DIGESTION AND ALLERGIC REACTIONS IN THE INTESTINE

Abstract

The physiological need for nutrient absorption requires the gut to have a large surface area, lined by a single layer of columnar epithelium. This means that the immune system of the gut is constantly bombarded by foreign proteins (food), while at the same time, the immune system is specifically designed to react to the foreign proteins of pathogens. To get around this problem, the gut immune system has acquired mechanisms to avoid excessive reactions to food, yet at the same time retain the ability to react to infectious agents. This system works efficiently in most individuals, but for an as-yet undefined reason, some people react to food antigens as though they were pathogens, and since food antigens are needed for nutrition, disease persists until the offending antigen is identified and eliminated from the diet. The clinical features of inappropriate immunological responses to food, including eczema, urticaria, vomiting, diarrhea, bloody stool, and failure to thrive, are defined as food allergies.
Ingested food is normally digested into peptides and amino acids by digestive enzymes before being absorbed by gut epithelial cells. Amino acids do not induce immunological reactions because they are too small to be recognized by antigen-presenting cells (APC). However, some proteins are poorly digested in the intestinal lumen and so intact macromolecules can be detected by immune cells and recognized as antigens.
In infants, non-specific defenses in the gut may be compromised. First, both intracellular and paracellular transport of macromolecules through the epithelium is increased. Second, insufficient secretion of digestive enzymes causes reduced digestion and increases the amount of undigested proteins in the intestinal lumen. Furthermore, frequent infections activate mucosal immune cells and induce further reactions.
Meanwhile, there is a function called “induction of tolerance”. TGF-β is an important cytokine that mediates active suppression against orally administered antigens, allowing the induction of non-responsiveness against transmitted antigens in breast milk. Lack of co-stimulatory and/or adhesion molecules on the surface of lymphocytes may partially explain the unresponsiveness of T cells to food, especially during infancy.
Identification and elimination of the proteins that cause these adverse reactions improves the condition of these patients. Elimination of the offending food from the diet should prevent further allergic reactions. Moreover, oral tolerance can be introduced only by continuing to eat a small amount of antigenic food after determining the amount and the way to eat such food after challenge tests, which we call Specific Oral Tolerance Induction (SOTI). Since challenge tests and SOTI can be dangerous and may introduce shock, these procedures should be performed under intensive surveillance by experienced doctors.