Oral allergy syndrome

An unusual, little known type of food allergy. Proteins in certain foods, usually fresh fruits and vegetables, can cross react with pollen proteins in patients with allergic rhinitis (aka "hay fever") and cause symptoms like oral itching.

This article does a good job summarizing the phenomena.

I am surprised how often I ask patients with allergic rhinitis is they get an itchy mouth with certain foods and they look dumbfounded and wonder how I knew that.

Cooking usually denatures the protein, so patients can often tolerate cherry pie but not cherries. Luckily, in general the symptoms are self-limited and patients can just avoid the food that gives them trouble.