Leukemias, cancers of the blood, are the most common malignancy in children. ALL comprises 75 percent of leukemia cases in children (another form, acute myeloid leukemia, accounts for the remaining cases). The five-year survival rate for ALL is nearly 80 percent.
ALL is characterized by abnormal, immature lymphocytes, a type of white blood cell found in the blood and bone marrow.
Marrow is a spongy tissue that is found in the center of bones. It is the source of stem cells, which evolve into the three kinds of blood cells in the body - red blood cells, white blood cells and platelets. Red blood cells carry oxygen to the body's tissues. White blood cells, of which there are three types - neutrophils, lymphocytes and monocytes - fight infection. Platelets enable the blood to clot.
The lymph system is comprised of a network of vessels that branch out into the body. These vessels carry lymph, a colorless fluid containing lymphocytes. Lymph nodes, small bean-shaped organs, are part of this network and are found in clusters in the abdomen, pelvis, underarm and neck. The spleen, which makes lymphocytes and filters blood; the thymus, an organ under the breastbone; and the tonsils in the throat are also part of the lymph system.
In people with ALL, the lymphocytes do not mature and are overproduced. These diseased lymphocytes collect in the blood and bone marrow, crowding out other blood cells. Lymphocytes also collect in lymph tissues, making them swell. The cancerous lymphocytes can also invade other parts of the body, including internal organs, the brain and spinal cord, and testicles.
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