White blood cells are made in your bone marrow - the soft, spongy centre of your bones. They are involved in your immune system, which defends your body against infection and other harmful substances. There are two main types of white blood cell. These are myeloid cells and lymphocytes.
The term leukaemia refers to a group of cancers of the white blood cells. If you have leukaemia, some of your white blood cells don't grow properly. They stay in your bone marrow and reproduce in an uncontrolled way. These abnormal white blood cells fill up your bone marrow and prevent it from making the normal blood cells (white blood cells, red blood cells and platelets). This means your body may be less able to fight off infections and you may develop anaemia or bruise and bleed abnormally.