molecules and invading microbes. The immune system’s policing job gets complicated by the porous nature of our bodies’ borders. Just about every organism that wants to get inside us can do so. Our body is a take-all-comers bash, a festival with open seating, coursing with every life-form that happens by—petty thieves and gangs; terrorists armed with nuclear suitcases; dumb drunk cousins and relatives; enemy agents cloaked as friends; and foes so unpredictable and alien that they seem as if beamed from another universe.
