**1. Introduction**

There is a tremendous interest in the understanding of immune response against pathogens and toxins on the gastrointestinal tract (GIT) of the birds due to in this specialized system, as it harbors 70 to 80 percent of the avian immune cells and molecules [1]. Additionally, there is an overwhelming interest in finding new alternatives to antibiotic growth promoters (AGP) because of regulations and consumer preference in many countries which these strategies are banned or regulated. The mucosal surface of the GIT is covered by a monolayer of columnar epithelial cells. This epithelium represents a vast surface that is vulnerable to foreign immunogens (i.e. food-borne antigens), microbial pathogens and toxins. By being in contact with a large number of potentially harmful substances and infectious organisms, the mucosal surface must provide a means to not only regulate active and passive absorption of macromolecules but also provide as a general and selective defenses

in part through secretory antibodies and other mucosal defense mechanisms. Consistent with these functions, the epithelial surface of the GIT is lubricated and protected by mucus secretion and by a highly specialized immune system underlying the epithelium which exports immunoglobulins into the intestinal mucosa. Secretory defenses are some of the most important means to protect the intestinal epithelium from enteric pathogens and toxins. Secretory IgA (sIgA) production, Goblet, Paneth, M cells and GALT tissues are the key cells in this defense. The objective of this review is to describe a variety of secretory immune responses against pathogens in GIT and the role of nutrients in immunomodulation.
