Chronic lymphocytic leukemia (CLL) is a type of leukemia—a cancer of the bone marrow and blood.
The American Cancer Society estimates that about 14,600 people are diagnosed with CLL each year in the U.S. CLL is the most prevalent type of leukemia in adults, accounting for about one-third of all cases. It is typically a disease of older adults. This type of leukemia does not occur in children.
What Is CLL?
In people with CLL, a type of blood cell called a lymphocyte begins to function abnormally. The majority of CLL cases (about 95 percent) start in B lymphocytes (B-cells); CLL can start in T lymphocytes (T-cells) and natural killer (NK) cells as well. Affected lymphocytes mature partially, but they never fully mature. These leukemic cells may look normal, and they can carry out some, but not all, of their normal infection-fighting functions.
Over time the leukemic cells build up in the bone marrow and blood, crowding out normal, healthy blood cells that the body needs. The reduction in the number of normal blood cells can lead to infection, anemia, and excessive bleeding. The leukemic cells also can travel through the bloodstream to other parts of the body, including the lymph nodes, liver, and spleen, interfering with the normal function of these organs.
Most cases of CLL are characterized by a chromosomal abnormality. Often this is a deletion—loss of part of a chromosome—but in some instances there may be an extra chromosome or some other abnormality.
Doctors have found that there are two different forms of CLL—one that progresses very slowly (indolent) and one that is fast growing (aggressive). Chromosomal changes (cytogenetics) are the most important factors for estimating the chance of recovering from CLL as well as the chance of having a recurrence of the disease. In addition, certain proteins, such as ZAP-70 and CD38, are markers for the aggressiveness of the disease.
CLL may take years before it causes symptoms. Once symptoms begin, CLL is often treated as a chronic disease. On rare occasions, CLL can transform into another cancer, such as a lymphoma or an acute leukemia.
Other Leukemias and Lymphomas that Arise from Lymphocytes
In addition to CLL, some rare types of leukemia and lymphoma arise from lymphocytes. They include:
- Prolymphocytic leukemia (PLL)—PLL tends to be more aggressive than most types of CLL. It can form in B-cells or T-cells, and it generally develops more quickly than CLL but not as fast as acute lymphoblastic leukemia.
- Large granular lymphocytic (LGL) leukemia—LGL leukemia tends to be slow growing, but some cases are more aggressive. It is characterized by enlarged lymphocytes with visible granules, and it can form in T-cells or NK cells.
- Hairy cell leukemia (HCL)—This slow-growing type is a cancer of the B-cells, but it is different from CLL. HCL gets its name from the way the cells look under the microscope: They have fine projections on their surface that make them look hairy. HCL is extremely rare, with fewer than 1,000 people diagnosed in the United States each year.
- Small lymphocytic lymphoma (SLL)—This disease is closely related to CLL. The difference is that in SLL the cancer cells are found in the lymph nodes and spleen, rather than in the bone marrow and blood. Approximately 5,000 new cases of SLL are diagnosed annually in the United States.
Understanding the Bone Marrow and Blood
To understand leukemia, it helps to have basic knowledge of the bone marrow and how healthy blood cells form and what they do.
Stem cells are cells in the body that have the potential to turn into any kind of cell, such as a skin cell, a liver cell, a brain cell, or a blood cell. Stem cells that turn into blood cells are called hematopoietic stem cells, or blood stem cells. Blood stem cells are mainly found in bone marrow (the soft, spongy tissue inside your bones), but some are also found in circulating blood. When blood cells become old or damaged, they die, and blood stem cells produce new blood cells to replace them.
Blood stem cells produce lymphoid stem cells and myeloid stem cells. Lymphoid stem cells produce lymphoblasts, which in turn produce several types of white blood cells, including lymphocytes and NK cells. Myeloid stem cells produce myeloblasts, which in turn produce white blood cells known as granulocytes. Myeloid stem cells also produce red blood cells and platelets.
- White blood cells (leukocytes) fight infection. The main types of white blood cells are B-cells, T-cells, NK cells, and granulocytes.
- Red blood cells (erythrocytes) carry oxygen from the lungs to the other parts of the body and take carbon dioxide back to the lungs to be removed.
- Platelets (thrombocytes) make the blood clot and slow or stop bleeding.
What Causes CLL?
Doctors do not know what causes CLL. The disease is more common in people who are middle-aged or older. The average age at diagnosis is 72. CLL is more common in males than females. Generally, it is also more common in North America and Europe than in Asia; however, people of Asian descent who live in the United States have the same (lower) risk as those living in Asia. This suggests to experts that the difference in risk has more to do with genetics than with factors in the environment.
There are only a few known risk factors for CLL. Factors that may increase your odds of developing CLL include:
- Being exposed to certain chemicals, such as herbicides and pesticides
- Having an immediate family member with CLL or a cancer of the lymph system
However, many people who develop CLL have none of the risk factors, and most people with the risk factors do not develop the disease.