A rapidly progressing cancer that starts in the blood-forming cells of the bone marrow. Leukemia results from an abnormal development of leukocytes (white blood cells) and their precursors. Leukemia cells look different than normal cells and do not function properly.
There are four main types of leukemia, which can be further divided into subtypes. When classifying the type of leukemia, the first steps are to determine whether the cancer is lymphocytic or myelogenous (cancer can occur in either the lymphoid or myeloid white blood cells) and whether it is acute or chronic (rapidly or slowly progressing).
In acute leukemia, the new or immature cells, called blasts, remain very immature and cannot perform their functions properly. The blasts rapidly increase in number