Answer: Proteins

When someone tells you they’re allergic to bananas, peanuts, shellfish, or any number of foods commonly associated with allergic reactions, what they’re really allergic to is one or more food proteins within that particular food. Exposure to those proteins causes an immune system response that, in the most mild forms, causes mere discomfort like itchy skin or an upset stomach, and, in the most extreme forms, causes swelling of the airways and potential death.

Further compounding the problem is the inherently unstable nature of food proteins. Many people are allergic to foods in their raw form (such as drinking a glass of milk or eating a certain kind of fruit), but are not as likely to be allergic to the same foodstuff when it has been cooked (such as milk used in baked goods or fruit that has been turned into jelly). This is because many food proteins that trigger allergic responses are altered, or denatured during the cooking process and the denatured form of the protein does not trigger an allergic response in some patients.