I like that idea a lot! For all I'm sure Alternia has its own horrible, rash-inducing plants, he wouldn't be too familiar with poison ivy, let alone how to treat it. ALL THAT ITCHING SURE IS FUN.

(For all Karkat is a different species, my idea of 'reaction vs no reaction' has bent towards 'which is funnier?' and Karkat miserably and itchy fits that option best.)