Macrophages, within the cytokine network, are a major source of many cytokines involved in immune response, hematopoiesis, inflammation and many other homeostatic processes. Upon stimulation by micro-organisms, microbial products or endogenous factors including cytokines, macrophages can de novo synthesize and release a large variety of cytokines (ie IL-1, IL-1ra, IL-6, IL-8, IL-10, IL-12, TNF alpha, IFN alpha, IFN gamma, MCP-1, MCP-3, MIF, M-CSF, G-CSF, GM-CSF, MIP-1, MIP-2, LIF, OSM, TGF beta). Some cytokines can upregulate the production of cytokines by macrophages (IL-3, GM-CSF, IFN gamma) while others can inhibit it (IL-4, IL-10, IL-13, TGF beta). In addition, these cytokines can modulate most of the macrophage functions and cell surface marker expression. Other cytokines (the chemokines such as MCP-1,2,3, MIP-1,2 and RANTES) contribute to the recruitment of circulating monocytes within tissues. It is worth noting that macrophages can be their own source of regulatory cytokines.