re: Poison Ivy Contagious Period

Poison ivy and the like are type 4 hypersensitivity rxn which basically means a cell mediated response. So, all those times you "weren't allergic" were just times when your body was building a response and expanding the population of cells that respond to the exposure and on subsequent exposure to the oil you got the rxn. So you're never not allergic, you just haven't had a response yet.