Hmm. I did some research on nightshade allergies a while back, and IIRC tomatoes, potatoes, and eggplant all contain a chemical that potatoes do not. So, you may be able to process potato but not the others. (And I think eggplant has it in small quantities than pepper or tomato)

Also, as an aside, if you have a blarg reaction to nightshades you may wish to keep in mind that tobacco is a nightshade and avoid second-hand smoke accordingly. :)