From Bone Marrow to the Bloodstream
Immune cells--also called white blood cells or lymphocytes--leave the bone marrow while still immature, and they migrate through the bloodstream on their way to the thymus and other lymphoid organs. During their journey, they mature into specialized T or B cells of the immune system. Other blood components, like red blood cells, completely mature in the bone marrow before being released into the blood.
In general, at any given moment, there is one blood stem cell circulating in a person's bloodstream for every 100 present in bone marrow. Although it is not precisely understood how the body signals for more blood stem cells to move out of the marrow, scientists are studying some factors that seem to increase the rate of release.