**1. Introduction**

The immune system is fundamental to protect the organism from pathogens and toxic exogenous agents, by discriminating between "self" and "nonself" antigens, and in normal physiological conditions it is programmed to react against "nonself." At the intestinal level, the gut-associated lymphoid tissue (GALT) is a key component of immune defense, protecting the body from foreign antigens and pathogens, while allowing tolerance to commensal bacteria and dietary antigens [1]. The intestinal microbiota, defined as the complex microbial community residing in the host's digestive tract, is recognized as an effective integral component of the host immune system, capable of finely tuning both the innate and adaptive immune responses during the entire lifespan. Indeed, the intimate relationship set up between microbiota and immune cells in the intestine is crucial for the maintenance of immunological homeostasis and, mostly, for the "education" of the immune system during the early

stages of life [2]. Diet has a strong influence on the gut microbiota, acting both as a modulator able to select specific microbial groups and providing substrates that can be metabolized by the microbiota producing metabolites that impact host health also through interaction with the immune system [3, 4]. Therefore, there is a close connection between diet, gut microbiota, and immune system, orchestrated by a fine-tuning of the complex mechanisms underlying this cross-talk.
