Why this happens is still largely shrouded in mystery, but we do have a pretty good idea of how it happens.

First, if foods are introduced too early into a child' s diet, their immature immune systems can mistake innocuous foods for harmful invaders and set off an immune response to protect them from the ' attack' .

The resulting symptoms are the allergic reaction. It' s no coincidence that dairy products and wheat, two of the most common foods fed to infants (as formula and rusks), cause problems in many of them.

Trouble can also hit further along the road. Some adults can eat certain foods for years with no problem, then suddenly start reacting to them. This can hap- pen for a variety of reasons.

Perhaps they' ve eaten too much of a given food - not a rare occurrence these days with wheat, for example.

Just think how many people start the day with toast or wheat-based cereal, grab a sandwich for lunch and then settle down to a plate of pasta for dinner! Also, as we age, our stom- ach acid levels fall and we tend not to chew and digest our food so well. Our digestive system is meant to break food down into molecules, which can be read- ily used by the body. Poor digestion, however, means fragments of food can enter the bloodstream, where they' re treated as hostile invaders by our immune system' s scout cells.

The same thing can happen if you have a ' leaky' gut, where the intestinal tract lining has become too permeable to do its job properly.

Stress, nutrient deficiencies, alcohol and some drugs can all contribute to this condition.

In all these ways, your body can become sensitive to certain foods, and continuing to eat them will mean your body goes on eliciting the same reaction due to its immunological memory.