Urushiol

The “Poison” in Poison-Oak

Chemically, urushiol is a mixture of eight 3-substituted pyrocatechols. The compound shown is the C17 diene, an important part of poison-oak’s arsenal.

Urushiol is a strong reducing agent. Oxygen in the epidermal layer of the skin transforms the urushiol to an orthobenzylquinone.

The immune systems of animals in the last 60 million years have ignored small metabolically-common molecules, like the six carbon ring of pyrocatechol. Animal immune systems have concentrated on foreign molecules larger than 18 carbons, and respond most briskly to compounds in the 21 to 23 carbon size range. Septadecylorthobenzylquinone is such a compound; it never originates in animals, and has 23 carbons. A few molecules of it can cause a rash!