The foliage of Poison Ivy can irritate the skin of most people, causing redness and blisters. This is caused by a reaction of the immune system to urushiol. People who are immune to Poison Ivy when they are young, can become sensitive to its irritating effects when they become older. Only primates and hamsters are known to have allergic reactions to Poison Ivy, while other animals appear to be immune to its effects. The growth habit of Poison Ivy and the appearance of its leaflets can be highly variable, which sometimes confuses people regarding its identity. At one extreme, Poison Ivy can assume the form of a low little-branched shrub with trifoliate leaves, while at the other extreme it can assume the form of a long woody vine that can swallow up trees. As a vine, its woody stems have a hairy appearance from the abundant aerial rootlets and no tendrils are produced across from the trifoliate leaves. Other woody vines produce simple leaves (e.g., Wild Grapes and Moonseed) or they have compound leaves with 5 or more leaflets (e.g., Virginia Creeper and Trumpet Creeper).