An Overview of Hairy Cell Leukemia

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Hairy cell leukemia is a rare, slowly growing blood cancer that affects about 950 Americans each year. The cancer is in the B cells (lymphocytes), a type of white blood cell that fights infection. Affected cells look “hairy” due to protrusions of cytoplasm that give the cells a ruffled outside border. The abnormal cells typically invade the bone marrow and spleen, causing low counts of circulating blood cells and spleen enlargement, or splenomegaly.

Hairy cell leukemia

Nephron / Wikimedia Commons / CC BY 3.0 


A combination of symptoms is common in people who are diagnosed with hairy cell leukemia. Most people have symptoms arising from an enlarged spleen (such as fullness or discomfort in the abdomen) or symptoms arising from reduced blood counts (abnormal bleeding/bruising/clotting, or infection). Fever, night sweats, fatigue, and weight loss, the so-called "constitutional symptoms," are also possible.

Symptoms at diagnosis may include any of the following:

  • Pain or fullness in the left side of the abdomen, beneath the ribs
  • Feeling full without eating or after eating only a small amount
  • Weakness
  • Fatigue
  • Infections that range in severity
  • Bruising or discoloration from bleeding beneath the skin

Many people with hairy cell leukemia have fullness or discomfort in the abdomen due to an enlarged spleen, or splenomegaly. The spleen may become massive, and sometimes it can even rupture, which is a medical emergency.

Many patients also have either bruising and bleeding because of low platelet counts, or recurrent infections due to low white blood cell counts. Having low blood counts can also cause weakness and fatigue.

It is possible to have hairy cell leukemia with few to no symptoms.

Finally, some people have essentially no symptoms but are diagnosed with hairy cell leukemia anyway. This may occur when a clinician notices an enlarged spleen, or when labs show low blood counts during a workup for something else.


The causes of hairy cell leukemia are not fully understood. Like all cancers, genetic mutations are thought to be important in the development of this malignancy. Exposures to ionizing radiation, pesticides, and farming might be triggers, but cigarette smoke, alcohol, and obesity do not appear to be risk factors.


Hairy cell leukemia is usually diagnosed by bone marrow biopsy and sampling of the liquid portion of the bone marrow, or a bone marrow aspirate. The sampled bone marrow tissue is analyzed to determine the microscopic appearance of cells, counts for each cell type, and the presence or absence of protein markers (CD markers) on the cell surface.

Some of the diseases that might look similar to hairy cell leukemia include:

The variant form of hairy cell leukemia is a rare chronic B cell malignancy that was previously thought to be a subtype of hairy cell leukemia but is now considered to be its own disease, distinct from hairy cell leukemia. The variant has less involvement of the bone marrow and is often associated with extreme elevation of the white blood cell count, often without the low blood counts seen in hairy cell leukemia.


Some people have hairy cell leukemia with no symptoms whatsoever. If you are in this category, you might be able to go with a watch-and-wait plan for months or even years after the diagnosis before you need treatment. Treating the disease earlier when you have no symptoms or other indications for treatment has not been shown to offer any advantages. 

However, treatment is usually required, and professional organizations and consensus groups have identified a number of problems or symptoms that indicate the need to treat, including the following:

  • Fever, night sweats, excessive fatigue, unexplained weight loss
  • Recurrent infection
  • Low red blood cell count
  • Low platelet count
  • Progressively increased white blood cell count or swelling of the lymph nodes

If you have an infection, it needs to be treated before your hairy cell leukemia is treated because treatment for hairy cell leukemia suppresses the immune system.

Modern therapy for hairy cell leukemia does not produce a cure, but it does alleviate symptoms and allows people to achieve a near-normal lifespan. Most people can have long-lasting remissions followed by further therapy when relapse occurs.

Purine Analogs: Cladribine or Pentostatin 

The treatment of hairy cell leukemia usually begins with a class of medications known as purine analogs, such as cladribine or pentostatin. If you receive a complete response, the recommendation is that you be observed until there is an indication for additional treatment. 

A complete response is defined as normalization of blood counts without transfusion, absence of hairy cell leukemia cells by bone marrow biopsy and peripheral blood sample, reduced spleen size by physical examination, and absence of disease symptoms.

Other Options

If you have not had a complete response initially to pentostatin or cladribine, contemporary guidelines suggest that you might consider a clinical trial in which purine analog with or without rituximab, interferon alpha, rituximab alone or vemurafenib are being investigated.

Treatments for relapsed disease depend upon what type of remission you had with your initial therapy. If you relapse after more than two years after achieving a complete response to initial therapy with purine analog, contemporary guidelines indicate that you may benefit from retreatment with the same purine analog or treatment with an alternative purine analog with or without rituximab. Rituximab alone is considered an option for patients unable to receive purine analog.

If available, a clinical trial may be considered for patients with disease relapse within two years after achieving a complete response to initial therapy and for those with progressive disease following second-line therapy.

Treatment with an alternate purine analog with or without rituximab, interferon alpha, rituximab alone (if unable to receive purine analog) or vemurafenib are also included as options for patients with disease relapse within two years after achieving a complete response to initial therapy.

Guidelines suggest ibrutinib, vemurafenib (with or without rituximab) or moxetumomab pasudotox are appropriate options if your disease progresses after second-line therapy. 

A Word From Verywell

Hairy cell leukemia is a rare disease, which means that studies with large numbers of patients are more challenging to conduct and to find in the biomedical literature so, at times, there might not be all the data that one would want to have in making a treatment decision. For example, there is no study comparing cladribine with pentostatin to determine whether one of these purine analogs is better than the other for patients with hairy cell leukemia.

Nonetheless, for a rare disease, hairy cell leukemia is quite treatable today, and many people with this cancer can expect to have long remissions, living full lives. When relapse occurs, there are many different avenues that can be pursued, including retreatment with the initial therapy in some instances and newer agents in others.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Leukemia and Lymphoma Society. Hairy cell leukemia facts.

  2. Hairy Cell Leukemia Foundation. Understanding hairy cell leukemia: Diagnosis, treatment and beyond.

  3. Bohn JP, Gastl G, Steurer M. Long-term treatment of hairy cell leukemia with interferon-α: still a viable therapeutic option. Memo. 2016;9:63-65. doi:10.1007/s12254-016-0269-1

Additional Reading

By Tom Iarocci, MD
Tom Iarocci, MD, is a medical writer with clinical and research experience in hematology and oncology.