NAET Theory
All things living and non-living have their own energy field. Sometimes, when a person comes in contact with an item for which they are not compatible, their energy field clashes with the energy field of that substance.
This interaction can cause blockages of energy in the subjects meridian system (the electromagnetic pathways that course through our bodies). These meridians are related to various bodily organs and systems. Blockages in these meridians can affect the organs/systems which they are related.
When this happens, the brain is alerted via the nervous system and the immune system is enacted, giving rise to a reaction/symptom. For example, if a person is not compatible to an apple and blockages of energy form in the lung meridian the reaction could be a cough or asthma attack.