Have you ever been in the field and either known, or suspected, you were exposed to poison ivy or poison sumac? If you have, and act promptly, you can prevent or minimize your bodies reaction to the exposure to the urushiol oils that cause the inflammation on your skin. Dr. Jim Brauker has a great video that summarizes how urushiol effects you skin, and how to treat it. If you are short on time or a TL:DW type person the short answer is this.
Wash the area thoroughly. Even if you do not have soap available at least use water and a cloth. The important thing is that your finish by using friction and a cloth to remove as much as the oil as possible.
In the video Dr. Brauker implies that for just washing, without using friction to remove the remaining oils, that Dawn dishwashing soap is more effective that Tecnu. I have reached out to http://www.teclabsinc.com, the makers of Tecnu to see if this conclusion is correct, or if the formula of Tecnu is such that it is more effective at removing urushiol, as opposed to motor oil/grease. I will update if I hear back from them.
The bottom line? Wash well and be sure to wipe with friction. If you are very sensitive this should be done within two hours of exposure.
Health warning from the Poison Ivy, Oak and Sumac Information Center: About 15 percent of the 120 million Americans who are allergic to poison oak, poison ivy, and poison sumac are so highly sensitive that they break out in a rash and begin to swell in 4 to 12 hours instead of the normal 24 to 48. Their eyes may swell shut and blisters may erupt on their skin. This is one of the few true emergencies in dermatolgy says William L. Epstein, MD. Get to a hosipital as soon as possible. A shot of corticosteroids will bring the swelling down.