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Leukemia is a malignancy (cancer) of blood cells. In leukemia, abnormal blood cells are produced in the bone marrow. Usually, leukemia involves the production of abnormal white blood cells the cells responsible for fighting infection. However, the abnormal cells in leukemia do not function in the same way as normal white blood cells. The leukemia cells continue to grow and divide, eventually crowding out the normal blood cells. The end result is that it becomes difficult for the body to fight infections, control bleeding, and transport oxygen.

Causes of Leukemia

Scientists don’t understand the exact causes of leukemia. It seems to develop from a combination of genetic and environmental factors.

For most people with leukemia, there’s no way to identify what causes it. In some cases, though, specific risk factors can be identified:

  • previous chemotherapy or radiation therapy
  • exposure to high doses of radiation or to benzene (found in unleaded gasoline, tobacco smoke, chemical production facilities)
  • family history
  • genetic abnormality, such as an abnormality on chromosome 22 (also known as the Philadelphia chromosome)
  • genetic disorders, such as Down syndrome and Fanconi anemia

All forms of cancer that can spread within the body (malignant), including leukemia, are thought to be due to genetic abnormalities (mutations). In leukemia, the damage occurs in the bone marrow stem cells. These special cells help to manufacture all the other cells in the blood. With this condition the production of these cells is out of control.

How Leukemia forms

In general, leukemia is thought to occur when some blood cells acquire mutations in their DNA  the instructions inside each cell that guide its action. There may be other changes in the cells that have yet to be fully understood could contribute to leukemia.

Certain abnormalities cause the cell to grow and divide more rapidly and to continue living when normal cells would die. Over time, these abnormal cells can crowd out healthy blood cells in the bone marrow, leading to fewer healthy white blood cells, red blood cells and platelets, causing the signs and symptoms of leukemia.

How Leukemia is classified

Doctors classify leukemia based on its speed of progression and the type of cells involved.

The first type of classification is by how fast the leukemia progresses:

  • Acute leukemia. In acute leukemia, the abnormal blood cells are immature blood cells (blasts). They can’t carry out their normal functions, and they multiply rapidly, so the disease worsens quickly. Acute leukemia requires aggressive, timely treatment.
  • Chronic leukemia. There are many types of chronic leukemias. Some produce too many cells and some cause too few cells to be produced. Chronic leukemia involves more mature blood cells. These blood cells replicate or accumulate more slowly and can function normally for a period of time. Some forms of chronic leukemia initially produce no early symptoms and can go unnoticed or undiagnosed for years.

The second type of classification is by type of white blood cell affected:

  • Lymphocytic leukemia. This type of leukemia affects the lymphoid cells (lymphocytes), which form lymphoid or lymphatic tissue. Lymphatic tissue makes up your immune system.
  • Myelogenous (my-uh-LOHJ-uh-nus) leukemia. This type of leukemia affects the myeloid cells. Myeloid cells give rise to red blood cells, white blood cells and platelet-producing cells.