Acute lymphocytic leukemia (ALL) is a type of leukemia, or cancer of the body’s blood-forming cells. Leukemia, which affects the body’s blood making system, including bone marrow and the lymphatic system, is the most common cancer in children. It accounts for approximately one-third of all cancers in children under the age of 15 and one-quarter of cancers occurring before the age of 20.

ALL is the most common form of leukemia in children. The American Cancer Society (ACS) predicts that about 3,800 children under age 19 will develop leukemia in 2007. Of these children, approximately 73 percent, or 2,800 children, will be diagnosed with ALL. Although the disease is more common in children than adults, the mortality rate is lower in children than in adults with ALL.

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 forms of blood cells are produced in the bone marrow from a cell called the stem cell. Blood cells produced in the bone marrow include:

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

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

Lymphocytes are one of the three main types of white blood cell. 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 bacteria and other foreign substances (antigens) by changing into plasma cells. The antibodies then mark the antigens for destruction.
   
• T lymphocytes (T-cells). T-cells destroy antigens and infected cells directly.

ALL can develop from early forms of either B-cells or T-cells. In children 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 can begin to build up, crowding out normal white blood cells, red blood cells and platelets. As a result, production of normal white blood cells is affected, and the body’s ability to fight off infection is impaired. In addition, decreased production of red blood cells and platelets results in anemia and bleeding disorders. Eventually, the cancer cells spill from the bone marrow 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 (acute vs. chronic). Acute leukemia develops from early cells known as blast cells. It progresses quickly and can be fatal within months if not treated. Leukemias that develop in children are usually acute. Chronic leukemia develops from more mature cells and progresses more slowly.
  
  
• The type of blood cells from which it developed (myeloid cells vs. lymphocytes).

ALL is classified as acute because it develops from blast cells and lymphocytic because it develops from bone marrow lymphocytes

ALL most commonly occurs in children, usually developing between ages 2 and 3. It develops more often in boys than girls, and is slightly more common in white children than African American or Asian American children. The five-year survival rate for ALL in children has increased over the years and is now 87 percent. The five-year survival rate refers to the percentage of children who live at least five years after their cancer is diagnosed. Many children live much longer than five years and the prognosis continues to improve due to advances in treatment.