Cancer starts when cells in a part of the body begins to grow out of control and can spread to other areas of the body. There are many kinds of cancer. Cells in nearly any part of the body can become cancer. To learn more about how cancers start and spread, see What Is Cancer?
Leukemias are cancers that start in cells that would normally develop into different types of blood cells. Here we will talk about acute myeloid leukemia (AML).
Acute myeloid leukemia (AML) has many other names, including acute myelocytic leukemia, acute myelogenous leukemia, acute granulocytic leukemia, and acute non-lymphocytic leukemia.
“Acute” means that this leukemia can progress quickly if not treated, and would probably be fatal in a few months. “Myeloid” refers to the type of cell this leukemia starts from.
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. The different types of AML are listed in “How is acute myeloid leukemia classified?”
AML starts in the bone marrow (the soft inner part of certain 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 testicles.
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 leukemias.
Normal bone marrow, blood, and lymphoid tissue
To understand the different types of leukemia, it helps to know about the blood and lymph systems.
Bone marrow is the soft inner part of some bones such as the skull, shoulder blades, ribs, pelvic (hip) bones, 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.
Inside the bone marrow, blood stem cells develop into new blood cells. During this process, the cells become either lymphocytes (a kind of white blood cell) or other blood-forming cells, which are types of myeloid cells. These other blood-forming cells can develop into red blood cells, white blood cells (other than lymphocytes), or platelets.
Types of 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. Having too few red blood cells in the body (called anemia) can make you feel tired, weak, and short of breath because your body tissues are not getting enough oxygen.
Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in stopping bleeding. They help plug up holes in blood vessels caused by cuts or bruises. Having too few platelets (called thrombocytopenia) may cause you to bleed or bruise easily.
White blood cells help the body fight infections. Having too few white blood cells weakens your immune system and can make you more likely to get an infection.
Types of white blood cells
Lymphocytes are mature, infection-fighting cells that develop from lymphoblasts, a type of blood stem cell in the bone marrow. 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 (a small organ behind the breast bone), the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow. There are 2 main types of lymphocytes:
- B lymphocytes (B cells) protect the body from invading germs by developing (maturing) into plasma cells, which make proteins called antibodies. The antibodies attach to the germs (bacteria, viruses, and fungi), which helps other types of white blood cells recognize and destroy them.
- T lymphocytes (T cells) can recognize cells infected by viruses and directly destroy these cells. They also help regulate the immune response.
Granulocytes are mature, infection-fighting cells that develop from myeloblasts, a type of blood-forming cell in the bone marrow. Granulocytes have granules that show up as spots 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.
Monocytes develop from blood-forming monoblasts in the bone marrow and are related to granulocytes. 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 also help lymphocytes recognize germs and make antibodies to fight them.
Any of the blood-forming myeloid or lymphoid cells from bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells no longer mature in a normal way. Leukemia cells often reproduce quickly, but in most cases the problem is that they don’t die when they should. They survive and build up in the bone marrow. Over time, these cells spill into the bloodstream and spread to other organs, where they can keep other cells in the body from doing their jobs.
Types of leukemia
Not all leukemias are the same. There are 4 main types of leukemia. Knowing the specific type helps doctors better predict each patient’s prognosis (outlook) and select the best treatment.
Acute leukemia versus chronic leukemia
The first factor in classifying a patient’s leukemia is whether most of the abnormal cells look like normal white blood cells (mature) or look more like stem cells (immature).
In acute leukemia, the leukemia cells are immature blood cells (called blasts). These leukemias are fast growing because normal blast cells divide quickly. But the leukemia cells don’t divide any more often than normal blast cells do. They just don’t stop dividing when normal blast cells would. 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 leukemia cells are more mature cells, but they are not completely normal. They generally don’t fight infection as well as normal white blood cells. And 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 with them for many years. But chronic leukemias are generally harder to cure than acute leukemias.
Myeloid leukemia versus lymphocytic leukemia
The other main factor in classifying leukemia is the type of bone marrow cells that are affected.
Myeloid leukemias start in immature forms of myeloid cells – white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes). They are also known as myelocytic, myelogenous, or non-lymphocytic leukemias.
Lymphocytic leukemias start in immature forms of lymphocytes. They are also known as lymphoid or lymphoblastic leukemias. Lymphomas are also cancers that start in lymphocytes. But whereas lymphocytic leukemias develop from cells in the bone marrow, lymphomas develop from cells in lymph nodes or other organs.
By considering whether leukemias are acute or chronic and whether they are myeloid or lymphocytic, they 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 focuses on acute myeloid leukemias in adults only. Chronic leukemias in adults and acute lymphocytic leukemia (ALL) in adults are discussed in other American Cancer Society documents. For information on AML in children, see Childhood Leukemia.
Last Revised: 02/22/2016