Phytoalexins are low molecular weight compounds produced in plants as a defense mechanism against microorganisms. They do, however, exhibit toxicity to humans and other animals in addition to microbes. Coumarins, glycoalkaloids, isocoumarins, isoflavonoids, linear furanocoumarins, stilbenes, and terpenes all fall into the category of phytoalexins
Because phytoalexins are natural components of plants, and because their concentration may increase as a response to production and management stimuli, it is useful to recognize the possible effects of phytoalexins in the human diet.
Linear furanocoumarins are potent photosensitizing agents in celery, parsley, parsnips, limes, and figs.
The most commonly reported symptoms include contact dermatitis and photodermatitis, particularly on the hands and fore arms .
Many linear furanocoumarins are present in these plantmaterials, all seem stable to cooking temperatures, and a few have even been reported to be mildly mutagenic .
Coumarin 13 and its derivatives are also present in many species of citrus, occurring primarily in the peel, and hence find their way into human diets largely through the use of cold-pressed citrus-peel oils which are used as flavoring agents .
Both linear furanocoumarins and coumarins are widely distributed and possess toxic properties, reported cases of human toxicity have been limited largely to contact dermatitis in individuals handling large quantities of plants containing these compounds. The chronic low level consumption of these two classes of compounds, as would occur in a normal varied diet, has not been conclusively associated with human illness.