What are the types of 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. The leukemia cells crowd out and replace normal blood and marrow cells.

There are four main types of leukemia:

• Acute myeloid leukemia (AML)
• Chronic myeloid leukemia (CML)
• Acute lymphocytic leukemia (ALL)
• Chronic lymphocytic leukemia (CLL)

Another type of leukemia, hairy cell leukemia, is a rare subtype of chronic lymphocytic leukemia (CLL). It is caused by an increased number of lymphocytes and progresses very slowly. It is called “hairy” because of the appearance of the leukemia cells under a microscope.

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 leukemia (also known as lymphoid or lymphoblastic leukemia) develops in the white blood cells called lymphocytes within the bone marrow. Myeloid leukemia (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 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.

Leukemia is categorized by two factors: how quickly it progresses and the types of cell it affects. The first category is broken down into two choices, acute or chronic, based on how rapidly symptoms appear and how quickly the disease develops in the body. The second category is also made up of two choices, lymphoblastic and myeloid, depending on which type of blood cell the disease targets. Combining the two categories yields four main sub-types of leukemia: acute lymphoblastic, chronic lymphoblastic, acute myeloid, and chronic myeloid. There are also other, much rarer forms of the disease, such as hairy cell leukemia, which fall outside the conventional categories.