Leukemia classification

# Classification
Leukemia is clinically and pathologically subdivided into several large groups.  The first division is between its acute and chronic forms:
- Acute leukemia is characterized by the rapid increase of immature blood cells. This crowding makes the bone marrow unable to produce healthy blood cells. Acute forms of leukemia can occur in children and young adults. In fact, it is a more common cause of death for children in the United States than any other type of malignant disease. Immediate treatment is required for acute leukemias due to the rapid progression and accumulation of the malignant cells, which then spill over into the bloodstream and spread to other organs of the body. Central nervous system (CNS) involvement is uncommon, but the disease can occasionally cause cranial nerve palsies.
- Chronic leukemia is distinguished by the excessive build up of relatively mature, abnormal blood cells. Typically taking months or years to progress, the cells are produced at a much higher rate than normal cells, resulting in many abnormal white blood cells in the blood. Chronic leukemia mostly occurs in older people but can theoretically occur in any age group. While acute leukemia must be treated immediately, chronic forms are sometimes monitored for a some time before treatment to ensure maximum effectiveness of therapy.
Additionally, the diseases are subdivided according to which kind of blood cell is affected.  This split divides leukemias into lymphoblastic or lymphocytic leukemias and myeloid or myelogenous leukemias:
- In lymphoblastic or lymphocytic leukemias, the cancerous change takes place in a type of marrow cell that normally goes on to form lymphocytes.
- In myeloid or myelogenous leukemias, the cancerous change takes place in a type of marrow cell that normally goes on to form red cells, platelets, and certain types of white cells.
Combining these two classifications provides a total of four main categories:
Within these main categories, there are typically several subcategories. Finally, hairy cell leukemia is usually considered to be outside of this classification scheme.
- Acute lymphoblastic leukemia (ALL) is the most common type of leukemia in young children. This disease also affects adults, especially those age 65 and older.  Standard treatments involve either chemotherapy or radiation.  The survival rates vary by age:  85% in children and 50% in adults.
- Chronic lymphocytic leukemia (CLL) most often affects adults over the age of 55. It sometimes occurs in younger adults, but it almost never affects children. Two-thirds of the affected individuals are men. The five-year survival rate is 75%. It is incurable, but there are many effective treatments.
- Acute myelogenous leukemia (AML) occurs more commonly in adults than in children, and more commonly in men than women.  AML is treated with chemotherapy.  The five-year survival rate is 40%.
- Chronic myelogenous leukemia (CML) occurs mainly in adults.  A very small number of children also develop this disease.  Treatment is with imatinib (Gleevec) or other drugs.  The five-year survival rate is 90%.
- Hairy cell leukemia (HCL) is sometimes considered a subset of CLL but does not fit neatly into this pattern.  About 80% of affected individuals are adult men.  There are no reported cases in young children.  HCL is incurable, but easily treatable.  Survival is 96% to 100% at ten years.