The mucosa

There is a slight problem with the mucosa- the places where mucus and acid is. (Like your nose and intestines.) The immune system just kills everything that isn't part of the body anywhere else, but here, if they just eat up random bacteria and stuff, they might damage the system. To protect a place, you don't just put the army in the front- you put horrible things that will cut open the intruders first, like those tipped walls. In here, it is the mucosa layer, which protects the skin beneath. IGA antibodies and salt are here to stop the intruders, so that it will be relatively calm but also not too chill.