Hi Belinda, 
There is an excellent explanation of leaky gut in this month's Scientific
American, by Alessio Fasano, head of pediatric gastroenterology and
nutrition at U. Maryland, Baltimore. The article is on page 54 and is titled
"Surprises from Celiac Disease". 

Food allergies occur when there is a leaky gut and undigested or partly
digested food particles pass through the weakened intestinal barrier and
into the bloodstream. These foreign proteins or partial proteins are
recognized by the immune system as non-self. The immune system mounts an
immune response as if these foreign proteins or peptides were invading
microbes. That is a food allergy. It is also the starting point for
autoimmune disease, but that's another issue. 

Best Wishes, 
Ron