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Leukemia is a type of cancer that starts in the soft, inner part of the
bones (bone marrow) and often moves quickly into the blood. It can then
spread to other parts of the body such as the lymph nodes, the spleen,
liver, central nervous system and other organs. In contrast, other
types of cancer can start in these organs and then spread to the bone
marrow (or elsewhere). Those cancers are not leukemia. Both children
and adults can get leukemia.
The term "acute" means that the cancer develops quickly, and
if not treated, could be fatal in a few months. "Lymphocytic" (limf-o-sit-ik) means that
the cancer starts from cells called lymphocytes (limf-o-sites). Acute
lymphocytic leukemia (ALL) is a type of cancer that starts from these
white blood cells in the bone marrow. ALL is sometimes called acute
lymphoblastic leukemia.
Leukemia is a complex disease with many different types and
sub-types. The kind of treatment given and the outlook for the person
with leukemia vary greatly according to the exact type of leukemia and
other factors.
Normal Bone Marrow, Blood, and
Lymphoid Tissue
In order to understand the different types of leukemia, it
helps to have some basic knowledge of the blood and lymph systems.
Bone Marrow
Bone marrow is the soft, spongy, inner part of bones. All of
the different types of blood cells are made in the bone marrow. Bone
marrow is made up of blood stem cells, blood-forming cells, fat cells,
and tissues that aid the growth of blood cells.
The blood-forming cells come from blood stem cells. They only
make new blood-forming cells and not other kinds of cells. (They are
different from embryonic stem cells which are formed from a developing
fetus and can grow to become other kinds of cells in the body.) The
blood-forming cells can develop into 1 of the 3 main types of blood
cell: red blood cells, white blood cells, or platelets.
Red Blood Cells
Red blood cells carry oxygen from the lungs to all other
tissues of the body. They also carry away carbon dioxide, a waste
product of cell activity. A shortage of red blood cells causes
weakness, shortness of breath, and tiredness.
Platelets
Platelets are actually pieces that break off from certain bone
marrow cells.. Platelets help stop bleeding by plugging up holes in
blood vessels caused by cuts or bruises. A shortage of platelets can
cause a person to bleed or bruise easily.
White Blood Cells
White blood cells help defend the body against germs such as
viruses and bacteria. There are quite a few types of white blood cells.
Each has a special role to play in protecting the body against
infection. The 3 main types of white blood cells are granulocytes,
monocytes, and lymphocytes.
The immune system is made up mainly of lymphoid tissue (also
known as lymphatic tissue). The main cell type that forms lymphoid
tissue is the lymphocyte,
a kind of white blood cell. The two main types of lymphocytes are
called B-cells and T-cells. Normal T-cells and B-cells do different
jobs within the immune system.
Any of the blood-forming cells can turn into a leukemic cell.
Once that happens, the cell can reproduce to form many new cancer
cells. These cells can overwhelm the bone marrow, spill out into the
bloodstream, and spread to other organs. Acute lymphocytic leukemia
starts from early forms of the lymphocytes.
Types of Leukemia
Not all leukemias are the same. Leukemias are divided into 4
main types. Knowing the exact type of leukemia can help doctors better
predict each patient's outlook (prognosis) and select the best
treatment.
The major types of leukemia are based on these factors:
- acute or chronic
- lymphocytic or myeloid
Acute Leukemia Versus Chronic Leukemia
The first factor to take into account to classify a patient's
leukemia is whether most of the abnormal cells look like normal mature
white blood cells or whether they look more like stem cells (have not
matured.)
Acute leukemia:
In acute leukemia, the bone marrow cells don’t mature properly. These
immature cells build up and crowd out normal cells. Without treatment,
most patients with acute leukemia would live only a few months. Some
types of acute leukemia respond well to treatment and many patients are
cured. People with other types often do not do as well.
Chronic
leukemia: In chronic leukemia the cells look mature, but
they are not really normal and they can’t fight infection they way they
should. They also live too long, so that they build up and crowd out
normal bone marrow cells. Chronic leukemias tend to progress over a
longer period of time, and most patients can live for many years.
However, chronic leukemias are generally harder to cure than acute
leukemias.
Myeloid Leukemia Versus
Lymphocytic Leukemia
The second factor to take into account is the type of bone
marrow cells that are involved.
Myeloid leukemia:
Myeloid leukemia mainly develops from white blood cells such as
granulocytes or monocytes, but can also form from the cells that mature
into platelets or red blood cells.
Lymphocytic
leukemia: Lymphocytic leukemias develop from lymphocytes
(a type of white blood cell) in the bone marrow.
Most cases of leukemia can be sorted into 1 of the 4 main
types shown in the table below.
| Acute lymphocytic leukemia
(ALL) |
Acute myeloid leukemia (AML) |
| Chronic lymphocytic
leukemia (CLL) |
Chronic myeloid leukemia
(CML) |
Although ALL is the most common of the 4 major types of
leukemia among children, it is the least common type among adults.
The rest of this document contains information on ALL of
adults only. Chronic leukemias of adults and acute myelocytic leukemia
(AML) of adults are discussed in other American Cancer Society
documents. For information about ALL in children, please see the
separate document, Childhood
Leukemias.
Revised: 10/04/2007
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