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Cancer starts when cells in the body begin to grow out of control. There are many kinds of cancer. Cells in nearly any part of the body can become cancer. To learn more about cancer and how it starts and spreads, see What Is Cancer?

Leukemias are cancers that start in cells that would normally develop into different types of blood cells. Most often, leukemia starts in early forms of white blood cells, but some leukemias start in other blood cell types.

There are several types of leukemia, which are divided based mainly on whether the leukemia is acute (fast growing) or chronic (slower growing), and whether it starts in myeloid cells or lymphoid cells. Knowing the specific type of leukemia helps doctors better predict each person’s prognosis (outlook) and select the best treatment.

Acute lymphocytic leukemia (ALL) is also called acute lymphoblastic leukemia. “Acute” means that the leukemia can progress quickly, and if not treated, would probably be fatal within a few months. "Lymphocytic" means it develops from early (immature) forms of lymphocytes, a type of white blood cell.

ALL starts in the bone marrow (the soft inner part of certain bones, where new blood cells are made). Most often, the leukemia cells invade the blood fairly quickly. They can also sometimes spread to other parts of the body, including the lymph nodes, liver, spleen, central nervous system (brain and spinal cord), and testicles (in males). Some cancers can also start in these organs and then spread to the bone marrow, but these cancers are not leukemia.

Other types of cancer that start in lymphocytes are known as lymphomas (either non-Hodgkin lymphoma or Hodgkin lymphoma). While leukemias like ALL mainly affect the bone marrow and the blood, lymphomas mainly affect the lymph nodes or other organs (but may also involve the bone marrow). Sometimes it can be hard to tell if a cancer of lymphocytes is a leukemia or a lymphoma. Usually, if at least 20% of the bone marrow is made up of cancerous lymphocytes (called lymphoblasts, or just blasts), the disease is considered leukemia.

Normal bone marrow, blood, and lymph tissue

To understand leukemia, it helps to know about the blood and lymph systems.

Bone marrow

Bone marrow is the soft inner part of certain bones. It is made up of blood-forming cells, fat cells, and supporting tissues. A small fraction of the blood-forming cells are blood stem cells.

Inside the bone marrow, blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into 1 of the 3 main types of blood cell components:

• Red blood cells
• Platelets
• White blood cells

Red blood cells

Red blood cells (RBCs) carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed.

Platelets

Platelets are actually cell fragments made by a type of bone marrow cell called a megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises.

White blood cells

White blood cells (WBCs) help the body fight infections. The main types of WBCs include lymphocytes, granulocytes, and monocytes.

Lymphocytes are the main cells that make up lymph tissue, a major part of the immune system. Lymph tissue is found in lymph nodes, the thymus, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. There are 2 main types of lymphocytes:

• B lymphocytes (B cells): B cells help protect the body by making proteins called antibodies. The antibodies attach to germs (bacteria, viruses, and fungi) in the body, which helps the immune system destroy them.
• T lymphocytes (T cells): There are several types of T cells, each with a special job. Some T cells can destroy germs directly, while others play a role in either boosting or slowing the activity of other immune system cells.

ALL develops from early forms of lymphocytes. It can start in either early B cells or T cells at different stages of maturity. This is discussed in Acute Lymphocytic Leukemia (ALL) Subtypes and Prognostic Factors.

Granulocytes are WBCs that 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.

Monocytes also help protect the body against bacteria. 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.