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Microorganisms establish a symbiotic relationship at all stages of growth in man, beginning from birth to adulthood. They are found in every part of the body, and they participate in the immune system against pathogen-mediated immune responses. Their actions are elicited by secreting microorganism associated-molecular proteins (MAMPs) which they use as signalling molecules to activate a cascade of immunological responses within the host cell. They maintain the barrier function of the intestinal wall as well as prevent colonisation of the intestine by pathogens. Using their MAMPs, they bind to specific pattern recognition receptors (PRR) activating immune mediators in response to pro-inflammation by pathogens. These immune mediators are either induced or suppressed (in the case of overproduced immune response). Some of these symbionts elicit their action by anaerobic fermentation of dietary fibres into byproducts of short-chain fatty acids (SCFAs). These bacterial metabolites functions by binding to G-protein coupled reactions (GPCRs) on colonic macrophages and Dendritic cells (DCs) and contributed to the increased production of interleukin 10 (IL 10) in response to pathogen-mediated immune response. These unique immunological actions of intestinal microbiota, have shown that microorganisms are beneficial to the host as against the widespread belief that they are disease-causing agents.