Acute lymphoblastic leukemia (ALL) is a cancer of the white blood cells, which are the cells in the blood that fight infections. There are two main types of white blood cells-lymphoid cells and myeloid cells. ALL affects the lymphoid cells.

ALL Leukemia cells are abnormal cells that cannot do what normal blood cells do. The abnormal cells are immature white blood cells that cannot help the body fight infections. Therefore, patients with ALL often get infections and have frequent fevers. The cancerous cells can replicate themselves very quickly and can take over the entire bone marrow system. This prevents the patient's body from producing the other types of blood cells - red blood cells which carry oxygen and platelets which stop bleeding. This causes oxygen deprivation (anemia) and excessive bleeding.

Like all blood cells, leukemia cells travel through the bloodstream. Depending on the number of cancer cells and where these cells collect, patients with ALL may have a number of symptoms. Patients with ALL frequently have low amounts of healthy red blood cells and platelets. With low red blood cells (a condition called anemia) patients may look pale and feel weak and tired just like altitude sickness. When there are not enough platelets, patients bleed and bruise easily. The cancer cells may also collect in the brain or spinal cord, also called the central nervous system (CNS).