Hairy cell leukemia is technically a type of lymphoma. Despite its name, the World Health Organization classifies it as a mature B-cell neoplasm, placing it in the same broad family as non-Hodgkin lymphomas. The “leukemia” label reflects how the disease was first discovered (abnormal cells circulating in the blood), but its biology is rooted in lymphocytes, the same white blood cells involved in lymphomas.
Why It’s Called Leukemia but Classified as Lymphoma
The distinction between leukemia and lymphoma has always been somewhat artificial. Leukemias were historically defined by cancer cells appearing in the blood and bone marrow, while lymphomas were defined by tumors in lymph nodes and other organs. Hairy cell leukemia blurs that line. The cancerous cells originate from mature B-lymphocytes, likely late-stage memory B cells that have already passed through the normal immune activation process. That cellular origin is the same as many lymphomas.
In the current WHO classification system, hairy cell leukemia sits under “splenic B-cell neoplasms,” a subcategory of mature B-cell neoplasms. It shares this neighborhood with splenic marginal zone lymphoma and other cancers that primarily affect the spleen. The WHO groups cancers by their cell of origin and biology rather than where they first show up, which is why hairy cell leukemia lands in a lymphoma-related category despite its name.
What Makes Hairy Cell Leukemia Distinct
The disease gets its name from the fine, hair-like projections visible on the surface of the abnormal cells under a microscope. These cells accumulate in the bone marrow, spleen, and blood, crowding out healthy blood cells. This leads to the hallmark problems: low red blood cell counts (causing fatigue), low white blood cell counts (causing frequent infections), and low platelet counts (causing easy bruising or bleeding). Most patients also develop a significantly enlarged spleen.
A genetic mutation acts as the disease’s fingerprint. More than 97% of classic hairy cell leukemia cases carry a specific mutation called BRAF V600E, a change in a gene that controls cell growth. This mutation is so consistently present that testing for it has become a key part of diagnosis. Doctors also confirm the diagnosis using a panel of surface markers on the abnormal cells, including CD25, CD103, and CD123, a combination that distinguishes hairy cell leukemia from other B-cell cancers that can look similar.
How It Differs From Other Lymphomas
Despite its classification alongside lymphomas, hairy cell leukemia behaves quite differently from most of them. It’s rare, accounting for roughly 2% of all leukemias and an even smaller fraction of B-cell cancers overall. It predominantly affects middle-aged men and tends to progress slowly.
Unlike aggressive lymphomas that require intensive chemotherapy regimens, hairy cell leukemia responds remarkably well to a single class of drugs. It also doesn’t typically cause the swollen lymph nodes that define many lymphomas. Instead, the spleen bears the brunt of the disease. Some patients don’t need treatment at all initially and can be monitored until symptoms develop, an approach that would be unusual for most lymphomas.
Treatment and Outlook
The prognosis for hairy cell leukemia is excellent compared to most blood cancers. First-line treatment with a purine analog (a type of chemotherapy given as a short course) produces overall response rates of 85% to 95%, with complete remission in 50% to 80% of patients. A large Israeli study of 203 patients found a 10-year overall survival rate of 94% after first-line therapy, and 75% at 20 years. From the time of diagnosis, 90% of patients were alive at 10 years and 86% at 20 years. Median overall survival was 21 years from diagnosis.
Treatment is typically brief, often just a single course lasting days to months depending on the drug used. Responses tend to be durable, and patients who eventually relapse often respond well to retreatment with the same drug. The median time to first relapse among responders is about 54 months, and many patients enjoy much longer remissions. For those who relapse after multiple treatments, the FDA has approved a targeted therapy that works by binding to a protein on the surface of the cancerous B cells and delivering a toxin that kills them from the inside.
The Practical Takeaway
If you or someone you know has been diagnosed with hairy cell leukemia, the lymphoma classification matters mainly for understanding the disease’s biology. It originates from the same type of immune cell as lymphomas, and oncologists who specialize in lymphomas often manage it. But its behavior, treatment approach, and prognosis are distinct. It’s one of the most treatable blood cancers, with survival rates that rival or exceed many common cancers caught at early stages.