G-CSF, or Granulocyte Colony-Stimulating Factor, is a naturally occurring protein in the body that stimulates the production of white blood cells, specifically neutrophils. These cells are crucial for fighting infections and maintaining a healthy immune system. G-CSF is produced by various cells, including macrophages and endothelial cells, in response to infection or inflammation. In medical settings, synthetic forms of G-CSF are used as a treatment to boost white blood cell counts in patients undergoing chemotherapy or those with certain blood disorders. This helps reduce the risk of infections and allows for more effective cancer treatment.