Leukaemia occurs when abnormal cells within the bone marrow grow in an uncontrolled way, affecting the production of white blood cells. 

White blood cells are important to help the body fight infections.

In leukaemia the bone marrow produces abnormal white blood cells. These cells can build up in the blood and stop the normal blood cells from working properly. The abnormal cells can collect in different parts of the body.
White blood cells develop from stem cells that are produced by the bone marrow. There are two types of stem cells that can form white blood cells – lymphoid stem cells and myeloid stem cells. Leukaemia can affect either of these types of cells.

Different types of leukaemia are named after the cells that are affected, and how quickly the abnormal cells grow:

• acute lymphoblastic leukaemia (ALL), also known as acute lymphocytic leukaemia, affects lymphoid stem cells and grows quickly;
• chronic lymphocytic leukaemia (CLL) affects lymphoid stem cells and usually grows slowly;
• acute myeloid leukaemia (AML) affects myeloid stem cells and grows quickly.
• chronic myeloid leukaemia (CML) affects myeloid stem cells and grows slowly