Regulation of hematopoiesis by cytokines that stimulate the proliferation and/or differentiation of various hematopoietic cells. In the absence of infection, bone-marrow stromal cells are the major source of hematopoietic cytokines. In the presence of infection, cytokines produced by activated macrophages and T-helper cells induce additional hematopoietic activity, resulting in rapid expansion of white blood cells that participate in fighting infection. IL = interleukin; CSF = colony-stimulating factor; EPO = erythropoietin. Stem cells are shown in blue; progenitor and immune cells in purple; stromal cell in yellow; and mature differentiated cells in tan.