Chronic lymphocytic leukemia is a type of blood cancer that starts in white blood cells called lymphocytes. These cells are part of your immune system and help fight infections. In CLL, the bone marrow makes too many abnormal lymphocytes that don't work properly. These abnormal cells build up slowly over time in the blood and bone marrow.
CLL is the most common type of leukemia in adults in the United States. It typically affects people over age 60, though younger adults can develop it too. The word chronic means the disease usually grows slowly over months or years. This is different from acute leukemia, which grows quickly and requires immediate treatment.
Many people with CLL live for years without needing treatment. Some may never need treatment at all. Others will need therapy when the disease progresses or causes symptoms. Regular blood tests help doctors monitor the disease and decide when treatment is needed.