The blood and bone marrow are intimately linked, since the cells circulating in the blood stream are formed in the bone marrow. The bone marrow is formed by stem cells, which are immature cells that can transform themselves into a variety of other cells: red blood cells that carry oxygen to the tissues, white blood cells that fight infection and form part of the immune system, and platelets that allow the blood to clot. Bone marrow that is actively producing blood cells can be found in the pelvic bones, shoulders, spine, ribs, breast bone, and skull. These cells are most commonly obtained from the pelvic and breast bones through a bone marrow biopsy, which is a small sample taken directly from the bone with a needle.
The bone marrow is responsible for making blood cells and is the spongy part of thick bones such as hip bones (pelvis), shoulder bones (scapula), bones of the spine (vertebrae), ribs, breast bone (sternum) and skull. The most common site to biopsy the bone marrow is the pelvis and the sternum.
A biopsy involves a needle taking a small sample from the bone marrow. The sample is then sent to the laboratory to be inspected under a microscope in order to diagnose bone marrow diseases including cancer. Among the cancers that can grow in the bone marrow are leukemias, lymphomas, and multiple myelomas.
There are four different types of white blood cells:
- B lymphocytes produce antibodies that attack foreign cells in the body such as bacteria, fungus, and viruses.
- T lymphocytes also fight infectious agents, and natural killer (NK) cells will attack any foreign cell.
- Granulocytes, such as neutrophils and eosinophils, are activated during an infection or allergic reaction, and attack the foreign object by surrounding it and destroying it.
- Monocytes are activated during an immune response, and they form macrophages which engulf and destroy pathogens and dead cells.
Leukemia is a generic term that includes cancers of a variety of blood cell types. Depending on their cell origin, the leukemia will be designated as lymphocytic or myelocytic. Lymphoid stem cells from the marrow give rise to B and T lymphocytes and NK cells, while myeloid stem cells can differentiate into red blood cells, granulocytes, monocytes, and platelets.
The lymphatic system is an arrangement of lymph vessels, lymph nodes, and lymphatic organs which are activated during the immune response to fight infection and disease. Lymphomas grow out of lymphocytes, the cells that circulate in the lymphatic system. Lymphomas can start just about anywhere in the body, because lymphatic cells are widespread. There are two main types: Hodgkin and non-Hodgkin lymphoma. Both usually start in the lymph nodes, and can spread to other organs via the lymphatic system.