Hi Sarah,
you correctly discriminate "poisoning" by altered or foreign substances from the "conditioned trigger" reaction in allergies.
We can consider signaling and its biochemistry. [https://www.naturessunshine.com/blog/en/10-high-histamine-foods/|Histamine] is involved in the "alarm triggering" mechanism, so substances that contain more of this create a flare-up rather than substances with no histamine in. Not many react to lettuce but it's possible to be sensitive to most kinds of food!