Immunophysiology and nutrition of the gut.

The intestinal surface comes into contact with a continuous fl ux of a vast variety of antigens. The primary function of the intestinal mucosa is absorption of nutrients. This function necessitates the transport of molecules across a single layer of epithelial cells and distinguishes the intestinal mucosa functionally from that of the respiratory or genitourinary tracts. The intestinal immune system must be able to recognize potential pathogens and inhibit their adherence and invasion by protective immune exclusion or infl ammatory responsiveness while allowing uptake and transport of dietary compounds and certain intact macromolecules (eg, growth factors and maternal immunoglobulins in breast milk) without inappropriate and detrimental immune reactions. Appreciation of this intricate balance is paramount to our understanding of intestinal immunophysiology. The intestinal immune system has unique have evolved to fulfi ll the dual task of immune defense and tolerance. An overview of the intestinal immune system with emphasis on immune responses directed toward dietary antigens is provided in the fi rst half of this chapter. The establishment and maintenance of nonresponsiveness or tolerance to dietary and commensal microbial antigens is the result of active and coordinated immune responses which mature postnatally upon contact with foreign antigens, particularly colonizing bacteria. The indigenous intestinal microbiota is established during the same period and commensal bacteria have a profound impact on the structural and functional maturation of the gut. Indeed, the intestinal microbiota may be considered an essential component of the intestinal immune system (Figure 1). The second half of this chapter is dedicated to the effects of nutritional factors to gut immune physiology. Malnutrition is still a major threat to infant and child health in large portions of the world. The effects of protein and energy malnutrition as well as the lack of certain individual nutrients on gut immunophysiology are overviewed in this chapter but discussed in detail in Chapter 8, “Trace Elements,” Chapter 9, “Iron,” Chapter 10, “Vitamins,” and Chapter 13, “Protein-Energy Malnutrition: Pathophysiology, Clinical Consequences, and Treatment.” The increasing evidence from clinical and experimental studies indicating that the route of nutrition, that is, enteral versus parenteral nutrition, affects intestinal immune functions and therefore child health is then discussed. In the fi nal paragraphs, an overview of novel nutritional approaches aiming to enhance intestinal and systemic immune functions, namely probiotics and prebiotics, is given.