Bone marrow is the spongy tissue inside certain bones including the ribs, vertebrae, pelvis, and the long bones of the arms and legs. These cells are few in number but can proliferate rapidly. Stem cells differentiate into precursor cells called colony-forming units that go on to produce red blood cells, white blood cells and platelets. Stem cells and later precursor cells are stimulated by various chemical messengers called cytokines. These messengers are released, for example, when the level of oxygen reaching the tissues is too low or when invading microorganisms are encountered.
Erythropoietin stimulates the production of red blood cells.
Cytokines called interleukins also influence the production of blood cells. These can be broken down into deficiencies due to inadequate blood cell production and those due to excessive blood cell destruction or loss. These causes will be discussed below in relation to specific deficiencies. For example, leukopenia is a lack of white blood cells and neutropenia is a low level of neutrophils. Pancytopenia refers to low levels of all types of blood cells.