For those interested, here are links to some Wikipedia pages about nightshades (aka Solanaceae), a plant family that includes tomatos, potatoes, green peppers and other less known plants:

http://en.wikipedia.org/wiki/Solanaceae

http://en.wikipedia.org/wiki/Nightshade (more technical info)

These are mentionned in the UltraSimple diet book as being a food family to be wary of, as they may cause delayed allergies. Thay are excluded in the U-S diet, and reintroduced afterward (that's the phase I'm currently in).  I am not done with the reintroduction process yet, but I have reason to believe that these foods may have been causing me problems.

BTW, the term "nightshade" in itself is often used to designate a plant bearing poisonous black shiny berries found in the wild. Other intesting note, tobacco is also part of this family, which seems to have lots of members... we know about tobacco's often unhealthy properties.