granulocyte-macrophage colony-stimulating factor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A naturally occurring protein that stimulates the production of granulocytes and macrophages by stem cells and is used as a drug by some immunosuppressed individuals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A cytokine protein secreted by macrophages, T cells, mast cells, endothelial cells and fibroblasts.