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. Lymphocytes are normally found in the blood, lymph nodes, bone marrow (the spongy, red tissue in the inner part of the large bones), spleen, and in a clear fluid called lymph that flows through small vessels in the body and collects in lymph nodes.

In people with CLL, mature lymphocytes grow abnormally and build up in the peripheral (circulating) blood, bone marrow, spleen, and lymph nodes. Over time, these malignant (cancerous) cells may crowd healthy blood-forming cells, resulting in fewer red blood cells (which deliver oxygen to the body), neutrophils (a type of white blood cell needed to fight infection), and platelets (which prevent bleeding). In about half of people, CLL grows and progresses slowly, and it may take years for symptoms to appear or for treatment to be needed. In fact, some patients never require treatment for their CLL. The other half has a more rapidly growing type of CLL, which requires treatment sooner.

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. T cells and B cells are specific types of lymphocytes. T cells normally help to fight infections by activating other cells in the immune system, while B cells help make antibodies to fight disease. The T-cell type of CLL is less common (about 1% of all CLL cases) and progresses more rapidly than the B-cell type of the disease (more than 95% of all CLL cases).

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

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