Bone Marrow

Bone marrow is the semi-solid tissue found in the centre of bones and is where circulating blood cells are produced. Approximately 500 billion blood cells are produced per day in an adult human, which includes cells of both the innate and adaptive immune system.

The bone marrow is one of two primary lymphoid organs (PLOs) in the human body, the other being the thymus. PLOs are where lymphocytes are formed and mature (lymphopoiesis). As well as producing a large repertoire of cells that are reactive against threats the PLOs also eliminate self-reactive lymphocytes, i.e those lymphocytes that would respond to our own tissue (autoreactive). T- and B-lymphocytes are "born" in the bone but only B-lymphocytes mature there, with T-lymphocytes maturing in the other primary lymphoid organ, the thymus.

Bone marrow transplants can be used to treat some diseases, such as leukaemia, which are forms of blood cancer that result in a high number of abnormal white blood cells. Normally, the bone marrow acts as a barrier to prevent cells that do not express specific surface markers from leaving the bone marrow. In leukaemia this process can become dysregulated allowing white blood cells that have not yet fully matured to enter the circulation.



Soluble Mediators