YES, you can get a systemic reaction to poison oak and ivy if the oils enter your blood or lymph system. The best systemic reaction description I've read is in the Zanfel product literature:
- The rash appears in new areas without additional exposure after more than four days
- Breakouts over more than 30% of your body
- Swelling joints
Zanfel will work to treat the itch temporarily; steroids appear to be the only "cure" for systemic symptoms in the long run. Otherwise, systemic cases can last months.