Acute Lymphoblastic Leukemia (ALL) is a form of leukemia, or cancer of the white blood cells.  Immature white blood cells are over produced in the bone marrow.  Lymphocytes are a type of white blood cell that the body uses to fight infections. In ALL, the bone marrow makes lots of unformed cells called blasts that normally would develop into lymphocytes. However, the blasts are abnormal. They do not develop and cannot fight infections. The number of abnormal cells (or leukemia cells) grows quickly. They crowd out the normal red blood cells, white blood cells and platelets the body needs.  ALL cause damage and death by crowding out normal cells in the bone marrow, and by spreading to other organs.  ALL’s peak incidence in children is at four to five years of age.  There are two types of ALL, type B (B cell) and type T (T cell).  B cell is the more common and has the higher success rate of a cure.  The overall cure rate for ALL in children is 85% at this time.

 http://leukemia.org/hm_lls