ON THIS PAGE: You will find some basic information about this disease and the parts of the body it may affect. This is the first page of Cancer.Net’s Guide to Chronic Lymphocytic Leukemia. To see other pages, use the menu on the side of your screen. Think of that menu as a roadmap to this full guide.
Leukemia is a cancer of the blood. Leukemia begins when healthy 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. In some 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. In other patients, the disease grows more quickly and needs treatment sooner.
Lymphocytes circulate in the bloodstream and are made in the lymph nodes, spleen, thymus, and bone marrow. Bone marrow is the spongy, red tissue in the inner part of the large and flat bones.
There are three different types of lymphocytes:
T cells, which fight infection by triggering other cells in the immune system and by destroying infected cells
B cells, which make antibodies
Natural killer (NK) cells, which fight microbes and cancer cells
In people with CLL, the abnormal cells crowd other types of cells in the bone marrow. This crowding prevents the production of the healthy blood cells, including:
Red blood cells that carry oxygen
Other types of white blood cells, such as neutrophils or granulocytes that fight infection
Platelets, which are needed for clotting
This means that people with CLL may have anemia from low levels of red blood cells, are more likely to get infections because they do not have enough white blood cells, and bruise or bleed easily because of a low level of platelets.
Most often, CLL is diagnosed when too many abnormal lymphocytes are found in the blood, also known as lymphocytosis. However, the same disease can occur when the abnormal lymphocytes are mostly in the lymph nodes but not in the blood. This is called small lymphocytic lymphoma, but it behaves very similarly to CLL.
Types of CLL
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.
B-cell CLL. More than 95% of people with CLL have the B-cell type.
T-cell prolymphocytic leukemia. The T-cell type of CLL is now called T-cell prolymphocytic leukemia . About 1% of people with CLL have the T-cell type.
Looking for More of an Overview?
If you would like additional introductory information, explore these related items. Please note that these links will take you to other sections on Cancer.Net:
ASCO Answers Fact Sheet : Read a one-page face sheet (available as a PDF) that offers an easy-to-print introduction to this type of cancer.
Cancer.Net Patient Education Video : View a short video led by an ASCO expert in leukemia that provides basic information and areas of research.
The next section in this guide is Statistics  and it helps explain how many people are diagnosed with this disease and general survival rates. Or, use the menu on the side of your screen to choose another section to continue reading this guide.