Food sensitivities develop over time after the GI tract begins to break down (becomes "leaky") and poorly digested food particles enter into the lumen of the intestine. The immune system identifies these leaked food particles as foreign invaders and marks them with an antibody, much like a flag, so they can be eliminated. Since the immune system is highly specific, the next time the offending food is consumed, the immune system is activated.
This process is different from having a food allergy (true genetic allergy or IgE-mediated response) but the offending foods must be identified and removed from the diet for a period of time while the gut is repaired. One of the most common symptoms associated with food sensitivities is headaches and/or migraines.
My Food Sensitivities - 90 Foods is our most comprehensive evaluation for a broad range of foods consumed in standard diets so offending foods can be properly identified.