|
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
In order to understand the different types of leukemia, it is helpful
to have some basic knowledge about the blood and lymph systems.
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
|