Bone Marrow
The bone marrow is the soft inner part of some bones such as the skull, shoulder blades, ribs, pelvis, and backbones. The bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow.
Blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into either lymphocytes (a kind of white blood cell) or other blood-forming cells. The other blood-forming cells can develop into 1 of the 3 main types of blood cell components:
Red blood cells carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed. 
Anaemia (having too few red blood cells in the body) typically causes a person to feel tired, weak, and short of breath because the body tissues are not getting enough oxygen.
Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets is called thrombocytopenia. A person with thrombocytopenia may bleed and bruise easily.
White blood cells help the body fight infections. 
Lymphocytes (L) are one type of white blood cell. The other types of white blood cells are granulocytes (G) (neutrophils, basophils, and eosinophils) and monocytes.
Any of the blood-forming or lymphoid cells from the bone marrow can turn into a leukaemia cell. Once this change takes place, the leukaemia cells fail to go through their normal process of maturing. Leukaemia cells may reproduce quickly, but in most cases the problem is that they don't die when they should. They survive and build up in the bone marrow. Over time, these cells spill into the bloodstream and spread to other organs, where they can keep other cells in the body from functioning normally.
