptic ulcer, osteoporosis.

## 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.

## Memory aid

Mycosis fungoides lymphocytes come to the epidermis like guests who never leave (epidermotropism) and gather in little parties (Pautrier); rubbing a mast cell lesion pops the histamine balloon (Darier).

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/general-pathology/mycosis-fungoides-sezary-syndrome-and-mastocytosis/)

[More Pathology questions →](https://durgical.com/en/practice/?f=pathology)

## Elsewhere in DurgiPedia

- [Mast cell and basophil](https://durgical.com/en/immunology/mast-cell-and-basophil/)
- [Lymphocyte recirculation and homing](https://durgical.com/en/immunology/lymphocyte-recirculation-and-homing/)
- [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/)

## Connections

## Related 4

- [T- and NK-cell leukaemias and extranodal lymphomas](https://durgical.com/en/general-pathology/t-and-nk-cell-leukaemias-and-extranodal-lymphomas/)
- [Nodal T-cell lymphomas](https://durgical.com/en/general-pathology/nodal-t-cell-lymphomas/)
- [Urticaria and angioedema](https://durgical.com/en/general-pathology/urticaria-and-angioedema/)
- [Psoriasis and seborrheic dermatitis](https://durgical.com/en/general-pathology/psoriasis-and-seborrheic-dermatitis/)

## Related conditions 4

- [Mycosis fungoides and Sézary syndrome (cutaneous T-cell lymphoma)](https://durgical.com/en/conditions/mycosis-fungoides-and-sezary-syndrome/)
- [Mastocytosis (cutaneous and systemic)](https://durgical.com/en/conditions/mastocytosis/)
- [Non-Hodgkin lymphoma](https://durgical.com/en/conditions/non-hodgkin-lymphoma/)
- [Urticaria and angioedema](https://durgical.com/en/conditions/urticaria-and-angioedema/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

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](https://www.cancer.gov/types/lymphoma/hp/mycosis-fungoides-treatment-pdq) (National Cancer Institute, 2025); [Systemic mastocytosis](https://medlineplus.gov/genetics/condition/systemic-mastocytosis/) (MedlinePlus, US National Library of Medicine, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/general-pathology/mycosis-fungoides-sezary-syndrome-and-mastocytosis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/mycosis-fungoides-sezary-syndrome-and-mastocytosis/