What is blood cancer? Blood cancers are also called haematological cancers. Most of haematological cancers start in the bone marrow. Therefore blood and bone marrow cancers are often discussed together. By affecting the blood cells and the bone marrow, these cancers change how blood cells work and behave. Blood cancers cause your bone marrow and the lymphatic system to produce blood cells that don't work properly. All the blood cancers affect different types of white blood cells in the blood, and all act differently. When abnormal blood cells start growing out of control, they interfere with the functions of the normal blood cells, and so they become cancerous.