S100A9 and neoplasm: S100A9 promotes the production of reactive oxygen species (ROS), and it interacts with RAGE and TLR4, triggering the inflammatory cytokine cascade including TNF-α, IL-1β, IL-6, and IL-8 and, finally, it promotes tumour metastatic spread and immunosuppression by invoking the myeloid-derived suppressor cell (MDSC) [66,67,68].