Leukaemia is a cancer that starts in blood-forming tissue. There are different types of leukaemia. Those that develop suddenly (over days or weeks) are called acute leukaemias. Leukaemias that develop over months or years are chronic leukaemias. Leukaemia is also named by the type of white blood cell affected, either the myeloid or lymphoid cells, which are distinguished by their appearance under the microscope. Myeloid white blood cells are the immune system's first line of defence against infection and are found mainly in the blood. [They] engulf and kill foreign organisms. Lymphoid white blood cells are found in the lymph nodes and in the blood. They lie in wait to detect foreign organisms and stimulate the body's defences to fight them. [CLL leukemia is Chronic Lymphoid Leukaemia. Non-CLL leukemia refers to any other type of leukemia, that is, chronic myeloid, acute myeloid or acute lymphoid leukemia]