III Innate and Adaptive Immune Cells: General Introduction

Iván López-Expósito

As an organ specialized in food digestion and nutrient absorption, the intestinal mucosa presents a huge surface area (almost 300 m2 in comparison with 2 m2 in skin) to the outside milieu and is continually exposed to foreign antigens derived from dietary constituents and the large number of microbes that reside within the intestinal lumen. In order to maintain the intestinal integrity, it is crucial to possess a fully functional associated immune system able to respond appropriately to such antigens and also to generate protective immunity to potential pathogens that employ the intestine as a primary site of entry and infection. Inappropriate responses to such antigens, apart from infections, are thought to underlie several intestinal pathologies including inflammatory bowel disease as well as food allergies (Bekiaris et al. 2014). Cells from both the innate and adaptive immune system can be found throughout the intestinal mucosa working together cooperatively with other cells and molecules in order to maintain intestinal functionality. Innate immunity provides effective initial defense mechanisms that take place even before infection and are poised to respond rapidly to microbes. These mechanisms react only to microbes and products of injured cells, and they are specific for structures that are common to a group of related microbes, not being able to distinguish fine differences between foreign substances. On the contrary, adaptive immune responses comprise responses that are stimulated by exposure to antigens of both microbial and non microbial origin and that increase in magnitude and defensive capabilities with each successive exposure. The main characteristics of adaptive immunity are a very high specificity for distinct molecules and the ability to “remember” and respond more vigorously to repeated exposures to the same antigen (Abbas et al. 2007).

When the immune response is triggered, a wide variety of cells from both the immune system and other tissues participate. These include epithelial and endothelial cells, neutrophils, monocytes, macrophages, dendritic cells, mast cells, natural killer cells, basophils, eosinophils, B and T cells. All of them have the ability to secrete a wide array of mediators responsible in part for their inflammatory effects (Si-Tahar et al. 2009). Based on the number of publications dealing with the immunomodulant/anti-inflammatory properties of food-derived compounds with bioactive properties, only monocytes, macrophages, dendritic cells, PBMCs and T cells will be covered in this part.

 
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