M-CSF, or macrophage colony-stimulating factor, is a protein that plays a crucial role in the immune system. It is primarily responsible for the growth and differentiation of monocytes into macrophages, which are essential cells that help the body fight infections and clear away dead cells. M-CSF is produced by various cells, including fibroblasts and endothelial cells, in response to inflammation or injury. By promoting the development of macrophages, M-CSF helps enhance the body's ability to respond to pathogens and maintain tissue homeostasis.