The short answer
Hairy Cell Leukemia means a rare, usually slow-growing B-cell leukemia. The exact diagnosis matters because abnormal cells can build up in bone marrow, blood, and spleen.
Hairy Cell Leukemia means a rare, usually slow-growing B-cell leukemia.
A typical evaluation may include blood counts, blood-cell review, bone marrow tests, immunophenotyping, and molecular testing.
Treatment categories may include observation for some people, targeted medicines, chemotherapy-based treatment, immunotherapy, and clinical trials.
Planning depends on symptoms, blood counts, infections, spleen size, molecular findings, and prior treatment.
Choose how you want to understand this
The full explanation.
A rare leukemia named for how the cells look
Hairy cell leukemia is a slow-growing cancer of B lymphocytes, a type of white blood cell. Under the microscope, the cells have fine projections around the edge. That is where the name comes from.
It is genuinely rare. The National Cancer Institute puts it at 1,200 to 1,300 new cases each year in the United States.
Rarity has a practical cost. Many community hematologists see very few cases. Ask whether your center treats this often. It is fair to seek an opinion at one that does.
How it usually shows up
NCI lists these signs and symptoms:
- Weakness or fatigue
- Frequent infections
- Easy bruising or bleeding
- Shortness of breath
- Pain or a feeling of fullness below the ribs
- Painless lumps in the neck, underarm, stomach, or groin
The fullness under the left ribs is the spleen. An enlarged spleen is called splenomegaly. It is a common feature here, and often what sends someone for a scan.
The tests that actually settle the diagnosis
NCI lists the workup: complete blood count, peripheral blood smear, bone marrow aspiration and biopsy, immunophenotyping, BRAF gene testing, and a CT scan.
Two of those deserve explanation.
The dry tap. NCI's clinician summary says the bone marrow here is usually fibrotic, meaning scarred. It does not aspirate easily. If the needle draws nothing, that is not a failed procedure. It is a finding that fits this disease. A core biopsy is taken instead.
Immunophenotyping. This uses antibodies to find which proteins sit on the cell surface. Hairy cells carry the B-cell markers CD19, CD20, and CD22. CD20 runs very high. NCI then names the trio that points to this diagnosis: CD11c, CD25, and CD103.
BRAF V600E. NCI calls this a disease-defining genetic feature. It is present in almost 100% of classic hairy cell leukemia. It is both a diagnostic anchor and a drug target.
The variant form behaves differently
About 10% of cases are variant hairy cell leukemia. NCI describes it as its own problem, not a mild version.
The differences show up on your own report. White blood cell count runs high, in the range of 15 to 50 × 10⁹/L. CD103 is variable. CD25 and CD23 are absent. BRAF variants are not present.
It matters for treatment. NCI reports that variant disease follows a more aggressive course. It also responds less well to purine nucleoside analog drugs, the standard first-line class. If your report says variant, expect a different conversation.
Not everyone is treated immediately
NCI describes watchful waiting as closely monitoring without treatment until signs or symptoms appear or change.
The clinician summary gives the triggers for starting treatment:
- Cytopenias, meaning low blood counts, especially when they cause symptoms
- An enlarging spleen
- Other complications, usually infections
Ask which of these your team is watching, and what number or change would tip the decision.
First-line treatment is short, and the results last
The backbone is a purine nucleoside analog, a drug class that interferes with DNA in dividing cells.
Cladribine. NCI describes it as a weight-based dose given by vein on five straight days, worked out by the treating team. Rituximab is an antibody against CD20. It may be added as eight weekly doses, either at the same time or later.
The response figures are strong. Complete response in 50% to 80% of patients, and overall response in 85% to 95%. Responses are described as durable.
The main toxicity is fever and a weakened immune system. NCI reports documented infection in 33% of treated patients. That is one in three. It is why the first month after treatment needs close attention.
Pentostatin. Given every other week for 3 to 6 months. Complete response runs 50% to 76%, overall response 80% to 87%. NCI reports 9-year relapse-free survival of 56% to 67%.
Targeting BRAF directly
Nearly every classic case carries BRAF V600E. So drugs aimed at that protein work here. NCI lists vemurafenib, dabrafenib, and encorafenib. Each can be given alone, or with rituximab or obinutuzumab.
The reported figures:
- Vemurafenib alone — 86% to 98% overall response, with 33% to 38% complete response
- Vemurafenib with rituximab — at 37 months of follow-up, 87% complete response and a 78% progression-free survival rate
- Dabrafenib with trametinib — 89% overall response, 65.5% complete response, and 94.5% progression-free survival at 2 years
Note the pattern. BRAF inhibitors alone give high overall response but fewer complete responses. Adding an antibody changes that.
If the disease comes back
NCI's patient summary makes an encouraging point. Re-treatment after relapse often produces another remission.
The clinician summary is more specific. The same purine analog, or a different one, can be used again, especially when the relapse comes after several years.
Other options carry their own numbers. Rituximab alone runs over 4 to 8 weeks. In relapsed or refractory disease it gives durable remissions, but rarely complete ones. Ibrutinib is a BTK inhibitor. In refractory disease it has shown a 54% response rate, with an estimated 73% progression-free survival at 36 months.
Splenectomy, removal of the spleen, remains an option in some situations.
Infection is the risk that needs a plan
Counts are low before treatment and the immune system is weak after it. So infection is the day-to-day danger. Do not wait to be told twice.
Go to an emergency department, or call 911, for either of these:
- A temperature of 100.4 °F (38 °C) or higher, the threshold CDC gives during cancer treatment. With neutrophils low, CDC says an infection during cancer treatment can be life-threatening and needs urgent medical attention, so this is a go-in now situation rather than a wait-for-a-call-back one.
- Shaking chills, even without a measured fever, or confusion, a racing pulse, or breathlessness alongside a temperature — the picture of sepsis.
Call the oncology team the same day for:
- A new cough, sore throat, or shortness of breath without fever
- Burning or blood when urinating
- Redness, swelling, or drainage around any catheter site
- Bruises appearing without injury
Bleeding that will not stop with firm pressure is an emergency too. Do not wait it out.
Keep a working thermometer at home and know the after-hours number before you need it.
Questions to bring to the appointment
- Is this classic or variant hairy cell leukemia, and what showed that?
- Was BRAF V600E tested, and what was the result?
- Am I being treated now, or watched, and what would change that?
- If treatment starts, is it cladribine or pentostatin, and why that one?
- Will rituximab be added, and at the same time or later?
- What is my neutrophil count, and what precautions follow from it?
More on hairy cell leukemia
Hairy Cell Leukemia: Diagnosis and Questions goes deeper into the workup. Hairy Cell Leukemia: Treatment Discussion covers choosing between options. Finding a Clinical Trial matters more than usual in a rare disease.
Sources
Words to know
Tap any term to see what it means.
Common questions
What is hairy cell leukemia?
It is a rare, usually slow-growing B-cell leukemia.
How is it diagnosed?
The evaluation may include blood counts, blood-cell review, bone marrow tests, immunophenotyping, and molecular testing; the exact sequence depends on the situation.
How is treatment planned?
Teams consider symptoms, blood counts, infections, spleen size, molecular findings, and prior treatment.
Should I seek a specialist opinion?
For an uncommon diagnosis, specialist pathology or treatment review can confirm a plan and clarify alternatives.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Your next step
Turn this guide into a short list for your care team.
Speak With Trained Specialists & Human Navigators
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Immediate emergency guidance for fever (>100.4°F during chemo), severe pain, or shortness of breath.
Help Us Improve This Guide
Did this explanation answer your question and help you determine your next step?
Know someone who needs this?
Plenty of people are looking for something like this and do not know where to start. If this would help a friend or someone you love, send it on — we have written an opening line so you do not have to stare at an empty message. You can change every word of it.
Your message is written and sent in your own email or messaging app — we never see who you send it to, and nothing is added to any list.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Sources last checked: 2026-07-22 what this meansLast updated: 2026-08-20Next planned review: 2027-07-22
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status — Source checked. This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
General education — varies by person. Answers genuinely differ between people. This page explains what commonly varies and points you to your care team for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.
Related articles
Still have questions?
Educational answers, plain language
Free to print and share