In healthy adults, the gut microbiome exists in a state of mutual symbiosis with its host. The environment of the gut dictates both the composition and functional productivity of the adult gut microbiota, which may interact with the host through presentation of various ligands such as pathogen-associated molecular patterns (PAMPs) and production of metabolites, e.g., SCFAs. These molecules modulate immune homeostasis in the GI tract and at remote mucosal surfaces and organs via their entry into the circulation.