The human immune system shows substantial interindividual immune variation that relates to genetic differences as well as environmental exposures. Our closest environment - our microbiota - has received considerable attention during the last decade(s). We know that the gut environment is instrumental for immune maturation in mice, but there is still a large knowledge gap regarding gut environment-immune crosstalk in humans, in particular regarding mechanisms. The general purpose of our work is to study how the gut environment is involved in immune modulation, in physiological and pathological conditions but also whether this impacts allergy and tolerance, and by what mechanisms this microbe-mediated immune instruction takes place. We study this in clinical material involving children ad young adults as well as in different in vitro models, where human (immune) cells are exposed to different microbial factors.