Dit attacks the nerves in the brain – but it obviously has its origin in another organ: the intestine. Billions of microbes adhere to its moist mucous membrane, which help with digestion. Immune cells also sit in the intestinal wall and are calibrated to the environment of a person in contact with the bacteria, their metabolites and food. “The gut is the Olympic training center for the immune system,” says Reinhard Hohlfeld, neurologist emeritus at the Ludwig Maximilians University in Munich. The immune system is constantly adjusted to germs. The training center also produces immune cells from the T cell family that can immigrate to the brain, for example when the brain is threatened by a large number of pathogens such as meningococci and streptococci and no longer works with its own protective mechanisms. Other causes – not yet known in detail – also guide the immune cells into the brain.