Native to western North America, and common throughout the North Coast, Western Poison Oak (Toxicodendron diversilobum) is familiar to most folks because of a learned avoidance. This woody shrub or climbing vine causes allergic skin reactions (dermatitis) in many people resulting in skin irritation, redness, and itching ... and in severe reactions blisters.
About 20% of people are immune to the allergen, but with 80% susceptible this plant causes more work-loss hours than any other plant in California. For further detailed information please refer to the UC IPM Online site: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7431.html (you will have to manually type this into your search engine).
So, abide by the old saying, "If leaves of three then let it be"... which came to be because of the leaves that normally consist of three leaflets (but occasionally consists of 5,7, or 9 leaflets). The leaves commonly turn red as fall approaches and the plants begin to shut down for winter dormancy.
