Hairy cell leukemia (HCL) is a rare, slow-growing cancer of the blood in which the bone marrow produces too many abnormal B cells, a type of white blood cell that normally helps fight infection. These cancerous cells develop unusual hair-like projections on their surface, which is how the disease gets its name. Under a microscope, a single affected cell looks like a “fried egg,” while clusters of them resemble a honeycomb pattern. HCL accounts for roughly 2% of all leukemias, and it’s diagnosed far more often in men than women, typically in middle age.
How the Disease Develops
In a healthy body, B cells mature in the bone marrow and then move into the bloodstream to help the immune system recognize and attack threats. In hairy cell leukemia, a genetic change causes certain B cells to grow out of control. These abnormal cells accumulate in the bone marrow and spleen, crowding out healthy blood cells. The result is a shortage of normal red blood cells, white blood cells, and platelets, a condition called pancytopenia.
Nearly all cases of classic HCL carry a specific genetic mutation called BRAF V600E. One study found this mutation in 100% of classic HCL samples, while another large analysis detected it in about 79%. This mutation is central to the disease: it sends a constant growth signal to the abnormal B cells, telling them to keep dividing. The variant form of the disease (more on that below) does not carry this mutation, which is one reason it behaves differently.
Symptoms and Early Signs
Hairy cell leukemia sometimes causes no symptoms at all and is discovered incidentally on a routine blood test. When symptoms do appear, they tend to develop gradually because the disease grows slowly. The most common complaints are fatigue and a general sense of weakness, both caused by declining blood cell counts.
Other symptoms include:
- A feeling of fullness in the belly, caused by an enlarged spleen pressing against the stomach, which can make it uncomfortable to eat more than a small amount at a time
- Easy bruising or bleeding, from low platelet counts
- Recurring infections, because the abnormal cells displace healthy white blood cells
- Unintentional weight loss
About 80% of patients have significantly low blood counts at the time of diagnosis. An enlarged spleen is one of the hallmark physical findings, though massive enlargement is less common today because the disease is often caught earlier through routine bloodwork.
Why Infections Are a Serious Concern
One of the more dangerous features of HCL is a profound drop in monocytes, a type of white blood cell that serves as a first-line defender against bacteria, fungi, and other pathogens. In healthy people, monocyte counts average around 442 cells per microliter of blood. In hairy cell leukemia patients, that number drops to an average of just 74 cells per microliter.
This severe monocyte deficiency leaves patients vulnerable to unusual infections that rarely affect people with normal immune systems. These include fungal infections like aspergillosis and histoplasmosis, a pneumonia caused by a fungus called Pneumocystis, and disseminated mycobacterial infections (related to the bacteria that cause tuberculosis). Between 4% and 9% of HCL patients develop mycobacterial disease specifically because of this immune gap.
How It’s Diagnosed
Diagnosis typically begins with a blood test that reveals low counts across multiple cell types. A doctor examining a blood smear under a microscope may spot the characteristic hairy cells directly. Bone marrow biopsy is usually needed to confirm the diagnosis, though it can be technically difficult because the abnormal cells often cause the marrow to become fibrous, making it hard to draw out a liquid sample.
A technique called flow cytometry is key to confirming the diagnosis. It identifies specific markers on the surface of the abnormal cells. Classic HCL cells express a combination of four surface proteins: CD103, CD123, CD25, and CD11c. More than 95% of classic HCL cases show all four markers. Doctors use a scoring system based on these markers, awarding one point for each, to distinguish HCL from other blood cancers that can look similar.
Testing for the BRAF V600E mutation provides further confirmation and helps rule out the variant form of the disease.
Treatment and Remission
The good news about hairy cell leukemia is that it responds exceptionally well to treatment. The standard first-line therapy is a class of drugs called purine analogs. A single course of treatment achieves a complete and durable remission in 70% to 90% of patients. In a large Italian study following 513 patients treated over nearly three decades, the overall response rate was about 92%, with roughly 65% achieving complete remission and another 19% achieving partial remission.
Treatment can be given intravenously or as a subcutaneous injection, and a typical course is short, often completed within a week. After successful treatment, patients enter a monitoring phase with periodic blood tests and physical exams. Five-year progression-free survival (meaning the disease has not returned) is around 78%. At 10 years, about 29% of patients remain progression-free, meaning the majority will eventually need retreatment, but retreatment often works well again.
For patients whose disease comes back after at least two prior treatments, the FDA approved a targeted therapy in 2018 that works differently. This drug attaches to a protein called CD22 on the surface of hairy cells and delivers a toxin directly into them. It was specifically approved for patients who had already received at least two systemic treatments, including a purine analog, and still had active disease.
The Variant Form
About 10% to 20% of people diagnosed with hairy cell leukemia have a variant form called HCL-V. Despite the similar name, it behaves quite differently. Variant HCL tends to present with a high white blood cell count rather than the low counts seen in the classic form. Patients with HCL-V typically have large spleens and a heavy burden of abnormal cells circulating in the blood.
The variant form lacks the BRAF V600E mutation and is missing some of the surface markers that define classic HCL, particularly CD25. This matters because the standard purine analog drugs that work so well against classic HCL are much less effective against the variant. Patients with HCL-V often fail to respond to first-line treatment, making it a more challenging disease to manage.
Living With Hairy Cell Leukemia
Because HCL is slow-growing and highly treatable, most people live for decades after diagnosis. Some patients with minimal disease and stable blood counts don’t need treatment right away and are monitored with a “watch and wait” approach. Treatment is typically started when blood counts drop low enough to cause symptoms or raise infection risk, or when the spleen becomes significantly enlarged.
Between treatments, the main practical concern is infection prevention. The combination of low monocytes, low neutrophils, and the immune-suppressing effects of treatment itself means patients need to take infections seriously. Fevers, lingering coughs, or unusual skin changes warrant prompt medical attention. Rarely, autoimmune complications like immune-related low platelets or anemia can develop alongside the leukemia, adding another layer to management.