[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [White cells, lymph nodes, spleen and thymus](https://durgical.com/en/general-pathology/areas/hematolymphoid/) › Hairy cell leukaemia: BRAF V600E, the dry tap and the red-pulp spleen

# Hairy cell leukaemia: BRAF V600E, the dry tap and the red-pulp spleen

- Lesion

A rare, slow B-cell leukaemia of middle-aged men with fine hair-like projections, a huge spleen and low blood counts.

Updated Nov 12, 2026

## Key points

- BRAF V600E in more than 90% of cases; BRAF inhibitors work in refractory disease.
- Markers: CD11c, CD25, CD103 and annexin A1, with B-cell markers; classically TRAP positive.
- Dry tap: marrow cannot be aspirated because of reticulin fibrosis.
- Massive splenomegaly with red-pulp infiltration; lymphadenopathy is rare.
- Pancytopenia and monocytopenia; atypical mycobacterial infections are typical.

## What it is

Hairy cell leukaemia is an uncommon indolent B-cell neoplasm named for the fine cytoplasmic projections of its cells. Nearly all cases carry the activating BRAF V600E mutation. The cells infiltrate the marrow, splenic red pulp and liver sinusoids rather than lymph nodes, causing massive splenomegaly and pancytopenia. Marrow aspiration often fails because the cells are trapped in reticulin fibres.

## Structure

- Blood: small numbers of cells with oval or bean-shaped nuclei and pale blue cytoplasm with hair-like projections.
- Marrow biopsy: cells with clear cytoplasm spaced apart (fried-egg look) in a reticulin mesh.
- Spleen: red pulp expanded, white pulp effaced, blood lakes.

## Step by step

- A mature B cell acquires BRAF V600E, constantly activating the MAPK/ERK pathway.
- The clone homes to marrow, splenic red pulp and liver.
- Marrow infiltration with reticulin fibrosis lowers production; the large spleen traps blood cells.
- Pancytopenia leads to infections, anaemia and bleeding.

## Markers

- CD11c
- CD25
- CD103
- Annexin A1
- BRAF V600E
- TRAP

## Clinical notes

- Middle-aged men (male-to-female ratio about five to one) with splenomegaly and cytopenias.
- Very responsive to purine analogue chemotherapy, with long remissions.

## Memory aid

Hairy cells: a man, a big spleen, a dry tap and a BRAF switch stuck on.

## Test yourself

Think of the answer, then tap the question to see it.

## Which mutation is nearly universal in hairy cell leukaemia?

BRAF V600E.

## Why is a marrow aspirate often a dry tap?

The tumour cells are enmeshed in reticulin fibres and cannot be aspirated; a biopsy is needed.

## Which part of the spleen do hairy cells infiltrate?

The red pulp, producing massive splenomegaly.

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## Elsewhere in DurgiPedia

- [B cell and plasma cell](https://durgical.com/en/immunology/b-cell-and-plasma-cell/)

## Connections

## Related 4

- [Oncogenes: RAS, MYC, HER2, BCR-ABL and the table to know](https://durgical.com/en/general-pathology/oncogenes-ras-myc-her2-bcr-abl-and-the-table-to-know/)
- [Splenomegaly and hypersplenism](https://durgical.com/en/general-pathology/splenomegaly-and-hypersplenism/)
- [Histiocytic disorders: HLH and Langerhans cell histiocytosis](https://durgical.com/en/general-pathology/histiocytic-disorders-hlh-and-langerhans-cell-histiocytosis/)
- [Essential thrombocythaemia and primary myelofibrosis](https://durgical.com/en/general-pathology/essential-thrombocythaemia-and-primary-myelofibrosis/)

## Related conditions 2

- [Non-Hodgkin lymphoma](https://durgical.com/en/conditions/non-hodgkin-lymphoma/)
- [Nontuberculous mycobacterial infections (MAC, M. abscessus, Buruli ulcer)](https://durgical.com/en/conditions/nontuberculous-mycobacterial-infections/)

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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: [Hairy Cell Leukemia Treatment (PDQ®)–Patient Version](https://www.cancer.gov/types/leukemia/patient/hairy-cell-treatment-pdq) (National Cancer Institute (NCI), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/hairy-cell-leukaemia-braf-v600e-the-dry-tap-and-the-red-pulp-spleen/