I have to be very careful around any of that stuff (ivy, oak, sumac). It seems like I don't even have to touch it and I break out.
In addition to Calamine lotion, oatmeal baths helped me when I got into that stuff all over. Works on poison oak and sumac as well. It's a cheap and easy way to relieve the itch.
Blend about a cup of oatmeal (instant, quick oats, slow cooking oats, doesn't matter) on the highest setting until it's a fine powder. You can test your blend by stirring a tablespoon of it in a glass of warm water. If the oats readily absorb the water and give it a milky look and a silky feel, you've blended long enough.
Sprinkle the blended oats into a tub of running water and stir the water with your hand several times to ensure even distribution. Feel along the bottom of the tub for clumps and break up any you find.
BE CAREFUL! The tub will be quite slippery when getting in.
Soak in the tub for 15-20 minutes and pat dry with a soft towel rather than rubbing. You can do this several times a day if you need.
Good Luck.