Hairy cell leukemia

by Jason Wasserman MD PhD FRCPC and David Li MD
May 29, 2025

Hairy cell leukemia (HCL) is a rare, slow-growing form of cancer that affects the blood and bone marrow. It occurs when your body produces an excessive number of abnormal white blood cells, specifically B cells. These abnormal cells have distinctive fine, hair-like projections visible under the microscope, which is why the disease is called “hairy cell” leukemia. Hairy cell leukemia typically involves the bone marrow, blood, and spleen.

What are the symptoms of hairy cell leukemia?

Symptoms of hairy cell leukemia can vary widely from person to person, but commonly include:

  • Fatigue
  • Weakness
  • Frequent infections
  • Easy bruising or bleeding
  • Fever
  • Night sweats
  • Weight loss
  • Discomfort or pain in the upper-left abdomen (due to an enlarged spleen)

Some patients may also experience an enlarged liver, although enlarged lymph nodes are uncommon at the onset of the disease.

What causes hairy cell leukemia?

The exact cause of hairy cell leukemia is not well understood. However, certain genetic mutations, especially a mutation known as BRAF V600E, are present in over 95% of cases. Environmental factors, such as exposure to pesticides or benzene, have also been associated with an increased risk of developing the disease. Additionally, there may be genetic susceptibility, as hairy cell leukemia can occasionally run in families.

Who gets hairy cell leukemia?

Hairy cell leukemia is very rare, accounting for about 2% of all leukemias. It is more common in men than women, occurring roughly four times more often in men. It typically affects adults around 60 years old and is extremely rare in children and young adults. The disease is more common among people of European descent compared to those of Asian, African, or Arab descent.

What parts of the body are typically affected by this disease?

Hairy cell leukemia primarily affects the bone marrow, blood, and spleen. In rare cases, it can involve other organs, such as the liver, lymph nodes, skin, bones, breasts, or brain, but these occurrences are uncommon.

How is this diagnosis made?

To diagnose hairy cell leukemia, your doctor may order blood tests and bone marrow tests. A pathologist examines the blood under a microscope, looking for the presence of abnormal “hairy” B-cells. A bone marrow biopsy is often necessary, where a small sample of bone marrow is taken and examined under a microscope. Pathologists look for hairy cells that exhibit the characteristic appearance, often described as having a “fried-egg” appearance due to the abundant cytoplasm surrounding the nuclei.

What additional tests may be performed to confirm the diagnosis?

Several specialized tests may help confirm hairy cell leukemia:

  • Immunohistochemistry (IHC): This test uses special antibodies that detect specific proteins on cells. Hairy cells typically show positive staining for proteins such as CD20, CD22, CD25, CD103, CD11c, CD123, and annexin A1 (ANXA1). These proteins help pathologists accurately identify hairy cells.
  • Flow cytometry: This test analyzes cells from blood or bone marrow to identify the unique combination of proteins expressed by hairy cells. Hairy cells exhibit a characteristic pattern, characterized by high levels of CD20, CD22, CD11c, CD103, CD25, and CD123.
  • Next-generation sequencing (NGS): This genetic test detects specific mutations in the DNA of cancer cells. In hairy cell leukemia, more than 95% of cases show the BRAF V600E mutation, confirming the diagnosis.

What is the prognosis for someone diagnosed with hairy cell leukemia?

Hairy cell leukemia typically has a very good prognosis, especially with modern treatments. Chemotherapy with drugs called purine analogues (like cladribine and pentostatin) is highly effective and often provides long-lasting remission. For patients who do not respond fully or who experience recurrence, newer targeted therapies, such as drugs that inhibit the BRAF mutation, have proven successful. Regular follow-up and monitoring for signs of recurrence or minimal residual disease can help ensure the best possible outcomes.

Questions to ask your doctor

  • What is the extent of my hairy cell leukemia?
  • What treatment options do you recommend?
  • What are the potential side effects of treatment?
  • How frequently will I need follow-up visits or testing?
  • What signs should I look out for that indicate the disease is coming back?
  • Are there any lifestyle changes I should make to improve my health during treatment?
  • Would participation in a clinical trial be beneficial for me?
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