In Australia about 2,300 adults and 200 children are diagnosed each year with leukaemia, a cancer of the white blood cells (WBCs). While exposure to radiation, benzene, and some anticancer drugs have been shown to increase the risk of developing leukaemia, and a few cases are associated with genetic disorders or rare viral infections, the cause of most leukaemias is not known.

What is it?

Leukaemia is a bone marrow disorder that arises when one abnormal white blood cell begins to continuously replicate itself. These cells do not function normally, they do not fight infection as they should, and they do not die at the same rate as other WBCs. As they accumulate, they inhibit the production of the other normal blood cells in the marrow, leading to anaemia, bleeding, and recurrent infections. Over time, the leukaemic cells spread through the bloodstream where they continue to divide, sometimes forming tumours and damaging organs such as the kidney and liver. Since the spleen is responsible for filtering the blood and destroying old cells, it may become enlarged and swollen with the abnormal cells, as can the liver and lymph nodes. If the cells reach the central nervous system and build up in the cerebrospinal fluid that supports the brain and spinal column, they can cause headaches and seizures.

The bone marrow, located in the soft center of the body's larger bones, produces precursors (immature versions) of red blood cells, platelets, and five different kinds of white blood cells. The most immature of these is called a blast. Most of these blood cells mature in the bone marrow before being released into the bloodstream. The WBCs created are grouped into two main categories: lymphocytes and myelocytes (also called granulocytes for the granules found inside the cell). Myelocytes (which include neutrophils, basophils, eosinophils, and monocytes) circulate in the blood, killing and digesting bacteria. Lymphocytes, which are found in both the blood stream and the lymphatic system, coordinate the immune response and produce antibodies. Leukaemia arises from one of these white blood cells. It is categorised both by the type of WBC involved and by how quickly it progresses. Although expanded classifications of the disease exist, the main types of leukaemia can be grouped as:

• Acute lymphocytic leukaemia (ALL). This is a rapidly developing disease that is characterised by large numbers of immature lymphocytes. It is the most common type of leukaemia found in children, although it affects both children and adults (usually adults age 65 and older).

• Chronic lymphocytic leukaemia (CLL). This disease progresses more slowly and is characterised by a mixture of mature and immature lymphocytes. It tends to be found in those over the age of 55 or 60.

• Acute myeloid (myelocytic) leukaemia (AML). Affecting people of all ages, this disease is characterised by production of large numbers of immature granulocyte myeloid cells (immature neutrophils -- the most common, monocytes, basophils, eosinophils, platelets or red blood cells) that replace other normal cells in the marrow.

• Chronic myelogenous (myelocytic or myeloid) leukaemia (CML). Chronic myelogenous leukaemia is rare in children. It is an acquired condition that begins in an immature stem cell in the bone marrow when pieces from two chromosomes (9 and 22) break off and switch places (translocation). This results in an altered, fused gene (bcr/abl) on chromosome 22 that produces a protein called tyrosine kinase that affects cell growth regulation. This leads to an overproduction of granulocytic white blood cells, many with the bcr/abl translocation, and the presence of both mature and immature cells in the bloodstream.