Milk and nuts. Patients can become allergic to chocolate. Also, besides peanut, milk and tree nuts are often added to chocolate candies.
Absolutely. Yes, as my colleagues have shared, chocolate contains many allergens. The one they didn't mention could be the most surprising of all: cockroach parts. Most chocolate bars contain at least 8. The fda rejects anything greater than 60 insect pieces/100 grams of chocolate. They can cause allergies. http://abcnews.go.com/blogs/lifestyle/2012/03/bugging-out-chocolate-allergy-linked-to-roaches/.
Yes. Most allergens are proteins (as opposed to polysaccharides or lipids). So most foods including chocolate are allergens and can cause allergic reactions.
Yes. Chocolate is often contaminated with peanut due to being packaged in plants where they also package peanut containing candies. If you are peanut allergic, never eat "grocery store aisle" chocolates.
Allergen evrywhere. Yes, all foods drugs etc have allergens in them. Luckily not a lot of people react 2 thm.