Leukaemia is a type of blood cancer. It starts in the blood-forming tissue, usually the bone marrow, and leads to the over-production of abnormal white blood cells, the part of the immune system that defends the body against infection.
To understand leukaemia, it helps to know more about your blood, which has three main cells groups. They have different jobs:
Your body is making fresh new blood cells all the time, to replace old ones that have worn out and died off. This happens mainly in your bone marrow. When you're healthy, this system is finely balanced in a process called haematopoiesis.
Leukaemia develops when this process goes wrong, and the white blood cells develop in an uncontrolled way. Your body is making too many white blood cells and they may not be fully formed or mature. This means your white blood cells can't do their job of fighting infection. They may also crowd out your red blood cells and platelets, so they can't work properly either.
The leukaemia name partly depends on whether it is chronic or acute:
The leukaemia name also tells you what specific type of white blood cell is affected, myeloid or lymphoblastic.
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Read more about the four main types of leukaemia: