Leukemia
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 3,800 children each
year in the US, accounting for about 30 percent of childhood cancers.
What causes leukemia in children?
The majority of
childhood leukemias are acquired genetic diseases. This means that gene
mutations and chromosome abnormalities in cells occur sporadically (by chance).
The immune system plays an important role in protecting the body from
diseases, and possibly cancer. An alteration or defect in the immune system may
increase the risk for developing leukemia. Factors such as exposure to certain
viruses, environmental factors, chemical exposures, and various infections have
been associated with damage to the immune system.
With the exception of specific genetic syndromes, little is known about the
causes of childhood leukemia.
The different types of leukemia
There are three main
types of leukemia, including the following:
- Acute lymphocytic leukemia (ALL). ALL, also called lymphoblastic or
lymphoid, accounts for about 75 to 80 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). AML, also called granulocytic,
myelocytic, myeloblastic, or myeloid, accounts for about 20 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). CML is 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 leukemia1.asp#9 breaks off and attaches itself to chromosome
leukemia1.asp#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 can also be present.
The difference between lymphocytic and myelogenous is the stage of
development of 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 (ALL). 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.