Intestinal epithelial cells are the cell boundary between the external environment and tissues of the gastrointestinal tract. From this interface the epithelial cells have evolved processes that help guide whether inflammation or an immune response will occur in the intestines. Indeed intestinal epithelial cells are active participants in the inflammatory and immune response because they (1) secrete inflammatory mediators, including cytokines, when they detect certain microbes; (2) secrete mediators when they become detached from the basement membrane; (3) recruit and engage dendritic cells, professional antigen-presenting cells which sample the intestinal lumen prior to presenting antigens to lymphocytes and (4) directly present antigens to T lymphocytes using a unique repertoire of molecules. Antigen presentation to lymphocytes is required for the initiation of a specific adaptive immune response, whether humoral (antibody) or cellular, and may be key to explaining many human chronic inflammatory diseases.

Key concepts

• Intestinal epithelial cells form a monolayer of cells that define the boundary between tissues of the intestine and the external environment.
  
• From this point of interface, intestinal epithelial cells receive and interpret insults/assaults from the intestinal lumen and signal to the tissues whether inflammation will ensue.
  
• Intestinal epithelial cells communicate through the secretion of protein and glycoprotein mediators, some of which launch the inflammatory response including by recruiting leucocytes into the tissues.
  
• Intestinal epithelial cells interact with and control lymphocyte activation using specialized molecules, some of which are used in antigen presentation.
  
• Antigen presentation to lymphocytes is critical to launching the adaptive (humoral and cellular) immune responses.
  
• Antigen presentation by intestinal epithelial cells most commonly results in the inhibition of activation of T lymphocytes, including CD8+ T lymphocytes.