Leukemia is cancer of the blood and develops in the bone marrow. The bone marrow is the soft, spongy center of the long bones that produces the three major blood cells: white blood cells to fight infection; red blood cells that carry oxygen; and platelets that help with blood clotting and stop bleeding. When a child has leukemia, the bone marrow, for an unknown reason, begins to make white blood cells that do not mature correctly, but continue to reproduce themselves. Normal, healthy cells only reproduce when there is enough space for them to fit. The body can regulate the production of cells by sending signals when to stop. With leukemia, these cells do not respond to the signals to stop and reproduce, regardless of space available.
These abnormal cells reproduce very quickly and do not function as healthy white blood cells to help fight infection. When the immature white blood cells, called blasts, begin to crowd out other healthy cells in the bone marrow, the child experiences the symptoms of leukemia (i.e., infections, anemia, bleeding).
Who is affected by leukemia?
Leukemia is the most common form of cancer in childhood. It affects approximately 3,250 children each year in the United States, accounting for about 30 percent of childhood cancers.
Leukemia can occur at any age, although it is most commonly seen in children between 2 and 6 years of age. The disease occurs slightly more frequently in males than in females, and is more commonly seen in Caucasian children than in African American children, or children of other races.
What are the different types of leukemia?
There are three main types of leukemia, including the following:
- Acute lymphocytic leukemia (ALL) – also called lymphoblastic or lymphoid, accounts for about 75 percent of the childhood leukemias. In this form of the disease, the lymphocyte cell line is affected. The lymphocytes normally fight infection. With acute lymphocytic leukemia, the bone marrow makes too many of these lymphocytes and they do not mature correctly. The lymphocytes overproduce, thus, crowding out other blood cells. Immature blood cells (blasts) do not work properly to fight infection. Acute leukemia can occur over a short period of days to weeks. Chromosome abnormalities (extra chromosomes and structural changes in the chromosome material) are present in the majority of ALL patients.
- Acute myelogenous leukemia (AML) – also called granulocytic, myelocytic, myeloblastic or myeloid, accounts for about 19 percent of the childhood leukemias. Acute myelogenous leukemia is a cancer of the blood in which too many granulocytes, a type of white blood cell, are produced in the marrow. The granulocytes normally fight infection. With acute myelogenous leukemia, the bone marrow makes too many of these cells and they do not mature correctly. The granulocytes overproduce, thus, crowding out other blood cells. Immature blood cells (blasts) do not work properly to fight infection. Acute leukemia can occur over a short period of days to weeks. Children with certain genetic syndromes, including Fanconi anemia, Bloom syndrome, Kostmann syndrome and Down syndrome, are at a higher risk of developing AML than other children.
- Chronic myelogenous leukemia (CML) – uncommon in children. Chronic myelogenous leukemia is cancer of the blood in which too many granulocytes, a type of white blood cell, are produced in the marrow. The granulocytes normally fight infection. With this disease, the bone marrow makes too many of these cells and they do not mature correctly. The marrow continues to produce these abnormal cells, which crowd out other healthy blood cells. Chronic myelogenous leukemia can occur over a period of months or years. A specific chromosome rearrangement is found in patients with CML. Part of chromosome 9 breaks off and attaches itself to chromosome 22, so that there is an exchange of genetic material between these two chromosomes. This rearrangement changes the position and functions of certain genes, which results in uncontrolled cell growth. Other chromosome abnormalities also can be present.
The difference between lymphocytic and myelogenous is the stage of development on what is called the pluripotent stem cell. The pluripotent stem cell is the first stage of development of all of the blood cells (white blood cells, red blood cells and platelets). This stem cell goes through stages of development until it matures into a functioning cell. The type of leukemia is determined by where the cell is in the stage of development when it becomes malignant, or cancerous.
The stem cell matures into either the lymphoid or myeloid cells. The lymphoid cells mature into either B-lymphocytes or T-lymphocytes. If the leukemia is among these cells, it is called acute lymphocytic leukemia. If the leukemia is found even further along this stage of development, it can be further classified as B-cell ALL or T-cell ALL. The more mature the cell, the more difficult it is to treat.
The myeloid cells develop into platelets, red blood cells and specialized white blood cells called neutrophils and macrophages. There are many classifications of AML. The type of leukemia is determined by the stage of development when the normal cells become leukemia cells.