Leukemia - Chronic Lymphocytic - CLL: Overview

Approved by the Cancer.Net Editorial Board, 07/2014

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.

About leukemia

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. 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.

About lymphocytes

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, B cells, and natural killer (NK) cells. Generally, T cells fight infections by triggering other cells in the immune system and by destroying infected cells, B cells make antibodies, and NK cells fight microbes 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 that carry oxygen, other types of normal white blood cells, such as neutrophils or granulocytes that fight infection, and platelets that 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.

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 now called T-cell prolymphocytic leukemia and much less common than the B-cell type of the disease. About 1% of people with CLL have the T-cell type. More than 95% of people with CLL have the B-cell type.

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.

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

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