Chronic Lymphocytic Leukemia (CLL)

What Is It?

Chronic lymphocytic leukemia (CLL) is a cancer that occurs when the bone marrow makes too many lymphocytes, a type of white blood cell. CLL usually grows slowly compared to other leukemias, and it may not cause symptoms for some time.

CLL is one of four main types of leukemia. It is the second most common type of leukemia in adults; most people with CLL are middle-aged or older. The disease is very rare in children.

Normally, a person's immature blood stem cells develop into myeloid and lymphoid stem cells. The myeloid cells become mature blood cells: white blood cells, red blood cells, and platelets. Lymphoid stem cells develop into three types of infection-fighting lymphocytes:

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