Dioxin, the chemical pollutant made infamous by Vietnam-era defoliant Agent Orange, has long been known to suppress immune function in humans and other animals. Surprisingly, this dangerous side effect has a scientific silver lining. While studying the toxin’s health effects, researchers discovered the genetic pathway to immune system malfunction. For people who would actually benefit from suppressed immunity — those suffering from autoimmune and allergic diseases — this clue may lead to better therapies.