There is more to psychology than just the brain. The average human brain weighs 3 pounds. The average adult carries around 2-6 pounds of microbes.
Understanding how these microbes communicate with our brain is just the beginning.
We know that many psychological disorders respond quite well to proper nutrition and repair of the gut lining. This includes depression, anxiety, bulimia, epilepsy, schizophrenia, autism and ADHD, to name a few. There are numerous potential immune mechanisms by which the microbiota could communicate with the brain.