How does the human microbiome influence immune system function?

The human microbiome, the community of bacteria and other microbes living mainly in our gut, trains the immune system by exposing it to harmless microbes that teach it to distinguish friend from foe. When microbes interact with gut cells, they release signals that help immune cells grow and learn to ignore harmless bacteria while staying ready to attack dangerous ones. This training also reduces inflammation, so the body is less likely to overreact to foods or harmless bacteria. For example, a child who grows up with a diverse gut microbiome is less likely to develop allergies because his immune system has learned to tolerate many different microbes. In this way, the microbiome acts as a daily teacher for our immune defenses.