Fig. 1: Schematic of the tridirectional gut‒immune‒brain axis.
From: Beyond the gut: decoding the gut–immune–brain axis in health and disease
The gut harbors a complex microbiota capable of producing metabolites that regulate host physiology. These metabolites, such as SCFAs, can influence host immune maturation and the balance between pro- and anti-inflammatory responses. On the other hand, immune signals, such as IgA and the Treg/Th17 balance, can regulate microbial tolerance. Microbial metabolites can modulate neuroinflammation and homeostasis by directly affecting brain-resident immune cells and neurons and by eliciting peripheral immune responses that cascade into the brain. Owing to bidirectional crosstalk, the brain itself can modulate the gut microbiota through the vagus nerve and the HPA axis
