Nightshades can cause inflammation and set off immune system responses that can result in flares for people with an autoimmune disease.
If you think that nightshades might be causing some issues for you, try removing them from your diet. The most common nightshades in the American diet are potatoes (found in chips, fries, gluten-free bread) and tomatoes (found in sauce, soups, juice). To help you, here is a list of other nightshades.
Members of the nightshade family can also be found in many condiments and seasonings:
Note that diligent reading of food labels and ingredient lists is necessary here. Many common packaged foods and flavorings contain nightshades.