Avoidance is the best prevention against the rash we get from coming in contact with poison ivy or its cousins, poison oak and poison sumac.
We are not born with an allergy to the oil in the poison ivy/oak/sumac leaft. After exposure to the oil, our body develops an allergy. This may take as little as one exposure or many exposures. Because it can take several exposures, you will meet people who brag that they can roll around in it and it does not affect them. (FYI- not the best idea!)
Some homeopathic physicians and naturopaths claim that you can become desensitized to poison ivy by taking small doses of it by mouth. These preparations are not standardized and could cause a severe allergic reaction. I do not recommend them. Besides, the poison ivy oil would travel all the way through your GI system. All the way. Think about that before you put it in your mouth.
Since the oil causes the rash, as soon as you wash the skin with soap and water, the oil is gone. After washing, you cannot “spread” the rash by scratching. The rash breaks out first in areas that had a large amount of the oil deposited, or where it stayed on the skin for a longer time before being washed off, it takes longer for it to break out in areas with little dose or a short contact time. Thus, it may appear that the rash is “spreading.” The liquid in the blisters is your own serum (like the plasma portion of blood) – it is not infectious or allergenic.
After working in the yard, or being out in the woods, make sure that you wash all your tools and clothing that was exposed to the poison ivy – the oil can remain on them for a long time and cause a rash next time you handle them.
Also, be aware that your dog may come in contact with the plant and carry the oil on his coat and expose you to it.