Hairy cell leukemia (HCL)

What is hairy cell leukemia (HCL)?

Hairy cell leukemia (HCL) is a rare, slow-growing leukemia that starts in a B cell (also called a B lymphocyte). B cells are white blood cells that help the body fight infection and are an important part of the body’s immune system. HCL gets its name from the short, thin projections that look like hair on its cells. 

HCL is an uncommon leukemia. There are approximately 1,200 new cases of HCL in the United States each year. While HCL cannot be cured, great progress in the treatment of HCL has resulted in prolonged survival for many patients. You should be treated by a hematologist-oncologist, a specialist who treats people with leukemia or other types of blood cancer. 

Find facts and statistics for HCL and other blood cancers. 

What should I do if I am diagnosed with HCL?

• Talk with your doctor about your diagnostic tests and what the results mean
• Talk with your doctor about all your treatment options and the results you can expect from treatment
• Ask your doctor whether a clinical trial is a good treatment option for you 

Learn more about clinical trials. 

How does HCL develop? 

Changes (mutations) in the genes of a B cell can cause it to develop into a leukemia cell. Normally, a healthy B cell would stop dividing and eventually die. In HCL, genetic errors tell the B cell to keep growing and dividing. Every cell that arises from the initial leukemia cell also has mutated DNA. As a result, the leukemia cells multiply uncontrollably. They usually go on to infiltrate the bone marrow and spleen, and they may also invade the liver and lymph nodes.  

When the leukemic hairy cells enter the bone marrow, they affect the production of healthy blood cells. Bone marrow is the soft, sponge-like tissue in the center of most bones where blood cells are made. As the leukemia cells build up in the bone marrow, they suppress the development of other blood cells, including red blood cells, platelets, and white blood cells. As a result, the number of healthy blood cells is usually lower than normal and can cause the following conditions: 

• Anemia: a condition when there is a low number of red cells in the blood which can cause fatigue and shortness of breath
• Neutropenia: a condition when there is a low number of white cells, which means the immune system can't effectively guard against infection due to a lack of neutrophils (a type of white cell)
• Thrombocytopenia: a condition when there is a low number of platelets, which can cause bleeding and easy bruising with no apparent cause
• Pancytopenia: occurs when there are low numbers of all three blood cell counts 

What are risk factors for HCL?

Doctors don't know why some cells become hairy cells and others don't. For most people who have HCL, there are no obvious reasons (risk factors) why they developed the disease. 

Some people with several risk factors may never get HCL, while others with no known risk factors may develop the disease. Risk factors associated with HCL include:  

• Age: HCL occurs most often in middle-aged to older adults—the median age at diagnosis is about 58 years
• Gender: more men than women are diagnosed with HCL—it is four times more common in men than in women
• Exposure to Agent Orange: HCL has been observed in patients following exposure to the herbicide Agent Orange used during the Vietnam War