Chronic lymphocytic leukemia (CLL) represents the other major form of lymphoid leukemia. CLL is the second most common type of leukemia in adults. It often occurs during or after middle age; it rarely occurs in children. As in ALL, too many blood stem cells develop into abnormal lymphocytes (leukemic cells) and do not become healthy white blood cells. These immature leukemic lymphocytes are not able to fight infection. Also, as the number of lymphocytes increases in the blood and bone marrow, there is less room for healthy white blood cells, red blood cells and platelets. This may result in infection, anemia and easy bleeding.

Unlike ALL, however, CLL usually progresses slowly, and often requires no treatment initially. This is known as an “indolent” or “smoldering” variety of CLL. However, depending on the individual patient’s genetics, CLL may progress rapidly and have a poorer prognosis. In addition to a complete blood work-up, cytogenetic testing using the fluorescence in situ hybridization technique is recommended to determine the best course of treatment. Most CLL cases are known as B-CLL, meaning that the cancer derives from B lymphocytes. Much rarer is T-CLL, in which the leukemic cells are T cells rather than B cells.