Does the quantity you ingest make a difference? Or do you get the same reaction, regardless. For example, eating babaganouj (eggplant puree) v.s. a couple of pieces of eggplant in a rice dish.
I'm interested in this, too. From what I've read, you can be sensitive to a chemical in nightshades called solanine and I believe that can build up.
In our case, our baby was fine with tomatoes and potatoes, but in the immediate days following an allergic reaction to eggplant, she suddenly became sensitive to tomatoes and potatoes. We've done allergy testing now and she's not IgE for any nightshades. So I'm wondering if solanine is the culprit. We haven't yet tried her on any nightshades again, and are planning on doing so in the coming weeks.