My teenage daughter developed peanut allergies in her mid teens while traveling with a drum corps one summer. She had noticed how ill she was feeling and systematically cut out foods until she narrowed the peanut butter sandwiches (she had been an avid peanut butter eater for years before). When she got back from tour and was tested she was confirmed for strong peanut allergies and had to carry an epi-pen. However, after about a year she got tested again and now had no reaction.

I still attribute this to something else was in her system that triggered reaction in conjunction with the peanut proteins.