Fantastic article explaining some of the science: http://www.westonaprice.org/food-features/nightshades The poisonous compounds in and drugs from nightshades include lycopene, calcitriol, solanine, nicotine, atropine, scopolamine, hyoscyamine and capsaicin. excerpts: regarding potatoes “Potatoes naturally produce solanine and chaconine, a related glycoalkaloid, as a defense mechanism against insects, disease and predators (humans included). Potato   Read More ...

In general, “Nightshades” are anything in the family Solanaceae. They contain neurotoxins for which most people naturally create an anti-toxin. Someone with a nightshade allergy doesn’t create the anti-toxin. The toxin then builds up in the body and  causes an enormous amount of problems, which can differ from person to   Read More ...