Chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL) are the same slow-growing cancer of B lymphocytes, named differently depending on where most of the abnormal cells are found. When they fill the blood and bone marrow, it is called CLL; when they gather mainly in the lymph nodes with few in the blood, it is called SLL. Diagnosis relies on a blood count and flow cytometry, and treatment ranges from careful monitoring to targeted oral drugs, which have transformed outlook for most patients.
CLL/SLL is one of many hematologic malignancies, and it is the most common leukemia in adults in Western countries. It mostly affects older adults, and many people live with it for years without needing treatment.
CLL vs SLL: One Disease, Two Presentations
Both conditions arise from the same mature B cell that multiplies and fails to die normally. The World Health Organization classifies CLL and SLL as a single disease entity. The difference is simply the pattern of spread.
| Feature | CLL | SLL |
|---|---|---|
| Main location | Blood and bone marrow | Lymph nodes and sometimes spleen |
| Monoclonal B cells in blood | 5,000 per microliter or more | Fewer than 5,000 per microliter |
| How it is confirmed | Blood flow cytometry | Lymph node biopsy |
| Staging system | Rai or Binet | Lugano (lymphoma) staging |
| Treatment | Same drugs and indications | Same drugs and indications |
CLL is quite different from acute leukemias. Unlike the fast-moving acute B-cell disease described in our article on B-cell ALL, another type of leukemia, CLL cells look mature and the illness usually progresses over years. A related finding, monoclonal B-cell lymphocytosis (MBL), describes a small clone of these cells without enough to meet the CLL threshold; it needs monitoring but not treatment.
Causes and Risk Factors
The exact cause of CLL/SLL is unknown. It is not caused by lifestyle choices, and it is not contagious. Recognized risk factors include:
- Age: most people are diagnosed in their 60s or 70s.
- Family history: first-degree relatives of people with CLL have a higher risk, though most never develop it.
- Sex: it is somewhat more common in men.
- Certain exposures: some agricultural chemicals, including Agent Orange, have been linked to CLL.
Inside the cancer cells, acquired genetic changes drive the disease. Common ones include deletion of part of chromosome 13 (del(13q)), an extra chromosome 12 (trisomy 12), and deletions of 11q or 17p. These changes are not inherited and strongly influence treatment choice.
Signs and Symptoms
Many people have no symptoms and learn they have CLL from a routine blood test showing a high lymphocyte count. When symptoms appear, they may include:
- Painless swollen lymph nodes in the neck, armpits, or groin
- Fullness under the left ribs from an enlarged spleen
- Fatigue from anemia, and easy bruising from low platelets
- Frequent or slow-to-clear infections, because immune function is weakened
- B symptoms: fevers, drenching night sweats, and unintended weight loss
CLL can also cause autoimmune problems, such as the immune system attacking red cells or platelets. People with CLL have a higher risk of skin cancers and other second cancers, so long-term surveillance matters.
How CLL and SLL Are Diagnosed
The first clue is usually a complete blood count with a raised lymphocyte count. A blood smear often shows small mature lymphocytes and fragile “smudge cells.”
Flow cytometry is the key test. It confirms that the B cells are a single clone and shows the typical CLL pattern: CD5 and CD23 positive, with dim CD20 and dim surface immunoglobulin. This pattern helps separate CLL from mantle cell lymphoma and other lymphomas. For SLL, a lymph node biopsy is usually needed.
A bone marrow biopsy is not always required. It helps when blood counts are low and the cause is unclear, because it shows how much of the marrow is involved. CT scans may be used to assess lymph nodes and organs, especially in SLL or before treatment.
Staging and Prognostic Tests
The Rai system (stages 0 to IV) and Binet system (A to C) stage CLL using lymph nodes, liver and spleen size, and blood counts. Before treatment, doctors also check:
- del(17p) and TP53 mutation: predict poor response to chemotherapy.
- IGHV mutation status: unmutated IGHV suggests more active disease.
- Beta-2 microglobulin: a blood marker linked to disease burden.
Treatment Options for CLL and SLL
Early CLL/SLL without symptoms is managed with active surveillance, often called watch and wait. Starting treatment early in this setting has not been shown to help people live longer. Treatment begins when the disease becomes “active,” for example with falling hemoglobin or platelets, bulky or growing lymph nodes or spleen, rapidly rising lymphocyte counts, or troublesome B symptoms.
Targeted Therapy
- BTK inhibitors: ibrutinib, acalabrutinib, and zanubrutinib are daily tablets that block survival signals in CLL cells. They are usually taken continuously while effective.
- BCL-2 inhibitor: venetoclax triggers CLL cells to die. Combined with the antibody obinutuzumab, it is often given for a fixed period of about a year.
- Anti-CD20 antibodies: rituximab and obinutuzumab target a marker on B cells and are often paired with other drugs.
Other Approaches
Chemoimmunotherapy is now used far less often but remains an option for selected fit patients with favorable genetics. For relapsed disease, newer BTK inhibitors, CAR T-cell therapy, and, rarely, allogeneic stem cell transplant may be considered. Supportive care, including vaccinations and prompt treatment of infections, is part of every plan.
Key Takeaways
- CLL and SLL are one B-cell cancer; the name depends on whether cells are mainly in the blood or lymph nodes.
- Flow cytometry showing CD5 and CD23 positive clonal B cells confirms the diagnosis.
- Many people need only monitoring for years.
- When treatment is needed, targeted drugs such as BTK inhibitors and venetoclax are the mainstays.
- Testing for del(17p), TP53, and IGHV guides the choice of therapy. See our leukemia guide for related conditions.
Frequently Asked Questions
Is CLL curable?
CLL is generally considered a chronic, controllable disease rather than a curable one for most patients. Modern treatments can keep it in check for many years, and many people have a normal or near-normal life span, particularly when diagnosed at an early stage.
Why is my doctor not treating my CLL right away?
Early CLL without symptoms often stays stable for years. Starting treatment early has not been shown to extend life, and it adds side effects. Your team will monitor your blood counts and start therapy when the disease becomes active.
What is Richter transformation?
Richter transformation is when CLL or SLL changes into a fast-growing lymphoma, most often diffuse large B-cell lymphoma. Warning signs include a rapidly enlarging lymph node, new fevers, or weight loss. It is uncommon but requires prompt evaluation.
Should my family members be tested?
Routine screening of relatives is not recommended, even though family members have a slightly higher risk. They should mention the family history to their own doctors and report symptoms such as persistent swollen glands.
