I've heard about people who weren't allergic to peanut butter or pollen previously in their lives, but suddenly develop an allergy to that substance. What is the mechanism behind an allergy "suddenly" appearing later in life?
From the MayoClinic, it isn't clear why adult allergies develop: http://www.mayoclinic.org/diseases-conditions/food-allergy/expert-answers/food-allergy/faq-20058483
However, there is evidence that food sensitivities develop when someone has a compromised gut. If the gut is compromised, undigested food particles "leak" into the blood stream and the body creates antibodies against this undigested food. These sensitivities can disappear if the gut is repaired. http://www.holistichelp.net/blog/why-do-food-sensitivities-develop-and-spread/