There are two classification systems that break acute lymphoblastic leukaemia down into various sub-types. These are the WHO (World Health Organisation) classification system and the FAB (French-American-British) classification system.
The WHO system is based on the type of lymphocyte (either B- or T-lymphocyte) that has become cancerous. This system is important as it is useful for planning treatment and predicting response. There are three different sub-types:
- early (precursor) B-lymphoblastic leukaemia (most adults with ALL have this type)
- mature B-lymphoblastic leukaemia (sometimes called Burkitt-type ALL because it is similar to Burkitt lymphoma)
- early (precursor) T-lymphoblastic leukaemia.
The FAB classification system is less helpful for planning treatment or predicting outcome. It looks at the appearance of the leukaemia cells under a microscope (morphology). It is divided into three different sub-types:
- L1 – the leukaemic cells (lymphoblasts) are quite mature and similar to normal lymphocytes
- L2 – the lymphoblasts are more immature
- L3 – the lymphoblasts are very immature and do not function well.
Philadelphia chromosome
Some people with ALL have a particular genetic abnormality known as a Philadelphia chromosome which can be detected by cytogenetic tests. The Philadelphia chromosome develops when part of chromosome 9 (the ABL gene) wrongly attaches to chromosome 22 (the BCR gene) during cell division. This creates a new gene, known as BCR-ABL, which produces a specific new protein. The protein causes the production of an enzyme called tyrosine kinase, which makes the bone marrow produce abnormal blood cells.
The Philadelphia chromosome is not inherited and cannot be passed on to your children.