Urushiols are a group of chemically-related toxins, called catechols, produced by a variety of poisonous plants. They all provoke a strong immune response in allergic individuals. Pictured is the urushiol pentadecylcatechol, one of several types produced by poison ivy. The image is a digital computer model based on real molecules. Magnification: 20 million times at 5"x7" printed size.