The prevalence of chronic autoimmune and inflammatory diseases, such as inflammatory bowel disease, allergies, or rheumatic diseases, is steadily increasing in developed countries. This increase is probably accelerated by environmental factors, such as decrease in infectious burden or changes in food processing. These lifestyle changes then strongly influence the strongest stimulus for the immune system – commensal microbiota. Despite the differences in the affected organ, the immune-mediated diseases have one or more factors in common – microbe either as a trigger or as a protector, mucosal barrier dysfunction, and dysregulation of the immune system. The core questions, which microbes are involved and how these diseases can be cured or even prevented still remain unsolved. Powered by the recent progress in technology, by new insights into the function of immune system, by advances in microbiome research, and extended use of gnotobiological techniques, these mechanisms are now being unravelled and new therapeutic possibilities are emerging. To secure their niche, the microbes devised many ingenious ways, how to dampen the inflammation. Nonpathogenic microorganisms or microbial components isolated from probiotic, commensal or even pathogenic microbes could be, therefore, used to interfere with the pathogenetic mechanisms of immune-mediated diseases.