Peripheral blood stem cells (PBSCs) are stem cells or progenitor cells from the bloodstream that develop and reside in the bone marrow. These cells are different from other cell types, characterized by the CD34 protein on their cell surface. Normally, very few PBSCs circulate in the bloodstream, but their numbers can be increased by stimulation from cytokines or appropriate bone marrow stimulants, such as G-CSF (a bone marrow stimulating agent that the human body can produce naturally). When the body needs to regenerate important blood components, especially various types of blood cells, these progenitor cells are stimulated out of the bone marrow. This allows for a standardized process of collection and quality preservation for future use, particularly for diseases related to the blood and immune systems.