What causes an allergic reaction to certain types of natural on processed foods such as peanuts?

Along with peanuts and shellfish, tree nuts are one of the food allergens most often linked to anaphylaxis — a serious, rapid-onset allergic reaction that may be ... to determine the presence ofallergen-specific immunoglobulin E, an antibody that binds to allergens and triggers the release of chemicals that cause symptoms. 
FOUND IN GOOGLE

Usually they are affected by a certain PROTEIN in a food or ingredient, such as peanuts and/or shellfish.