Leukemia is cancer of the body’s blood-forming cells. It affects the body’s blood-making system, including bone marrow and the lymphatic system. Although it is often thought of as a children’s disease, it affects about 10 times as many adults as children. The disease also occurs more often in males than in females.

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 mother cell called the stem cell. Blood cells produced in the bone marrow include:

• White blood cells (cells that fight infection)
  
  
• Red blood cells (cells that carry oxygen to, and carbon dioxide from, the body's cells)
  
  
• Platelets (cells that help develop blood clots and control bleeding)

Leukemia develops from bone marrow cells. In patients with leukemia, the bone marrow manufactures a large number of abnormal white blood cells. These cells are unable to defend the body from disease because they are defective. As the disease progresses the abnormal cells build up, crowding out normal white blood cells, red blood cells and platelets. 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.  

The disorder develops in the bone marrow but typically spreads quickly into the blood. Eventually it can spread into the lymph nodes, the liver, the covering of the spinal cord and brain, spinal fluid, the spleen and other organs.

The lymph nodes are small bean-shaped glands located in various parts of the body. The spleen is an organ that is located near the stomach. The spleen and lymph nodes are components of the lymphatic system, a network of organs, nodes and vessels. The lymphatic system removes lymph (fluid containing white blood cells, plasma and other substances) from tissues and returns it to the bloodstream. It is a major component of the immune system and defends the body from infections and disease.

In 2007, the American Cancer Society (ACS) estimates there will be more than 44,000 new cases of leukemia diagnosed in the United States. Although leukemia is often considered a childhood disease, it is actually 10 times more common in adults. In children, acute lymphocytic leukemia (ALL) accounts for the majority of the cases. In adults, acute myeloid leukemia (AML) and chronic lymphocytic leukemia (CLL) are the most common types of the disease. Certain forms of leukemia, such as CLL have decreased over the past 15 years while others (AML) have increased. Overall, death rates from leukemia in both males and females have decreased by about 0.6 percent since 1991.