Sel’skokhozyaistvennaya Biologiya [Agricultural Biology], 2013, № 4, p. 3-25 UDC 636.5:591.1:577.27 GUT IMMUNITY IN BIRDS: FACTS AND REFLECTIONS (review) 1 All-Russia Research and Technological Institute of Poultry Breeding, RAAS, Sergiev Posad 141311, Moscow province, Russia e-mail: fisinin@land.ru V.I. Fisinin1, P. Surai 2, 3 3Scottish Agricultural College (SAC), Edinburgh, EH9 3JG, UK, Scotland Received March 12, 2013 S umma r y 2Feed-Food. <...> The role of protective mechanisms in the gut is difficult to overestimate. <...> Therefore, gut status determines the chicken health, utilization of nutrients and biologically active substances (FCR) and other important commercially relevant parameters of the poultry production. <...> This review summarises recent knowledge about the development and functioning of protective immunological mechanisms in the gut. <...> In post-hatch period a chick grows so fast that its weight increases 4.4 times in the 1st week of life, and by 5000 % after 5 weeks when it amounts 2 kg. Such high productive indices now are attainable owing to intense selection for growth rate, efficient healthcare and livestock management, and improved nutrition satisfying needs in all basic nutrients and bioactive substances. <...> For example, changes in microstructure of the intestine, particularly in its mucosa, may reduce assimilation of nutrients, which affects the general health, the efficiency of utilization of nutrients and bioactive substances, and, therefore, growth, development, feed conversion, and other important economic parameters of poultry industry. <...> The intestine is the first level of defense from exogenous pathogens that colonize host cells and tissues, and this is also the largest organ with immune properties. <...> In this regard, protective mechanisms in the gut including its immune system are so particular that deserve a detailed consideration. <...> For example, the acute phase of immune response in chickens is associated with the decline in feed intake and productivity, and up to 10% extra consumption of nutrients that otherwise could be utilized in growth and development (1). <...> In a fast-growing broiler about 12 % of newly synthesized proteins are spent on the maintenance of homeostasis in the digestive tract. <...> Intense proliferation reduces the age and maturity of goblet cells, which may affect <...>