AML is a cancer that starts in cells that would normally develop into different types of blood cells. Most cases of AML develop from cells that would turn into white blood cells (other than lymphocytes), but some cases of AML develop in other types of blood-forming cells. (Acute leukemia that develops in lymphocytes is called acute lymphocytic leukemia (ALL). For more information on this type of leukemia, see the American Cancer Society document, Leukemia--Acute Lymphocytic.)

AML starts in the bone marrow (the soft inner part of the bones, where new blood cells are made), but in most cases it quickly moves into the blood. It can sometimes spread to other parts of the body including the lymph nodes, liver, spleen, central nervous system (brain and spinal cord), and testes.

Other types of cancer can start in these organs and then spread to the bone marrow. But these cancers that start elsewhere and then spread to the bone marrow are not leukemia.

Normal Bone Marrow, Blood, and Lymphoid Tissue

In order to understand the different types of leukemia, it is helpful to have some basic knowledge about the blood and lymph systems.

Bone Marrow

Bone marrow is the soft inner part of some bones such as the skull, shoulder blades, ribs, pelvis, and backbones. The bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow.

The blood-forming cells come from blood stem cells. These stem cells only make new blood-forming cells and not other kinds of cells. (This makes them different from embryonic stem cells, which are formed in a developing fetus and can grow into most other cell types in the body.)

Stem cells go through a series of changes. During this process, the cells develop into either lymphocytes (a kind of white blood cell) or other blood-forming cells. The other blood-forming cells can develop into 1 of the 3 main types of blood cell components: red blood cells, white blood cells (other than lymphocytes), or platelets.

Red Blood Cells

Red blood cells carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed. Anemia (having too few red blood cells in the body) typically causes weakness, fatigue, and shortness of breath because the body tissues are not getting enough oxygen.

Platelets

Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets is called thrombocytopenia. A person with thrombocytopenia may bleed and bruise easily.

White Blood Cells

White blood cells are important in defending the body against infections. Lymphocytes are one type of white blood cell. The other types of white blood cells are granulocytes (neutrophils, basophils, and eosinophils) and monocytes.

Lymphocytes are the main cells that make up lymphoid tissue, a major part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. The 2 types of lymphocytes are known as B lymphocytes (B cells) and T lymphocytes (T cells).

• B lymphocytes protect the body from invading germs by developing (maturing) into plasma cells, which make antibodies. These antibodies attach to the germs, such as bacteria, viruses, and fungi. Once the germ has been coated in this way, other white blood cells can recognize and destroy it.
• T lymphocytes can recognize cells infected by viruses and directly destroy these cells.

Granulocytes are white blood cells that have granules in them, which are spots that can be seen under the microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria. The 3 types of granulocytes--neutrophils, basophils, and eosinophils--are distinguished by the size and color of their granules. Granulocytes develop from blood-forming cells called myeloblasts to become mature, infection-fighting cells.

Monocytes, which are related to granulocytes, also are important in protecting the body against bacteria. They start in the bone marrow as blood-forming monoblasts and develop into mature monocytes. After circulating in the bloodstream for about a day, monocytes enter body tissues to become macrophages, which can destroy some germs by surrounding and digesting them. Macrophages are also important in helping lymphocytes to recognize germs and start making antibodies to fight them.

Any of the blood-forming or lymphoid cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells fail to go through their normal process of maturing. Although leukemia cells may reproduce quickly, in most cases the problem is that they don't die when they should. They continue to survive and accumulate. Over time, these cells spill into the bloodstream and spread to other organs, where they can keep other cells in the body from functioning normally.

Types of Leukemia

Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the specific type of leukemia can help doctors better predict each patient's prognosis (outlook) and select the best treatment.

Acute leukemia versus chronic leukemia: The first factor to consider in classifying a patient's leukemia is if most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).

In acute leukemia, the bone marrow cells cannot mature properly. Immature leukemia cells continue to reproduce and build up. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured.Other types of acute leukemia have a less favorable outlook.

In chronic leukemia, the cells can mature partly but not completely. These cells are not really normal. They generally do not fight infection as well as do normal white blood cells. And, of course, they survive longer, build up, and crowd out normal cells. Chronic leukemias tend to progress over a longer period of time, and most patients can live for many years. However, chronic leukemias are generally harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia: The second factor to consider in classifying leukemia is the type of bone marrow cells that are affected.

Leukemias that start in early forms of myeloid cells--white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes)--are myeloid leukemias (also known as myelocytic, myelogenous, or non-lymphocytic leukemias).

If the cancer starts in early forms of lymphocytes, it is called lymphocytic leukemia (also known as lymphoid leukemia). Lymphomas are also cancers of lymphocytes. But, unlike lymphocytic leukemias, which develop in the bone marrow, lymphomas develop from lymphocytes in lymph nodes or other organs.

By considering whether they are acute or chronic and whether they are myeloid or lymphocytic, leukemias can be divided into 4 main types:

• acute myeloid (or myelogenous) leukemia (AML) 
• chronic myeloid (or myelogenous) leukemia (CML) 
• acute lymphocytic (or lymphoblastic) leukemia (ALL) 
• chronic lymphocytic leukemia (CLL)

The rest of this document contains information on acute myeloid leukemias of adults only. Chronic leukemias of adults and acute lymphocytic leukemia (ALL) of adults are discussed in other American Cancer Society documents. For information on AML in children, please see the separate document, Childhood Leukemias.

Last Medical Review: 08/03/2007
Last Revised: 05/14/2009