Editing Growth factor (section)

== Comparison to cytokines ==
''Growth factor'' is sometimes used interchangeably among scientists with the term ''[[cytokine]].''<ref name="YorioClark2007">{{cite book | first1 = Thomas | last1 = Yorio | first2 = Abbot F. | last2 = Clark | first3 = Martin B. | last3 = Wax | name-list-style = vanc |title=Ocular Therapeutics: Eye on New Discoveries|url=https://books.google.com/books?id=7jVNuIt8vm4C&pg=PA88|year=2007|publisher=Academic Press|isbn=978-0-12-370585-3|page=88}}</ref> Historically, cytokines were associated with [[hematopoietic]] (blood and lymph forming) cells and [[immune system]] cells (e.g., [[lymphocyte]]s and tissue cells from [[spleen]], [[thymus]], and [[lymph node]]s). For the [[circulatory system]] and [[bone marrow]] in which cells can occur in a liquid suspension and not bound up in solid [[tissue (biology)|tissue]], it makes sense for them to communicate by soluble, circulating protein [[molecule]]s.   However, as different lines of research converged, it became clear that some of the same signaling proteins which the hematopoietic and immune systems use were also being used by all sorts of other cells and tissues, during development and in the mature organism.

While ''growth factor'' implies a positive effect on [[cell proliferation]], ''cytokine'' is a neutral term with respect to whether a molecule affects proliferation. While some cytokines can be growth factors, such as [[Granulocyte colony-stimulating factor|G-CSF]] and [[Granulocyte macrophage colony-stimulating factor|GM-CSF]], others have an inhibitory effect on [[cell growth]] or cell proliferation. Some cytokines, such as [[Fas ligand]], are used as "death" signals; they cause target cells to undergo programmed [[cell death]] or ''[[apoptosis]]''.

The [[nerve growth factor]] (NGF) was first discovered by [[Rita Levi-Montalcini]], which won her a [[Nobel Prize in Physiology or Medicine]].