Leukaemia is a cancer which starts in blood-forming tissue, usually the bone marrow. It leads to the over-production of abnormal white blood cells, the part of the immune system which defends the body against infection.
A further in-depth explanation of what leukaemia is can be found on the Leukaemia Care website here.
There are four main types of Leukaemia:
Acute myeloid leukaemia (AML)
Acute lymphoblastic leukaemia (ALL)
Chronic myeloid leukaemia (CML)
Chronic lymphocytic leukaemia (CLL)
There are also a number of rarer types of leukaemia and leukaemia sub-types. You can find more information on these types on the Leukaemia Care website.
In the following video, leading haematologist (blood cancer specialist) Mary Frances McMullin explains what leukaemia is. (Video length: 1 minute 25 seconds)