**Abstract**

The Gastrointestinal tract (GIT) is a very complex environment which converges a lot of players including nutrients, microorganisms, pathogens, cells, and peptides which determine the type of outcome against threats affecting feed efficiency and body weight gain. Traditionally, GIT is examined as a selective barrier which permit or deny the entrance of molecules, but beyond of that, it is a crucial place to produce important proteins for the host which, at least in part, determine the outcome for a threat such as microorganisms, toxins, anti-nutritional factors, among others. During the non-antibiotic promotants time, there is a necessity to understand how this system works and how we can modulate through nutrition, in part to take advantage of this, and support a better immune response and nutrient absorption in challenged poultry environments. The goal of this chapter is to review the different mechanism of immunity in the GIT emphasizing on secretory defense response and the nutritional strategies including fiber and fatty acids to improve it.

**Keywords:** antibiotic growth promotants, mucins, M cells, paneth cells, goblet cells, host defense peptides, trefoil factors, IgA, dietary fiber short chain fatty acids, medium chain fatty acids
