Colony-stimulating factors or CSFs are glycoproteins that bind to receptors on hematopoietic stem cells or HSCs to regulate their growth and differentiation. CSFs circulate in the blood and bone marrow as soluble factors. Stromal cells also secrete membrane-bound CSFs called stem cell factors to keep quiescent HSCs inactive and maintain the stem cell pool. As active HSCs divide and differentiate into committed progenitors, CSFs stimulate the progenitors to proliferate and differentiate to form colonies of specific blood cell types. Upon microbial infection, CSFs induce progenitors to produce immune cells that are released into the bloodstream to attack the invading microbes. In the absence of CSFs, the immune cells undergo apoptosis, preventing the cancerous growth of blood cells.