PFAS, also called oral allergy syndrome, occurs when pollen sensitive individuals eat raw fruits, vegetables, or some tree nuts. It happens because the offending protein in the pollen is also found in the food (or something that closely resembles it). Thus, eating the food sets off a reaction because the immune system thinks it's found the pollen it's sensitized to. Cooking the food changes the protein structure so sensitive people can usually eat the food without suffering a reaction.
Dr. Teryl Boothe and selected guests.