The mind is made up of two distinct parts; the conscious mind and the sub-conscious mind. These two parts of the mind are very different to each other. The conscious mind is the rational, discerning portion of the mind and the sub-conscious portion is the emotional, protective mind. The protective portion of the mind will protect us against everything, real or imagined. If the sub-conscious mind perceives a threat while it is experiencing a particular experience it may begin to identify any similar experience as a threat. In this way certain experiences become "triggers", creating allergic reactions to substances or situations that are normally considered harmless.
Imagine a child having recently been discharged from hospital sneezes loudly while playing with her pet dog. The child notices the worried expressions her parents faces having seen her sneeze and she becomes afraid of being sent back to hospital because for being ill. This situation has inadvertently sensitised the little girl's protective sub-conscious mind against playing with dogs and from this time on, each time the child plays with a dog she has a sneezing fit. This sneezing fit is identified as an allergy. Later this allergy spreads to other pets such as cats.
Using hypnosis, it's possible to rapidly retrain the sub-conscious mind so that it no longer perceives specific situations as a threat to the point that allergies are no longer a concern.