Cancer starts when cells start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about how cancers start and spread, see What Is Cancer? For information about the differences between childhood cancers and adult cancers, see Cancer in Children.
Leukemia is a cancer that starts in early blood-forming cells found in the bone marrow, the soft inner part of certain bones. Most often, leukemia is a cancer of the white blood cells, but some leukemias start in other blood cell types.
Any of the blood-forming cells from the 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 might reproduce quickly, and not die when they should. These cells build up in the bone marrow, crowding out normal cells. In most cases, the leukemia cells spill into the bloodstream fairly quickly. From there they can go to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testicles, or other organs, where they can keep other cells in the body from doing their jobs.
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 bones. New blood cells (red blood cells, white blood cells, and platelets) are made there. In infants, active bone marrow is found in almost all bones of the body, but by the teenage years it is found mainly in the flat bones (skull, shoulder blades, ribs, and hip bones) and vertebrae (the bones that make up the spine).
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. Blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into 1 of the 3 main types of blood cell components.
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 (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 by plugging up holes in blood vessels. Having too few platelets (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 the lymph nodes, thymus (a small organ behind the breast bone), spleen, tonsils and adenoids, and bone marrow. It is also scattered through the digestive system and respiratory system. There are 2 main types of lymphocytes:
- B lymphocytes (B cells) help protect the body against germs such as bacteria and viruses. They make proteins called antibodies that attach to the germ, marking it for destruction by other parts of the immune system.
- T lymphocytes (T cells) also help protect the body against germs. Some types of T cells destroy germs directly, while others play a role in either boosting or slowing the activity of other immune system cells.
Acute lymphocytic (lymphoblastic) leukemia (ALL), the most common type of childhood leukemia, develops from early forms of lymphocytes. It can start in either early B cells or T cells at different stages of maturity. Although both B cells and T cells can develop into leukemia, B-cell leukemias are much more common than T-cell leukemias. For more information, see the section “How is childhood leukemia classified?”
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 under the microscope 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 start making antibodies to fight them.
Types of leukemia in children
Leukemia is often described as being either acute (fast growing) or chronic (slow growing). Almost all childhood leukemia is acute.
The main types of acute leukemia are:
- Acute lymphocytic (lymphoblastic) leukemia (ALL): About 3 out of 4 childhood leukemias are ALL. This leukemia starts from early forms of lymphocytes in the bone marrow.
- Acute myelogenous leukemia (AML): This type of leukemia, also called acute myeloid leukemia, acute myelocytic leukemia, or acute non-lymphocytic leukemia, accounts for most of the remaining cases. AML starts from the myeloid cells that form white blood cells (other than lymphocytes), red blood cells, or platelets.
- Hybrid or mixed lineage leukemia: In these rare leukemias, the cells have features of both ALL and AML. In children, they are generally treated like ALL and usually respond to treatment like ALL.
Both ALL and AML can be further divided into different subtypes. For more on these subtypes, see the section “How is childhood leukemia classified?”
Chronic leukemias are much more common in adults than in children. They tend to grow more slowly than acute leukemias, but they are also harder to cure. Chronic leukemias can be divided into 2 types.
- Chronic myelogenous leukemia (CML): This leukemia rarely occurs in children. Treatment is similar to that used for adults (see “Treatment of children with chronic myelogenous leukemia”). For more detailed information on CML, see Leukemia--Chronic Myeloid.
- Chronic lymphocytic leukemia (CLL): This leukemia is extremely rare in children. For more information on CLL, see Leukemia--Chronic Lymphocytic.
Juvenile myelomonocytic leukemia (JMML)
This rare type of leukemia is neither chronic nor acute. It begins from myeloid cells, but it usually doesn’t grow as fast as AML or as slow as CML. It occurs most often in young children (under age 4). Symptoms can include pale skin, fever, cough, easy bruising or bleeding, trouble breathing (from too many white blood cells in the lungs), and an enlarged spleen and lymph nodes.
Last Revised: 02/03/2016