In order to understand what a blood or marrow transplant is it will be useful to first know about bone marrow and bone marrow function.

Bone marrow function — Bone marrow is the soft, spongy area in the centre of some of the larger bones of the body. The marrow produces all of the different cells that make up the blood, such as red blood cells, white blood cells (of many different types), and platelets. All of these cells develop from a type of precursor cell (a cell that is the forerunner of the other cells) found in the bone marrow, called a haemopoietic stem cell.

The body is able to direct haemopoietic stem cells to develop into the blood components needed at any given moment. This is a very active process, with the bone marrow producing millions of different cells every hour. Most of the stem cells stay in the marrow until they are transformed into the various blood components, which are then released into the blood stream. Small numbers of stem cells, however, can be found in the circulating blood, which allows them to be collected under certain circumstances (see below). Various strategies can be employed to increase the number of haemopoietic stem cells in the blood prior to collection.

Bone marrow transplantation — Some of the most effective treatments for cancer, such as chemotherapy and radiation, are toxic to the bone marrow. Usually the higher the dose, the more toxic the effects on the bone marrow.

In bone marrow transplantation used for treatment of cancer, you are given very high doses of chemotherapy or radiation therapy, which kills cancer cells and destroys all the normal cells developing in the bone marrow, including the critical stem cells, this is called conditioning. After the conditioning, you must have a healthy supply of stem cells reintroduced, or transplanted. The transplanted cells then re-establish the blood cell production process in the bone marrow.

When bone marrow transplantation is used for non-cancerous, haematological conditions such as aplastic anaemia, the transplanted cells replace the failing bone marrow cells with new ones from a matching donor. The multipotent stem cells in the bone marrow reconstitute all three blood cell lines, giving the patient a new immune system, red blood cells, and platelets. Conditioning therapy is not needed prior to the donated cells being transplanted as there are no cancer cells to kill.

The cells that will be transplanted can be taken from the bone marrow (called a bone marrow transplant), from the bloodstream (called a peripheral blood stem cell transplant, which requires your donor to receive a daily injection for 5 days to boost the number of haemopoietic stem cells in the blood), or occasionally from blood obtained from the umbilical cord at the time of birth of a normal newborn (called an umbilical cord blood transplant).

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