Food sensitivities vs food allergies
For some people, certain foods are perceived as foreign invaders in the body. When these foods are consumed, the immune system mounts an attack by releasing antibodies to deal with them. In the case of a food allergy, IgE is released (Immunglobulin E). In the case of a food sensitivity (sometimes called an intolerance), IgG is released (Immunoglobulin G). It's these antibodies that allergy and food sensitivity tests look for.
This sounds confusing, but the difference between these two immune responses is very apparent once you've learned it. A great mnemonic is this: