Growth factors are proteins that bind to receptors on the cell surface, with the primary result of activating cellular proliferation and/or differentiation. They are very versatile, and stimulate cellular division in numerous different cell types. Eg: Flt-3/Flk-2 show proliferative effects on primitive hematopoietic cells. Stem Cell Factor is a hematopoietic growth factor that stimulates the growth cells of numerous lineages. Cytokines are a unique family of growth factors. Secreted primarily from leukocytes, they stimulate the humoral and cellular immune responses, as well as the activation of phagocytic cells. Eg: Colony Stimulating Factors (CSFs) are cytokines that stimulate the proliferation of specific pluripotent stem cells of the bone marrow in adults. Granulocyte-CSF (G-CSF) is known for its specific proliferative effects on cells of the granulocyte lineage as well as differentiation and activation of hematopoietic cells of neutrophilic granulocyte lineage. Macrophage-CSF (M-CSF) is specific for cells of the macrophage lineage. Granulocyte-macrophage-CSF (GM-CSF) has proliferative effects on both granulocyte and macrophage classes of lymphoid cells. IL-3 interacts with very early multipotent hematopoietic progenitor cells.