Leukaemias are grouped by the type of blood cell affected, and how quickly the disease develops and gets worse. ^[1]

The body naturally produces blood stem cells (this happens in the bone marrow). These blood stem cells are young cells that can develop into white blood cells, red blood cells or platelets ^[2]. The 2 broad groups of these blood stem cells are: 

• lymphoid cells – which develop into of a type of white blood cell called lymphocytes 
• myeloid cells – which develop into red blood cells, platelets and white blood cells other than lymphocytes. 

Leukaemia is grouped based on whether it starts in the lymphoid cells or myeloid cells. 

Fastgrowing leukaemia is referred to as ‘acute’, and slowergrowing leukaemia is referred to as ‘chronic’. 

The most common forms of leukaemia are: 

• acute lymphoblastic leukaemia (ALL), also called acute lymphocytic leukaemia, starts in lymphoid cells, causing them to develop into abnormal lymphocytes. ALL develops quickly, and is more common in children than in adults. The main subtypes of this cancer are Bcell ALL, Tcell ALL and mixed lineage ALL ^[3]. 
• acute myeloid leukaemia (AML), also called acute myelogenous leukaemia, starts in myeloid cells, causing them to develop into abnormal white blood cells, red blood cells or platelets. Most of the time it occurs in the myeloid cells that form white blood cells other than lymphocytes. This type of leukaemia develops quickly and is most common in older people. There are different subtypes of this cancer, including a subtype known as acute promyelocytic leukaemia (APML) that is treated differently ^[4]. 
• chronic lymphoblastic leukaemia (CLL), also called chronic lymphocytic leukaemia, starts in lymphoid cells, causing them to develop into abnormal lymphocytes. Unlike ALL, the cancer cells build up slowly, and there may be no symptoms for several years. However, there is a second type of CLL that grows faster and is more serious. CLL accounts for about two-thirds of leukaemias and mainly affects older adults. 
• chronic myeloid leukaemia (CML), also known as chronic myelogenous leukaemia, starts in myeloid cells, causing them to develop into abnormal while blood cells, red blood cells or platelets. It is slow growing, but can change into a fastgrowing leukaemia. CML makes up about 10% of leukaemias and mainly occurs in adults. 

Less common types of leukaemia include: 

• hairy cell leukaemia starts in lymphoid cells, causing them to develop into abnormal lymphocytes ^[5]. The name of the disease comes from the hairy appearance of the leukaemia cells under a microscope. This type of leukaemia gets worse slowly or not at all. It may not require treatment for many years. 
• chronic myelomonocytic leukaemia (CMML) starts in myeloid cells, causing a large number of monocytes (a type of white blood cell) in the blood. About 15–30% of people with CMML go on to develop AML.