Tomatoes are one of the most commonly reported eczema triggers, but they work through chemical irritation rather than a classic immune-system allergy. They cause mast cells (immune cells in the skin) to release histamine directly, making skin itchy and inflamed — a landmark study found that 76% of people with chronic skin reactions responded to whole tomato in testing. They also contain salicylates (aspirin-like compounds) and a natural compound called alpha-tomatine that can irritate the gut lining.

Canned and concentrated tomato products (paste, ketchup, jarred sauce) are more problematic than fresh because processing increases histamine levels. Peeling fresh tomatoes reduces allergen load, since the compounds are concentrated in the skin. The reaction typically takes 2–4 hours to develop and is dose-dependent, which makes tomato particularly difficult to identify as a trigger.