It is classified according to how quickly the abnormal changes happen and by the type of blood cell that is affected.
Acute leukemia gets worse quickly, with fast multiplication of abnormal, immature blood cells.
Chronic leukemia worsens gradually. Abnormal cells are present, but they are more mature than they are in acute leukemia and can carry out at least some of their functions. However, they do not fight infection as well as normal white blood cells do. Also, they tend to live much longer than normal white blood cells, which results in an abnormal accumulation of cells. Lymphocytic leukemia affects the white blood cells called lymphocytes, which control the body's immune response by finding and destroying foreign substances.