I ate about 1 teaspoon. Within 30 mins - 2 hours, started feeling symptoms: itchy and red all over, slightly nauseous and generally feeling ill. Back was covered in red. Was told it "looked like hamburger" - due to red raised spots.
Started treatment right away with a 100 g bottle of pure ascorbic acid crystals. Applied powder with a little moisture to affected skin areas, while drinking water with capfuls of ascorbic acid crystals dissolved in it.
Repeated this treatment for 20 minutes - 1 hour until the full 100 g bottle was finished. Result was I was back to normal, and so was my skin and feeling fine with no more itching. Red and raised bumps faded.
2) Went on a shopping trip. Ate a grape at a grocery store. Got on the bus to go to another store. Approx 20 minutes after eating the grape, my feet started itching, badly. Instantly recognised it as an allergic reaction. Started itching in my mouth shortly after. Immediately went home and started Ascorbic Acid treatment, which relieved itching and red bumpy skin like the first treatment described above did.
2.1) Researched possible links between pistachios and grapes. I had eaten grapes and pistachios quite often before, in various forms: pistachio- baklava, Indian fudge, whole nuts. Grapes - as fruit, as jam, juice and wine. Never had before the slightest reaction.
3) 6 months later went on a shopping trip. Ate 2 or 3 organic grapes from Metro. 20 minutes later when we arrived home, felt ill and threw up. Felt better and took C.
4) Wild grapes growing nearby in our neighborhood. Ate a few. No reaction whatsoever. Will still conibue to eat wild grapes occasionally from same spot.
Neighbour has tons of wild grapes growing. We picked bags full and made grape juice which we turned into various jelly desserts. Ate all of them and was fine.