G-CSF hormone molecule. Molecular model showing the primary (rods) and secondary (helices) structures of the human granulocyte colony- stimulating factor (G-CSF) hormone. G-CSF is a growth factor produced by a number of immune system cells and endothelial cells that stimulates the bone marrow to produce granulocyte white blood cells. There are three types of granulocytes; neutrophils, eosinophils and basophils. These cells help to destroy invading organisms, and are involved in allergic and inflammatory responses.