According to the Centers for Disease Control, as of 2006, 33.1% of women were choosing to exclusively breastfeed their newborn from 0-3 months of age. At the one-year mark, only 22.7% of women were still breastfeeding their baby (non-exclusively).
The American Academy of Pediatrics Committee, the World Health Organization (WHO), the Canadian Pediatric Society, the Pediatric Society of New Zealand, and other similar organizations in various countries worldwide have all made statements on infant feeding and the appropriate time to introduce solid foods into a baby's diet. The current consensus is that solid food should not be introduced until at least the age of 4-6 months, if not later.
Before this age, the epithelial lining of a child's stomach is not developed enough to digest solid foods. This creates a leaky gut condition where particles of solid foods can leak from the gut, undigested, into the child's bloodstream. Once foreign (food) particles are present in the bloodstream, the body begins to develop antibodies to those particles. These antibodies initiate the food allergy reaction and response (physically, behaviorally, mentally). Breastfeeding Anthropologist, Katherine Dettwyler, notes that,
"Nonhuman primates and children in traditional cultures worldwide normally experience several years of a transitional diet, with steadily increasing amounts of solid foods in addition to breast milk. The breast milk component of the diet continues to provide an excellent, uncontaminated source of protein as well as of immunological factors, and may be the only food the child desires or can tolerate during illnesses."
If a still-developing child is regularly eating a food that is especially damaging to the gut (even in healthy adults), the chances of developing antibodies to such common foods increases. Gluten is especially destructive to many people's gut lining, and because it is often found in infant foods and cereals, infants who are not breastfed are being exposed to large doses of this hard-to-digest grain protein, and at younger and younger ages. Such exposure can affect the rate at which a child develops a food allergy or even Celiac Disease Autoimmunity (CDA), as a 2005 study from JAMA indicates:
"...children exposed to gluten-containing food products) in the first three months of life had a fivefold increased risk of CDA, compared with children exposed to gluten-containing foods at four to six months", and, "[the] risk of CDA was marginally increased for children not exposed to gluten until the seventh month or later."
These findings indicate that breastfeeding an infant for as long as possible, while their gut and epithelial lining are still developing, lowers their chances of developing autoimmune (allergic) responses to food, based on early exposure.