Leukemia or leukaemia (Greek leukos λευκός, "white"; aima αίμα, "blood") is a cancer of the blood or bone marrow and is characterized by an abnormal proliferation (production by multiplication) of blood cells, usually white blood cells (leukocytes). Leukemia is a broad term covering a spectrum of diseases. In turn, it is part of the even broader group of diseases called hematological neoplasms.

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 US than any other type of malignant disease). Immediate treatment is required in 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, although the disease can occasionally cause cranial nerve palsies.
• Chronic leukemia is distinguished by the excessive build up of relatively mature, but still 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. Whereas acute leukemia must be treated immediately, chronic forms are sometimes monitored for 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 took place in a type of marrow cell that normally goes on to form lymphocytes.
• In myeloid or myelogenous leukemias, the cancerous change took place in a type of marrow cell that normally goes on to form red cells, some types of white cells, and platelets.

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.