Western poison oak occurs only on the Pacific Coast of North America, where it is common. The plant toxin produced by members of the genus Toxicodendron, called urushiol, is known for causing an uncomfortable, and sometimes painful, skin reaction. The active components of urushiol have been determined by Billets (1975) to be unsaturated congeners of 3-heptadecylcatechol with up to three double bonds in an unbranched C17 side chain. "Leaves of three, leave them be. Leaves of four, eat some more."
Text from: http://en.wikipedia.org/wiki/Toxicodendron_diversilobum
Picture from: http://bapd.org/hamilton-gulch-long-sequence.html