The simplistic answer is that the immune system tries out new antigen-detecting cells in a safe environment before letting them run around with a loaded gun. An

The simplistic answer is that the immune system tries out new antigen-detecting cells in a safe environment before letting them run around with a loaded gun. Any cell that activates in that safe environment is destroyed, as it recognized one of your own antigens. The other mechanism is that the immune system doesn't react only to foreign antigens, there also needs to be some general activation of the immune system in that area for a real response. The full answer is that this is really, really complex like pretty much everything involving the immune system. And auto-immune diseases indicate that the immune system doesn't always get this right.