system
Given the following post, respond as if you're commenting on Reddit:
human
How does our immune system "know" not to attack our gut flora?

What mechanism allows our gut flora to exist without being attacked by our immune system? Especially bacteria such as E. coli that are potentially pathogenic?
gpt
The gut is outside the body as though the body was a torus / donut, the epithelium of the gut forms a barrier between the contents of the gut and the body and blood stream. In places there is mucus also contributing to the barrier, eg stomach. There are some immunoglobulins (igA) in the gut secretions and the acid environment of the stomach is bacteriocidal but in general there are no immune cells within the gut lumen, in the fluids in the gut. There are immune cells in the gut walls (lymphoid tissue and tissue macrophages) but they are not in contact with the bacteria within the lumen of the gut.

If the bacteria breach the barriers of the gut the immune system will respond. For example, pathogenic E Coli have *fimbriae* (little fingers), a mechanism to attach to the intestinal wall. They can also produce toxins. "Normal" E.Coli does not.