I would estimate I've had poison ivy over a 100 times. Most of these have been very minor, but I've had a few bad ones. I've done a lot of reading on the subject.
My understanding is the extract doesn't work, but who knows?
My understanding is that the more you are exposed to it, the more likely you are to get it. Getting exposed to it a lot doesn't help. If you stay away from it for years, you can gain some imunity back. Here is a link, look towards the bottom of the page for the myths.
Poison Ivy, Oak, & Sumac - Fast Facts - (www.poisonivy.us)