In 1994 I was diagnosed with celiac disease, which led me to create Celiac.com in 1995. I created this site for a single purpose: To help as many people as possible with celiac disease get diagnosed so they can begin to live happy, healthy gluten-free lives. Celiac.com was the first site on the Internet dedicated solely to celiac disease. In 1998 I founded The Gluten-Free Mall, Your Special Diet Superstore!, and I am the co-author of the book Cereal Killers, and founder and publisher of Journal of Gluten Sensitivity.
Breast milk contains antibodies against all the antigens the mothers immune system has met prior to or during the pregnancy and has produced antibodies to them.
This system is the wisdom of nature and this is the way that mothers milk protects babies from all the antigens (infectious agents, toxins etc.) occurring in the environment where the mother lives. These antigens without this protection may enter the body through the digestive or respiratory systems. The best example is that breast milk protects babies from bacteria causing diarrheas in the underdeveloped countries.
The antibodies are produced by the cells (plasma cells) localized in the gut and the lung. These cells are migrating to the lactating breast-tissue for hormonal trigger (enteromammal plasma cell circle) and they continue producing these antibodies in the breast. These antibodies appear in the breast milk. In brief, the breast milk may contain all the antibodies the mother has in her digestive and respiratory systems. The function of these antibodies is to block the entrance of antigens infectious agents, toxins, allergens etc) across the digestive or respiratory tract of babies.
In case of CD, it means that if the mother has circulating antibodies to gliadin, these antibodies appear in the milk. If the breast fed baby ingests gliadin (or the mother ingests accidentally and traces of gliadin appear in the milk) the antibodies in the milk blocks the gliadin and it will not able to cross the intestinal wall and meet with the babys immunosystem. Theoretically, the breast-fed infant do not have any immunoreaction to gliadin. If the mother accidentally ingests gliadin during breast feeding it is likely that the concentration of antigliadin antibodies become higher in the breast milk.
To answer the question: the antibodies in breast milk are protective and do not "trigger" celiac disease in genetically predisposed babies. There are several data showing that breast-feeding has a protective effect in case of celiac disease. Furthermore, it is well documented that breast-feeding in the first year of life decreases the risk of allergies by 50% in babies whose parents have allergies.
As far as the reaction after weaning concerned: it is also known that babies may have some reaction (loose stool or spit up or discomfort) transiently after introducing a new food, however, this is a temporary symptom and not allergy or immunoreaction to the food. It is likely that their digestive system should accommodate to the new foods.