ALL is cancer of the lymphocytes (see Figure 3–1). A normal lymphoid stem cell matures into one of three types of white blood cells (WBCs):

• B lymphocytes that make antibodies to fight infection
• T lymphocytes that help the body fight infection and disease
• Natural killer cells that kill cancer cells or viruses

In 85% of children with ALL, B lymphocytes become cancerous. In the other 15% of children with ALL, T lymphocytes become cancerous. The cancer cells multiply rapidly and have no ability to develop into mature WBCs. After accumulating in the bone marrow, cancerous lymphocytes (called leukemic blasts) spill over into the blood and enlarge the liver, spleen, and lymph nodes. If left unchecked, the cancerous cells can invade the central nervous system (CNS)—which includes the brain and spinal cord—and other organs, such as testes in boys.

Figure 3–1: ALL is a disease of B or T lymphocytes

When leukemic blasts begin to multiply and pack the bone marrow, fewer normal red blood cells (RBCs), platelets, and WBCs are produced. As the number of normal blood cells decreases, symptoms appear. Low RBC counts cause fatigue and pale skin. Low platelet counts may result in bruising and bleeding problems. If healthy WBCs are crowded out by blasts, the child will have little or no defense against infections. To read more about blasts, blood, and genetics, see Chapter 2, Overview of Childhood Leukemia.