S100A9 and rheumatoid arthritis: Monocytes/macrophages express many specific molecules which play a role in RA pathogenesis i.e.: TNF-α, IL-1β, S100A8, S100A9, GM-CSF, IL-6, HMGB, SAA, MMP-3, MMP-12, MCP-1/CCL2, CCL3, CCL5, CX3CL1 and IL-8.