Leukemia--Chronic Lymphocytic Overview

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What Is Leukemia - Chronic Lymphocytic (CLL)? TOPICS

What is chronic lymphocytic leukemia?

Leukemia is a type of cancer that starts in cells that form new blood cells. These cells are found in the soft, inner part of the bones called the bone marrow. Any blood-forming cell can turn into a leukemia cell. Once that happens, the cell can grow and divide to form many new cancer cells. These cells can take over the bone marrow, spill out into the bloodstream, and spread to other organs.

Normal bone marrow, blood, and lymph tissue

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.) One type of blood stem cell makes new lymphocytes (lymphocytes are a type of white blood cell). The other type of blood stem cell 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.

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.

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.

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.

Chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) 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. Many 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.
  • The other kind grows faster and is more serious.

The leukemia cells from these 2 types look alike. But certain 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.

CLL isn’t the only kind of leukemia. There are 4 main types of leukemia:

Knowing the exact type can help doctors better predict each patient's outlook (prognosis) and select the best treatment.

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: 08/05/2013
Last Revised: 11/04/2014