CLL is a blood cancer that starts in white blood cells and grows slowly

Chronic lymphocytic leukemia (CLL) is a cancer of the blood and bone marrow in which abnormal white blood cells called lymphocytes multiply over time. Unlike some cancers that form tumors in one place, CLL spreads through the bloodstream and collects in the lymph nodes, spleen, and liver. It is called "chronic" because it typically grows slowly—some people live for years with few or no symptoms before treatment becomes necessary.

CLL almost always affects B lymphocytes, a type of white blood cell that normally helps fight infection. In CLL, these cells become abnormal and crowd out healthy blood cells, which can lead to anemia, bleeding problems, and a weakened immune system. The disease is most common in adults over 70, though it can occur at any age.

Key Takeaways

  • CLL develops when bone marrow produces too many abnormal B lymphocytes that accumulate in the blood, lymph nodes, and spleen.
  • Many people with CLL have no symptoms when the disease is first found, often discovered by chance during a blood test for another reason.
  • CLL progresses slowly in most cases, and some people never need treatment during their lifetime.
  • Doctors use blood tests and bone marrow biopsy to diagnose CLL and staging systems to predict how quickly it will advance.
  • Treatment options include watch-and-wait monitoring, chemotherapy, targeted drugs, and immunotherapy, depending on the stage and how fast the disease is growing.

How CLL develops in the bone marrow

CLL begins when a single B lymphocyte in the bone marrow becomes abnormal and starts to divide repeatedly. The bone marrow is the spongy tissue inside bones where blood cells are made. Normally, the body tightly controls how many white blood cells are produced, and old cells are cleared away when they die. In CLL, this control breaks down—the abnormal cells multiply without stopping and live much longer than normal lymphocytes.

These abnormal cells accumulate first in the blood, then spread to lymph nodes (small bean-shaped organs that filter fluid throughout the body), the spleen, and sometimes the liver. As the number of abnormal cells grows, they crowd out healthy red blood cells and platelets, which can cause anemia (too few red blood cells) and bleeding or bruising problems. The abnormal cells also interfere with the immune system's ability to fight real infections.

The exact reason a B lymphocyte becomes abnormal is not fully understood. Age is the strongest known risk factor—the disease is rare before age 50 and becomes more common with each decade after 70. Some research suggests genetic factors and exposure to certain chemicals may play a role, but no single cause has been proven.

Symptoms often do not appear until the disease is advanced

Many people with CLL have no symptoms at all when the disease is first found. The cancer is often discovered by chance when a blood test done for another reason shows an unusually high white blood cell count. This is one reason CLL is sometimes called an "incidental" diagnosis.

When symptoms do develop, they may include fatigue, shortness of breath, easy bruising or bleeding, frequent infections, swollen lymph nodes (felt as lumps in the neck, armpit, or groin), or an enlarged spleen (felt as fullness or pain on the left side of the abdomen). Some people experience night sweats or unintended weight loss. These symptoms can develop gradually over months or years, or sometimes more rapidly.

Because CLL grows slowly in most cases, many people live with the disease for years without needing treatment. Others experience faster progression and require treatment sooner. The rate of progression varies widely and cannot be predicted with certainty at diagnosis.

Diagnosis uses blood tests and bone marrow examination

Diagnosis begins with a complete blood count (CBC), a standard blood test that measures the number of different types of blood cells. In CLL, the CBC shows an elevated number of lymphocytes—usually more than 5,000 lymphocytes per microliter of blood, compared to the normal range of 1,000 to 4,800. A doctor may order additional blood tests called flow cytometry, which identifies the specific characteristics of the abnormal cells and confirms they are B lymphocytes.

To confirm the diagnosis and gather more information, a doctor may perform a bone marrow biopsy. In this procedure, a needle is inserted into the hip bone to remove a small sample of bone marrow, which is then examined under a microscope. The biopsy shows how many abnormal cells are present in the marrow and whether they have certain genetic features that affect prognosis.

Doctors also order imaging tests such as a CT scan or ultrasound to check whether lymph nodes or the spleen are enlarged. Blood tests may measure kidney and liver function and check for signs of infection. All of this information helps determine the stage of the disease and guide treatment decisions.

Staging systems predict how quickly CLL will progress

Once CLL is diagnosed, doctors assign a stage based on how many abnormal cells are present and which organs are affected. The most common staging system divides CLL into three stages. Stage 0 means abnormal lymphocytes are found only in the blood. Stage 1 includes enlarged lymph nodes in addition to abnormal cells in the blood. Stage 2 means the spleen or liver is enlarged, or both. Stage 3 involves anemia (low red blood cell count) caused by the cancer. Stage 4 includes low platelet count.

Doctors also look at genetic features of the cancer cells to predict prognosis. Certain genetic changes, such as deletion of chromosome 17p or mutation of the TP53 gene, are associated with faster progression and may influence treatment choices. Other genetic markers, such as deletion of chromosome 13q, are associated with slower progression.

These staging and genetic findings help doctors and patients understand what to expect and decide whether to start treatment immediately or monitor the disease with regular blood tests and physical exams.

Treatment depends on stage, genetics, and how fast the disease is growing

Not everyone with CLL needs immediate treatment. Many people, especially those with early-stage disease and no symptoms, are managed with "watch and wait"—regular blood tests and doctor visits to monitor the cancer without starting treatment. This approach avoids the side effects of treatment for people whose disease is not causing problems. Treatment begins when the cancer starts to progress rapidly, symptoms develop, or blood counts drop significantly.

When treatment is needed, options include chemotherapy drugs (such as fludarabine or bendamustine), targeted drugs that attack specific features of the cancer cells (such as ibrutinib or venetoclax), and immunotherapy drugs that help the immune system recognize and kill cancer cells (such as rituximab). Many patients receive a combination of these drugs. The choice depends on the patient's age, overall health, genetic features of the cancer, and how the disease has responded to previous treatments.

Stem cell transplantation, in which bone marrow or blood stem cells from a donor are infused to replace the patient's diseased marrow, is an option for some younger patients with high-risk disease, though it carries significant risks and is not suitable for everyone.

Prognosis varies widely depending on individual factors

Survival times for CLL vary considerably. Some people live for many years with minimal progression and never require treatment. Others experience faster disease growth and need treatment within months of diagnosis. Median overall survival—the point at which half of patients are still alive—has improved over the past two decades due to newer drugs, but individual outcomes depend on age, genetic features of the cancer, overall health, and how well the cancer responds to treatment.

People diagnosed with CLL at an early stage and without high-risk genetic features often have longer survival times than those with advanced disease or unfavorable genetics. However, prognosis is not certain, and some people with seemingly unfavorable features live longer than expected, while others with favorable features progress faster than predicted.

Regular follow-up with an oncologist who specializes in blood cancers allows for monitoring of the disease and adjustment of treatment plans as needed. Many people with CLL live productive lives for years after diagnosis, balancing treatment with work, family, and other activities.

Frequently Asked Questions

Is CLL the same as acute lymphoblastic leukemia?

No. CLL is chronic (slow-growing) and occurs mainly in adults, while acute lymphoblastic leukemia (ALL) is acute (fast-growing) and occurs mainly in children. Both are blood cancers affecting lymphocytes, but they behave very differently and are treated differently.

Can CLL turn into another type of cancer?

In a small percentage of cases, CLL can transform into a more aggressive lymphoma called Richter transformation. This occurs in roughly 5 percent of people with CLL and usually happens after several years. Symptoms of transformation include rapid enlargement of lymph nodes, fever, and worsening fatigue.

What does it mean if my CLL has a deletion of chromosome 13q?

Deletion of 13q is considered a favorable genetic marker associated with slower disease progression and longer survival times. People with this genetic feature often have a better response to treatment and may not need treatment for many years after diagnosis.

Will I need chemotherapy if my CLL is not causing symptoms?

Not necessarily. If your CLL is early-stage and not causing symptoms, your doctor may recommend watch and wait instead of immediate treatment. Treatment is typically started when the disease begins to progress, symptoms develop, or blood counts drop significantly.

Can CLL run in families?

CLL is not inherited in the traditional sense, but having a family member with CLL slightly increases your risk of developing it. The increase is modest, and most people with a family history of CLL never develop the disease.