Chronic lymphocytic leukemia (CLL) is a type of cancer that starts in the white blood cells (called lymphocytes) in the bone marrow. It then invades the blood. Leukemia always starts in the bone marrow. It can spread to the lymph nodes, the spleen, liver, and other parts of the body. Leukemia cells tend to build up in the body over time. In many cases people don't have any symptoms for at least a few years. Compared to other types of leukemia, CLL usually grows slowly

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

• One kind grows very slowly and rarely needs to be treated. People with this kind of CLL survive an average of 15 years or more.

• The other kind grows faster and is more serious. People with this form of CLL survive an average of about 8 years.

The leukemia cells from these 2 types look alike. But certain new lab tests can tell them apart.

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

Normal bone marrow, blood, and lymph tissue

In order to understand the different types of leukemia, it helps to know something about the blood and lymph systems.

Bone marrow

Bone marrow is the soft inner part of some bones, such as bones of the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made up of blood-forming stem cells, fat cells, and tissues that help cells grow.

Early blood cells are called stem cells. Blood stem cells go through a series of changes to make new blood cells and lymphocytes. They cannot make any other kinds of cells. (This makes them different from embryonic stem cells, which are formed from a developing fetus and can develop into most other cells in the body.)

The blood-forming cells can develop into 1 of the 3 main types of blood cells:

• red blood cells
• white blood cells (other than lymphocytes)
• platelets

Red blood cells: Red blood cells carry oxygen from the lungs to all other cells in the body. They also carry away carbon dioxide, a waste product of cell activity. A shortage of red blood cells (called anemia) causes weakness, shortness of breath, and tiredness.

Platelets: Platelets help prevent bleeding by plugging up holes of blood vessels caused by cuts or bruises. A person with a shortage of platelets can bruise or bleed easily.

White blood cells: White blood cells help the body fight infections. Lymphocytes are one type of white blood cell, but there are other types, too. Lymphocytes are the main cells that make up lymphoid tissue, an important part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

There are 2 types of lymphocytes:

• B lymphocytes protect the body from invading germs. B lymphocytes are the cells that most often develop into chronic lymphocytic leukemia cells.

• T lymphocytes destroy cells that are infected with viruses.

Any of the blood-forming or lymphoid cells from the bone marrow can turn into leukemia cells. Once this happens, the cells don't go through their normal growth process and don't work the way they should. These cells also do not die as they should. So they build up, spill into the bloodstream, and spread to other organs.

Types of leukemia

Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the exact type of leukemia can help doctors better predict each patient's outlook (prognosis) and select the best treatment.

Acute leukemia versus chronic leukemia

The first factor in classifying leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells). In acute leukemia, the cells grow rapidly but they are not able to mature the way they should. The cells keep on splitting into new cells and build up in the blood. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured.

Chronic leukemia refers to a condition where the cells look mature but they are not really normal. The cells live too long, build up, and crowd out normal cells. Chronic leukemias tend to grow over a longer time, and most patients can live for many years. But as a rule, they are harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia

The second factor to consider in classifying leukemia is the type of bone marrow cells that are involved: lymphocytic or myeloid (also called myelogenous).

Lymphocytic leukemias start from the white blood cells in the bone marrow called lymphocytes. Myeloid leukemias start from any other type of white blood cell, red blood cell, or platelet-making cell in the bone marrow.

By looking at whether a leukemia is acute or chronic and whether it is myeloid or lymphocytic, most cases of leukemia can be sorted into 1 of the 4 main types shown in the table below:

Along with these main types, there are a few other, less common, types of leukemia. The information here is only about chronic lymphocytic leukemia (CLL) of adults. For information about other types of leukemia please see the separate American Cancer Society documents on these topics.

Last Medical Review: 08/13/2009
Last Revised: 08/13/2009