What is childhood leukemia?
Leukemia is a cancer of the early blood-forming cells. Most often, leukemia is a cancer of the white blood cells, but some leukemias start in other blood cell types.
Leukemia starts in the bone marrow (the soft inner part of certain bones, where new blood cells are made). In most cases, the leukemia invades the blood fairly quickly. From there it 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.
Some other childhood cancers, such as neuroblastoma or Wilms tumor, start in other organs and can spread to bone marrow, but these cancers are not leukemia.
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. It is where new blood cells (red blood cells, white blood cells, and platelets) are made. 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 pelvis) 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:
- Red blood cells
- White blood cells (which include lymphocytes, granulocytes, and monocytes)
Red blood cells
Red blood cells carry oxygen from the lungs to all other tissues of the body, and take carbon dioxide back to the lungs to be removed.
Platelets are actually cell fragments that are made by a type of bone marrow cell called a megakaryocyte. They are released into the blood, where they are important in stopping bleeding by plugging holes in blood vessels caused by cuts or bruises.
White blood cells
White blood cells, also known as leukocytes, help the body fight infections. The 3 main types of white blood cells are lymphocytes, granulocytes, and monocytes.
Lymphocytes: These are the main cells that make up lymphoid tissue, a major part of the body's immune system. Lymphoid tissue is found in many places in the body, including the lymph nodes, thymus, spleen, tonsils and adenoids, and bone marrow. It is also scattered through the digestive system and respiratory system.
Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. There are 2 main types of lymphocytes:
- B lymphocytes (B cells) help protect the body against germs such as bacteria and viruses. When a B cell comes into contact with one of these germs, it matures into a plasma cell, which releases proteins called antibodies. The antibodies 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. There are several types of T cells, each with a special job. Some T cells can 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: These white blood cells 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: These white blood cells, which are related to granulocytes, also help protect 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.
Development of leukemia
Any of the cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells fail to go through the normal process of maturing. Leukemia cells might reproduce quickly, and not 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 functioning normally.
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.
There are 2 main types of acute leukemia:
Acute lymphocytic (lymphoblastic) leukemia (ALL): About 3 out of 4 cases of childhood leukemia are ALL. This leukemia starts from the lymphoid cells 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 leukemias: In these rare leukemias, the cells have features of both ALL and AML. In children, they are generally treated like ALL and respond to treatment like ALL.
Both ALL and AML can be further divided into different subtypes. For more information 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 also be divided into 2 types.
- Chronic myelogenous leukemia (CML): This leukemia rarely occurs in children. Treatment is similar to treatment in adults (see “Treatment of children with chronic myelogenous leukemia [CML]”). For more detailed information on CML, see our document, Leukemia--Chronic Myeloid.
- Chronic lymphocytic leukemia (CLL): This leukemia is extremely rare in children, so it is not discussed further in this document. For more information on CLL, see our document, Leukemia--Chronic Lymphocytic.
Juvenile myelomonocytic leukemia (JMML)
This rare type of leukemia is neither chronic nor acute. It begins from myeloid cells, but it 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 Medical Review: 06/11/2012
Last Revised: 01/18/2013