GM-CSF is a cytokine that functions as a white blood cell growth factor. GM-CSF stimulates stem cells to produce granulocytes (neutrophils, eosinophils, and basophils) and monocytes. Monocytes exit the circulation and migrate into tissue, whereupon they mature into macrophages. Thus, it is part of the immune/inflammatory cascade, by which activation of a small number of macrophages can rapidly lead to an increase in their numbers, a process crucial for fighting infection. The active form of the protein is found extracellularly as a homodimer.
GM-CSF is required for the growth and development of marrow progenitor cells. It also stimulates proliferation and differentiation of granulocytes and macrophages. GM-CSF has a stimulative effect on mature macrophages, neutrophilic granulocytes and acidophilic granulocytes.

From:http://www.genetic-tests.net/Recombinant-Proteins/29/