The vast majority of our immune system (about 80 per cent) is located in the gut. And with good reason. This is where the main stage at the bacterial Woodstock is situated, and any immune system worth its salt must be there, or be square. The bacteria are confined to a fenced-off area — the mucous membrane of the gut — preventing them from getting too dangerously close to the cells of the gut wall. The immune system is able to play with the cells without ever posing a danger to the body. This allows our defender cells to get acquainted with many previously unknown species.
