3- Immune system
Your gut is the main area in the body where the immune system interacts with what's brought in from the outside world. Thus, the interaction between gut bacteria and your own cells appear to play an important role in the development of a fully-functioning immune system. According to a 2003 review paper in the Lancet, lymphatic tissue in the intestine contains the largest pool of cells capable of producing an immune response.
A 2012 study found that whether babies are fed breast milk or formula influences the composition of their gut bacteria, and in turn, the development of their immune system. Babies fed only breast milk had more diversity in their gut bacteria than babies who were fed only formula. There was also a link between the genes that were "turned on" in the babies' gut bacteria, and the genes that were "turned on" in their immune system.https://www.livescience.com/39444-gut-bacteria-health.html