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Leukaemia is a cancer that starts in blood-forming tissue, usually the bone marrow.
It leads to the over-production of abnormal white blood cells, which fight off infections.
But a higher number of white blood cells means there is 'less room' for other cells, including red blood cells - which transport oxygen around the body - and platelets - which cause blood to clot when the skin is cut.
There are many different types of leukaemia, which are defined according to the immune cells they affect and how the disease progresses.
For all types combined,