What Is Chronic Lymphocytic Leukemia?

Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Chronic lymphocytic leukemia (CLL) is a type of cancer that starts from cells that become certain white blood cells (called lymphocytes) in the bone marrow. The cancer (leukemia) cells start in the bone marrow but then go into the blood.

In CLL, the leukemia cells often build up slowly over time, and many people don't have any symptoms for at least a few years. In time, the cells can spread to other parts of the body, including the lymph nodes, liver, and spleen.

What is leukemia?

Leukemia is a cancer that starts in the blood-forming cells of the bone marrow. When one of these cells changes and becomes a leukemia cell, it no longer matures normally. Often, it divides to make new cells faster than normal. Leukemia cells also don't die when they should. This allows them to build up in the bone marrow, crowding out normal cells. At some point, leukemia cells leave the bone marrow and spill into the bloodstream, often causing the number of white blood cells in the blood to increase. Once in the blood, leukemia cells can spread to other organs, where they can prevent other cells in the body from functioning normally.

Leukemia is different from other types of cancer that start in organs like the lungs, colon, or breast and then spread to the bone marrow. Cancers that start elsewhere and then spread to the bone marrow are not leukemia.

Not all leukemias are the same. Knowing the specific type of leukemia helps doctors better predict each patient's prognosis (outlook) and select the best treatment.

What is a chronic leukemia?

Whether leukemia is acute or chronic depends on whether most of the abnormal cells are immature (and are more like stem cells) or mature (and are like normal white blood cells).

In chronic leukemia, the cells can mature partly but not completely. These cells may look fairly normal, but they are not. They generally do not fight infection as well as normal white blood cells do. The leukemia cells survive longer than normal cells, and build up, crowding out normal cells in the bone marrow. Chronic leukemias can take a long time before they cause problems, and most people can live for many years. But chronic leukemias are generally harder to cure than acute leukemias.

What is a lymphocytic leukemia?

Whether leukemia is myeloid or lymphocytic depends on which bone marrow cells the cancer starts in.

Lymphocytic leukemias (also known as lymphoid or lymphoblastic leukemia) start in the cells that become lymphocytes. Lymphomas are also cancers that start in those cells. The main difference between lymphocytic leukemias and lymphomas is that in leukemia, the cancer cell is mainly in the bone marrow and blood, while in lymphoma it tends to be in lymph nodes and other tissues.

Are there different types of CLL?

Doctors have found that there seem to be 2 different kinds of CLL:

  • One kind of CLL grows very slowly and so it may take a long time before the patient needs treatment.
  • The other kind of CLL grows faster and is a more serious disease.

The leukemia cells from these 2 types look alike, but lab tests can tell the difference between them. The tests look for proteins called ZAP-70 and CD38. If the CLL cells contain low amounts of these proteins, the leukemia tends to grow more slowly.

What are the other types of leukemia?

There are 4 main types of leukemia based on whether they are acute or chronic, and myeloid or lymphocytic:

In acute leukemias, the bone marrow cells cannot mature properly. Immature leukemia cells continue to reproduce and build up. Without treatment, most people with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured. Other types of acute leukemia have a less favorable outlook.

Myeloid leukemias (also known as myelocytic, myelogenous, or non-lymphocytic leukemias) start in early forms of myeloid cells -- white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes).

Rarer forms of lymphocytic leukemia

The common form of CLL starts in B lymphocytes, but there are some rare types of leukemia that share some features with CLL.

Prolymphocytic leukemia (PLL): In this type of leukemia the cancer cells are similar to normal cells called prolymphocytes -- immature forms of B lymphocytes (B-PLL) or T lymphocytes (T-PLL). Both B-PLL and T-PLL tend to be more aggressive than the usual type of CLL. Most people will respond to some form of treatment, but over time they tend to relapse. PLL may develop in someone who already has CLL (in which case it tends to be more aggressive), but it can also occur in people who have never had CLL.

Large granular lymphocyte (LGL) leukemia: This is another rare form of chronic leukemia. The cancer cells are large and have features of either T lymphocytes or another type of lymphocyte called natural killer (NK) cells. Most LGL leukemias are slow-growing, but a small number are more aggressive. Drugs that suppress the immune system may be helpful, but aggressive cases are very hard to treat.

Hairy cell leukemia (HCL): This is another cancer of lymphocytes that tends to progress slowly. It accounts for about 2% of all leukemias. The cancer cells are a type of B lymphocyte but are different from those seen in CLL. There are also important differences in symptoms and treatment. This type of leukemia gets its name from the way the cells look under the microscope -- they have fine projections on their surface that make them look "hairy." Treatment for HCL can be very effective and is described in How is Chronic Lymphocytic Leukemia Treated?

The rest of this document focuses mainly on CLL in adults, with some limited information on hairy cell leukemia. For information on other types of leukemia in adults and children, please see our separate documents on these topics.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: January 6, 2015 Last Revised: April 11, 2016

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