What is leukemia?
Leukemia is cancer that originates in blood-forming tissue. The disease is characterized by the uncontrolled growth of blood cells, usually white blood cells (leukocytes), in the bone marrow. White blood cells are a fundamental component of the body's immune response. The leukemia cells crowd out and replace normal blood and marrow cells.
In the United States, about 47,666 adults and 4,103 children, adolescents and young adults under 20 were diagnosed with leukemia in 2014 (Leukemia and Lymphoma Society). The most common types of leukemia in adults are AML and CLL. ALL is very rare in adults, and is the most common type of leukemia in children.
Leukemia incidences are more common in men and boys than girls and women, and also more likely to occur in white people than black. Although people of any age can get leukemia, it is most common in adults over 60 years of age.
How leukemia develops
Leukemia is classified in two ways. One way is by the type of white blood cell that is affected (lymphoid or myeloid cells). Another way is by how quickly the disease develops and gets worse, meaning whether it is acute (fast-growing) or chronic (slow-growing).
Lymphocytic versus myeloid leukemia
Lymphocytic leukemia (also known as lymphoid or lymphoblastic leukemia) develops in the white blood cells called lymphocytes within the bone marrow. Myeloid (also known as myelogenous) leukemia may also start in white blood cells other than lymphocytes (e.g., monocytes), as well as red blood cells and platelets.
Acute versus chronic leukemia
Acute leukemia is rapidly progressing and results in the accumulation of immature, functionless blood cells in the bone marrow. With this type of leukemia, cells reproduce and build up in the marrow, decreasing the marrow’s ability to produce enough healthy blood cells. Chronic leukemia progresses more slowly and results in the accumulation of relatively mature, but still abnormal, white blood cells.