TNF and collagenous colitis: Anti-TNF, anti-tumor necrosis factor; CyA, cyclosporin A; CD, Crohn's disease; IBD, inflammatory bowel disease; NOD2, nucleotide-binding oligomerization domain 2; CC, collagenous colitis; UC, ulcerative colitis; PROTECT, Predicting Response to Standardized Colitis Therapy; IL-33, interleukin-33; GPR43, G protein-coupled receptor 43; SCFA, short-chain fatty acid; KCD, Korean Crohn's disease; 16S rRNA, 16S ribosomal ribonucleic acid