Chronic lymphocytic leukemia (also called CLL) is a blood and bone marrow disease that usually gets worse slowly. CLL is the second most common type of leukemia in adults. It often occurs during or after middle age; it rarely occurs in children. Normally, the body produces bone marrow stem cells (immature cells) that develop into mature blood cells. There are 3 types of mature blood cells: Red blood cells that carry oxygen and other materials to all tissues of the body. White blood cells that fight infection and disease. Platelets that help prevent bleeding by causing blood clots to form. In CLL, too many stem cells develop into a type of white blood cell called lymphocytes. There are 3 types of lymphocytes: B lymphocytes that make antibodies to help fight infection. T lymphocytes that help B lymphocytes make antibodies to fight infection. Natural killer cells that attack cells with cancer or a virus.

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