From TAMU
When we talk about the reactions to these plants, I must first explain that they are as diverse as the leaflet variations. It is the urushiol (pronounced oo-roo-she-ol) resin, from oleoresin found in the sap, that causes contact dermatitis in humans. Urushiol is found in all parts of the plant but the hairs, wood cells, anthers and pollen. This is why honey made by bees who feed on poison ivy, isn't poisonous. Even though the urushiols often produce a profound reaction in humans, birds eat the berries and bears, cattle, deer, goats, hogs and horses eat the leaves and stems with no ill effects. It is perplexing to note that rabbits, squirrels and rodents eat the very berries that can so debilitate a human.
From WVU
poison ivy
Rhus radicans
Perennial, high climbing, woody vine. Leaves alternate, deciduous, pinnately compound; leaflets three, thin, bright green, shiny, ovate to elliptic, entire to serrate to shallowly lobed, 2 to 12 cm long, 2 to 12 cm wide. Flowers small, yellowish green, in clusters of 2 to 6 in lower leaf axils. Fruit a scarcely fleshy drupe, glabrous to short pubescent, .4 to.5 cm broad. Found throughout the southern states east of the Mississippi River; most abundant in moist woods but also in pastures, fence rows, roadsides and waste places.
poison oak Rhus toxicodendron
Low shrub, .3 to 2 m tall; very similar in appearance to R. radicans; however, it does not climb and leaflets are thicker, dull green, hairy on both surfaces and are more often lobed or coarsely serrate. Fruit is densely pubescent rather than glabrous or short pubescent. Found throughout the south; most abundant on relatively dry, sunny sites in woodlands, thickets and old fields.
TOXICITY
The toxic principle is a phenolic compound called urushiol. It is a skin and mucous membrane irritant and is found in all parts of the plant. Some humans are quite sensitive to the effects of the toxin while others show no ill effects from coming into contact with the plant. The toxin has little or no effect on animals but pets may carry the irritating substance on their hair and thereby transmit it to humans.
SYMPTOMS
Susceptible humans exhibit intense itching with inflammation and the formation of blisters at the areas of contact. Animals are rarely affected. Burning maybe dangerous because the irritant may be transmitted by smoke.