Serotonin in seed shells (think anaphylactic) would trigger an immune system response because its how they have evolved in order to be pooped out by animals, serotonin irritates the digestive system and aids fast passage, ergo the seed is spread further afield.

Fruits could, but its really a good idea to quantify the chemicals in each and their subsequent response, because the phrase "trigger an immune response" is so vague its like trying to quantify legislation.