S100A9 and Gastrointestinal inflammation: Calprotectin, the stable heterodimer formed by S100A8 and S100A9 monomers, is abundantly expressed in neutrophils and used as a non-invasive diagnostic tool to distinguish gastrointestinal inflammation; it can also attenuate ROS damage and promote autophagy and apoptosis of macrophages, lymphocytes, endothelial cells, and tumor cells [27].