It builds up in your system over time too....
I never used to be allergic to silky oak (Grevillea robusta) but a lot of people are. I used to go in and cut them down when other tree workers were allergic to them, but I recently did a big one and had a bad reaction. Swelling, rash, itching, constricted throat, etc etc... ended up going to hospital and getting some drugs to deal with it. I won't be doing any more silky oaks though.
If it's just a rash, an immuno suppressant combined with an anti imflammatory/anti hystamine will see you through, but think twice before going after another black walnut. If you have any difficulty breathing, or even start to feel your throat is closing up then get to an emergency room quick smart, preferrably have someone drive you there. That kind of reaction can kill you.
Shaun