Overview

Leukemia is a cancer of the blood. Leukemia begins when normal blood cells change and grow uncontrollably. Chronic lymphocytic leukemia (CLL) is a cancer of the lymphocytes, a type of white blood cell involved in the body’s immune system. For about half of people with CLL, the disease grows and progresses slowly, and it may take years for symptoms to appear or for treatment to be needed. In fact, some patients may never need treatment for their CLL. The other half of people who develop CLL have a type of CLL that grows more quickly and need treatment sooner.

About lymphocytes

Lymphocytes are made in the bone marrow, the spongy, red tissue in the inner part of the large bones. Lymphocytes are found in the blood, lymph nodes, and spleen. There are three different types of lymphocytes: T cells, B cells, and natural killer (NK) cells. Generally, T cells fight infections by activating other cells in the immune system and by destroying infected cells, B cells make antibodies, and NK cells fight microbial cells and cancer cells.

About CLL

In people with CLL, the abnormal cells crowd other types of cells in the bone marrow, preventing the production of red blood cells (which carry oxygen), other types of white blood cells, and platelets (parts of the blood needed for clotting). This means that people with CLL may be anemic (because they do not have enough red blood cells), more likely to get infections (because they do not have enough of the type of white blood cells called neutrophils that fight bacteria), and bruise or bleed easily (because of a low level of platelets).

There are two general types of CLL, and it is important for doctors to find out whether the disease is caused by the overgrowth of T cells or B cells. The T-cell type of CLL is less common (about 1% of people with CLL have this type) and grows faster than the B-cell type of the disease (more than 95% of people with CLL have the B-cell type).

Learn more about other, rare types of chronic T-cell leukemia and types of B-cell leukemia.

Looking for More of an Overview?

If you would like additional introductory information, explore these related items on Cancer.Net:

• ASCO Answers Fact Sheet: Read a one-page face sheet (available in PDF) that offers an easy-to-print introduction for this type of cancer.
  
  
• Cancer.Net Patient Education Video: View a short video led by an ASCO expert in this type of cancer that provides basic information and areas of research.
  
  
• Cancer.Net En Español: Read about CLL in Spanish.

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Find out more about basic cancer terms used in this section.