When your immune system thinks something in a specific food is a danger to your body, it triggers a response. In most cases, it’s a protein in the food that the body thinks is harmful. The response can affect the respiratory tract, cardiovascular system, gastrointestinal tract, and skin.Hives are the most common symptom of a food allergy. That’s because your skin contains something called mast cells, which are part of the immune system. When the mast cells become active, they release histamine, which causes hives to form.