"The sap causes immediate primary irritation of the skin, the degree of burning and irritation depending on the amount of oil involved. Meanwhile, the resin quickly penetrates the skin and triggers an allergic reaction. It penetrates thin skin (eyelids, between the fingers and toes, the back of the knees) most rapidly, and these areas may remain highly sensitive to urushiol for up to one year. Even minute amounts of the oil may trigger flare-ups weeks after the rash is healed. Once the sap of the poison ivy plant touches your skin, you become an allergic time bomb. If the resin isn't washed off within 5 to 10 minutes, it's only a matter of hours (usually 24 to 72) before you break out in a rash. Any area that comes into contact with the resin will react, except the mucous membranes, such as the lips, mouth, and inside of the nose. (There's always the exception that proves the rule. A doctor in Washington reported recently that he developed poison oak urethritis after clearing brush.) The rash starts off as red, swollen patches, with a few small fluid-filled blisters. As the reaction intensifies, the blisters become larger, and then break down and weep. The whole area becomes covered with an oozing, scaling crust."