Most infants are immunologically active and are able to develop a tolerance to oligoclonal antigens by producing IgA, along with activation of regulatory T cells, in early infancy. Cytokines and their signaling molecules are important mediators in the intestine, regulating both oral tolerance and mucosal inflammation. This system works efficiently in most individuals, but for an as yet undefined reason, some people react to food and other proteins as though they were pathogens, with induction of chronic inflammation in the mucosa. The adverse reaction caused by ingested foods is defined as food intolerance. The clinical features of food intolerance include vomiting, diarrhea, bloody stool, eczema, failure to thrive, and a protean range of other symptoms. Intolerance can be divided into two categories depending on whether or not they are immunologically mediated. Food intolerance and mucosal inflammation are deeply related because tolerance cannot be established when there is an inflammation in the intestinal mucosa. Mast cells, eosinophils, mucosal lymphocytes, and epithelial cells are deeply involved and related to each other in the development of mucosal inflammation. Meanwhile, rectal bleeding in infancy is related to lymphoid hyperplasia with eosinophil infiltration into the colonic mucosa facilitated by C-C motif ligand 11 (CCL11, known as eotaxin-1) and C-X-C motif chemokine ligand 13 (CXCL13). Rectal bleeding in infancy may not be simply caused by allergic reactions against specific antigens, but may be due to migrated lymphocytes developing immunological tolerance; including IgA synthesizing, in the intestinal mucosa.
