S100A9 and inflammatory bowel disease: Here, we studied the quaternary protein structures (termed configuration) of S100A8 and S100A9 and their biological function in inflammatory bowel diseases (IBD).<h4>Methods</h4>We dissected fecal S100A8 and S100A9 configurations in patients with IBD by size-exclusion chromatography coupled with tandem mass spectrometry and systematically defined human S100A8 and S100A9 homodimer functions compared with the calprotectin heterotetramer (CP) in the intestine of mice and in human epithelium and T cells.