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), 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

In order to understand the different types of leukemia, it helps to know about the blood and lymph systems.

Bone marrow

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 either lymphocytes (a kind of white blood cell) or other blood-forming cells. The 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 a person to feel tired, weak, and short of breath because the body tissues are not getting enough oxygen.

Platelets

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 plugging holes in blood vessels caused by cuts or bruises. A shortage of platelets in the blood is called thrombocytopenia. A person with thrombocytopenia may bleed and bruise easily.

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 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.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. There are 2 main types of lymphocytes:

• B lymphocytes (B cells)
• 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 also help lymphocytes to recognize germs and start making antibodies to fight them.

Any of the blood-forming or lymphoid 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. 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 (growing quickly) or chronic (growing slowly). Almost all childhood leukemia is acute.

Acute leukemias

There are 2 main types of acute leukemia:

Acute lymphocytic (lymphoblastic) leukemia (ALL): This type of 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): This type of leukemia, 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: In these rare leukemias, 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 these subtypes, see the section, "How is childhood leukemia classified?"

Chronic leukemias

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 treat. Chronic leukemias can also be divided into 2 types.

• Chronic myelogenous leukemia (CML): This leukemia 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): This leukemia is extremely rare in children and is not discussed further in this document. For more information on CLL, see the American Cancer Society document, Leukemia--Chronic Lymphocytic.

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, 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: 08/19/2007
Last Revised: 05/14/2009