+ -Text Size

Leukemia is a type of cancer that starts in cells that form new blood cells. These cells are found in the soft, inner part of the bones called the bone marrow.

Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Normal bone marrow, blood, and lymphoid tissue

To understand the different types of leukemia, it helps to know something about the blood and lymph systems. Any blood-forming cell can turn into a leukemia cell. Once that happens, the cell can grow and divide to form many new cancer cells. These cells can take over the bone marrow, spill out into the bloodstream, and spread to other organs.

Bone marrow is the soft, spongy, inner part of bones such as the skull, shoulder blades, ribs, pelvis, and bones in the spine. All of the different types of blood cells are made in the bone marrow. Bone marrow is made up of a small number of blood stem cells, blood-forming cells, fat cells, and tissues that help the blood cells grow.

Blood stem cells go through a series of changes to make new blood cells. They can develop into 1 of the 3 main types of blood cell:

  • Red blood cells, which carry oxygen from the lungs to the rest of the body and carry carbon dioxide from the body to the lungs.
  • Platelets, which are pieces of cells that help your blood clot
  • White blood cells, which fight infection

There are different types of white blood cells. Each has its own role in fighting infection. The 3 main types of white blood cells are granulocytes, monocytes, and lymphocytes.

Lymphocytes are the main cells that make up lymphoid tissue, which is a major part of the immune system. The 2 main types of lymphocytes are called B cells and T cells. Normal T cells and B cells do different jobs within the immune system. ALL starts in early forms of B cells or T cells.

Acute lymphocytic leukemia

Acute lymphocytic leukemia (ALL) is also called acute lymphoblastic leukemia. The term acute means that the leukemia grows quickly, and if not treated, could be fatal in a few months. People with chronic leukemias can live years without treatment. Lymphocytic means it develops from early forms of lymphocytes, a type of white blood cell. This is different from acute myeloid leukemia (AML), which starts in other blood cell types found in the bone marrow. To learn about AML, see Leukemia--Acute Myeloid.

Other types of cancer that start in lymphocytes are known as lymphomas (non-Hodgkin lymphoma and Hodgkin disease). The main difference between these types of cancers is that the cancer cells in ALL are mainly in the bone marrow and blood (although they may spread to other places), while lymphomas are mainly in lymph nodes or other organs. For more information on lymphomas, see Non-Hodgkin Lymphoma and Hodgkin Disease.

Leukemia starts in the bone marrow and then moves into the blood where it can spread to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (brain and spinal cord), and other organs. Other types of cancer can start in these organs and then spread to the bone marrow (or elsewhere). Those cancers are not leukemia. Both children and adults can get leukemia.

ALL isn’t the only kind of leukemia. There are 4 main types of leukemia:

Knowing the exact type can help doctors better predict each patient's outlook (prognosis) and select the best treatment.

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

Last Medical Review: 12/08/2014
Last Revised: 02/22/2016