Leukemia is a cancer of the early blood-forming cells. Most often, the leukemia is a cancer of the white blood cells, but leukemia can involve other blood cell types as well.

Leukemia starts in the bone marrow and then spreads to the blood. From there it can go to the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testes (testicles), or other organs.

Some 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

Bone Marrow

The 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, pelvis) and vertebrae (back bones).

Bone marrow is made up of blood-forming cells and supporting tissues that help the blood-forming cells grow. The earliest blood-forming cells are called 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.)

Blood 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)
• platelets

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. When the marrow doesn't make enough red blood cells to replace those wearing out, it can lead to anemia (having too few red blood cells in the body). Anemia can cause weakness, tiredness, and shortness of breath.

Platelets

Platelets are fragments that break off from a type of bone marrow cell called a megakaryocyte. They are released into the blood, where they are important in plugging holes in blood vessels caused by cuts or bruises. Not having enough platelets is called thrombocytopenia and can result in excess bleeding and bruising.

White Blood Cells

White blood cells, also known as leukocytes, defend the body against infections from germs like bacteria, viruses, and fungi. The 3 main types of white blood cells are lymphocytes, granulocytes, and monocytes.

Lymphocytes: Lymphocytes are the main cells that make up lymphoid tissue, a major part of the immune system. The immune system helps the body fight off infections.

Lymphoid tissue is found in many places in the body, including the lymph nodes, thymus gland, spleen, tonsils and adenoids, and bone marrow. It is also scattered through the digestive system and respiratory system. The extensive system connecting all lymphoid tissues is called the lymphatic system. Lymphocytes circulate in this system and eventually flow into the bloodstream.

There are 2 main types of lymphocytes: B lymphocytes (B cells) and T lymphocytes (T cells). Normal B cells and T cells do different jobs in the immune system.

B cells help protect the body against 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 cells also help protect the body against foreign substances (those not normally present in the body). They recognize specific chemicals, such as those found on the outside of virus-infected cells.

There are several types of T cells, each with a specialized job to do. Some destroy abnormal cells by releasing substances that punch holes in them, causing them to become leaky. Some T cells release substances called cytokines that attract other types of white blood cells, which then kill the abnormal cells. Still other T cells act as a check on the immune system to ensure it does not attack normal cells.

Lymphocytes are the cells from which acute lymphocytic (or lymphoblastic) leukemia develops. Although both B cell and T cells can develop into leukemia, B-cell leukemias are much more common than T-cell leukemias.

Normal B cells and T cells can be recognized by lab tests that look for specific chemicals on their surfaces. Some substances are found only on B cells, and others are found only on T cells. Lab tests can also help determine how mature the B cells or T cells are. Both the type of lymphocyte and its stage of maturity are important because leukemias that arise from these different kinds of cells have different characteristics.

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 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.

Types of Leukemia in Children

Leukemia is often described as being either acute (growing quickly) or chronic (growing slowly).

Acute Leukemias

Almost all childhood leukemia is acute. There are 2 main types of acute leukemia:

• Acute lymphocytic leukemia (ALL, also called acute lymphoblastic leukemia) accounts for about 3 out of 4 cases of childhood leukemia. This leukemia starts from the lymphoid cells in the bone marrow.
• Acute myelogenous leukemia (AML, also called acute myeloid leukemia, acute myelocytic leukemia, or acute non-lymphocytic leukemia [ANLL]) accounts for most of the remaining cases. This leukemia starts from the cells that form white blood cells (other than lymphocytes), red blood cells, or platelets.
• Hybrid or mixed lineage leukemias are rare. The cells have features of both ALL and AML. 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 the subtypes of ALL and AML, see the section, "How Is Childhood Leukemia Classified?"

Chronic Leukemias

Chronic leukemias can also be divided into 2 types. Chronic myelogenous leukemia (CML) is rare in children, but it does occur. It is treated the same as in adults. For more information on CML, see the American Cancer Society document, Leukemia--Chronic Myeloid. Chronic lymphocytic leukemia (CLL) is extremely rare in children and is not discussed further in this document.

Juvenile Myelomonocytic Leukemia (JMML)

This rare type of leukemia is neither chronic nor acute. It begins from myeloid cells, but isn't as fast growing as acute myelogenous leukemia or as slow as chronic myeloid leukemia. It occurs most often in young children (under age 4). Symptoms can include pale skin, fever, cough, trouble breathing (due to too many white blood cells in the lungs), and an enlarged spleen and lymph nodes.

Last Medical Review: 08/19/2007
Last Revised: 05/14/2009