Grapes contain a heat-stable protein called LTP (lipid transfer protein) that is resistant to cooking, digestion, and processing — meaning wine, raisins, grape juice, and balsamic vinegar all retain it. People with grape allergy are typically part of a broader 'LTP syndrome' where the primary sensitiser is peach, and the immune system then cross-reacts with similar LTP proteins in grapes, apples, cherries, hazelnuts, and other foods. This syndrome is most common in Southern Europe.

Grapes also contain salicylates (aspirin-like compounds). For most people with eczema, grapes are not a significant trigger. A useful diagnostic clue: if wine triggers a reaction but fresh grapes do not, the histamine, sulfites, or alcohol in wine — not the grape protein itself — are the likely cause.