Acute lymphocytic leukemia (ALL) is a type of leukemia, or cancer of the body’s blood-forming cells. It affects the body’s blood-making system, including bone marrow and the lymphatic system.

Also known as acute lymphoblastic leukemia, ALL develops from lymphocytes, a type of white blood cell, in the bone marrow. Bone marrow is the soft, inner component of bones. It is composed of blood-forming cells, fat cells, and tissues that support the growth of blood cells. All blood cells are produced in the bone marrow from stem cells and include:

• White blood cells (cells that fight infection).
  
  
• Red blood cells (cells that carry oxygen to tissues all over the body).
  
  
• Platelets (cells that help develop blood clots and control bleeding).

ALL initially develops in the bone marrow but typically spreads quickly into the blood. Eventually it spreads further to other organs including the liver, brain, spinal cord and testicles.

Lymphocytes are one of the three main types of white blood cells. They are the chief type of cell found in the lymphoid tissue, a major part of the immune system. Lymphoid tissue is found in the bone marrow, as well as the lymph nodes, thymus, spleen, tonsils and adenoids. The two main types of lymphocytes are:

• B lymphocytes (B-cells). B-cells defend the body from foreign substances (e.g., germs, viruses) by changing into plasma cells, which produce antibodies.   The antibodies attach themselves to foreign substances or threats which mark them for destruction.
   
• T lymphocytes (T-cells). T-cells recognize the infected cells and destroy them directly.

ALL can develop from early forms of either B-cells or T-cells. In patients with leukemia, the bone marrow manufactures a large amount of abnormal white blood cells. Since they are defective, these cells are unable to defend the body from disease. As the disease progresses they build up, crowding out normal white blood cells, red blood cells and platelets. As a result, the production of normal white blood cells is affected, and the body’s ability to fight infection is impaired. In addition, the decrease in production of red blood cells and platelets results in anemia and bleeding disorders. Eventually, the cancer cells spill into the bloodstream and spread to other areas of the body.

There are many different types and subtypes of leukemia. In general, the disease is first classified according to how quickly it progresses. The second classification is based on the type of white blood cell from which the leukemia developed. The abnormal cells may be considered mature or immature depending on the cell type.

Leukemia can develop in two families of blood cells, lymphocytes or myeloid cells at two different progressions, acute and chronic. ALL is classified as a lymphocytic leukemia because it develops from bone marrow lymphocytes. It is classified as acute because most of the abnormal cells are immature. These immature cells are also known as blasts. Acute forms of leukemia develop quickly and, without proper treatment, are often fatal within a few months.

According to the American Cancer Society (ACS), approximately 44,000 new cases of leukemia will be diagnosed in the United States in 2007. Of these new cases, about 5,200 will be ALL. Although the disease mainly occurs in children, about one-fifth of cases occur in adults. The ACS estimates that about 1,400 people will die from ALL in 2007. The mortality rate is higher for adults, who account for two-thirds of deaths from ALL each year in the United States. In addition, the disease is slightly more common in males than in females and much more common in whites than African Americans.