Leukemia is cancer of the blood cells. It begins in the bone marrow when abnormal cells multiply out of control to such an extent that normal blood cells are unable to develop. Leukemia starts in the bone marrow—the soft, spongy material inside bones where blood cells develop from stem cells. Leukemia most often affects the white blood cells, which protect the body against infection. The disease begins when normal development of stem cells into white blood cells goes wrong, creating abnormal white blood cells. Eventually the abnormal white cells overtake other types of blood cells, including red blood cells, which carry oxygen to the body tissues; and platelets, which enable the blood to clot. Leukemia therefore interferes with the blood’s ability to clot and carry oxygen.
The abnormal white cells may also spread to other parts of the body, including the lymph nodes, skin, spleen, liver, brain, and spinal cord.
Leukemia is not one disease but a complex group of diseases. There are four main types of leukemia: acute lymphocytic leukemia (ALL), acute myeloid (or myelogenous) leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML).
- Acute lymphocytic leukemia (ALL). ALL develops from white blood cells called lymphocytes. Immature cells that normally grow into lymphoctyes become cancerous and build up rapidly in the bone marrow, replacing normal stem cells. The blood transports these abnormal cells to the lymph nodes, spleen, liver, kidneys, and other organs. These cells can cause meningitis, kidney and liver failure, anemia, and other conditions.
- Acute myeloid leukemia (AML). AML develops from cells in the bone marrow that normally grow into specific white blood cells called granulocytes. Instead of developing normally, the cells become cancerous and multiply rapidly in the bone marrow. These abnormal cells may grow under the skin, forming small tumors, and result in anemia, meningitis, and damage to the liver, kidneys, and other organs.
Chronic lymphocytic leukemia (CLL). In CLL, mature lymphocytes become cancerous and begin to multiply, starting in the lymph nodes. The abnormal cells spread to the bone marrow, and then the spleen and liver, which become enlarged. The shortage of normal blood cells and an abnormal reaction by the immune system causes anemia. The immune system attacks normal tissues, destroys normal red blood cells and platelets, and causes joint inflammation and thyroid gland inflammation (thyroiditis).
Subtypes of CLL are named according to the type of lymphocyte affected. The most common type of CLL is B-cell leukemia. Less common are T-cell leukemia, Sézary syndrome, and hairy cell leukemia, in which the abnormal cells have hair-like projections when viewed under a microscope.
Chronic myeloid leukemia (CML). In CML, bone marrow cells become cancerous and produce large amounts of abnormal granulocytes that may be either immature or mature. These abnormal cells crowd out the normal bone marrow cells. As larger numbers of immature granulocytes enter the bloodstream and bone marrow, the number of red blood cells and platelets decreases.
Most often, CML progresses to a stage called blast crisis, in which the bone marrow produces only immature granulocytes. This is a sign that the disease has worsened.
The cause of leukemia is unknown. At present, no one knows what causes leukemia. However, scientists have discovered some risk factors that increase a person’s chance of getting the disease. These may include excess exposure to radiation, or prior treatment for a malignancy, or having had a previous blood disorder. However, most people who develop leukemia do not have any of these risk factors.
CLL is rare in China and Japan, and remains rare among Japanese people who move to the U.S. This suggests that genetic factors play some role in the development of CLL.
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