Overview of the immune dynamics of the digestive system

The digestive tract of the chicken is a major site of pathogen exposure. Although the bird has a multifaceted set of tools to prevent or resist infection, any activation of the immune system can divert nutrients away from production. Therefore, prevention of pathogenic exposure is preferred. However, it is unlikely that the bird can escape exposure to all pathogens during its life, thus the ability to respond to immunologic challenges is essential. The immune system of birds is similar to that of mammals in terms of structure and function, although some differences do exist, particularly in regulatory aspects. The innate immune system responds nonspecifically to foreign molecules and is essential for the induction of the specific (acquired) immune response. Cells of the innate immune system include macrophages, dendritic cells, heterophils, and natural killer cells. The acquired immune response involves recognition of a specific antigen and response by lymphocytes. Cytotoxic T lymphocytes are especially effective at inducing cells infected with intracellular pathogens to undergo apoptosis. Helper T lymphocytes increase the effectiveness of innate immune cells in combating extracellular pathogens and are also essential for activating B lymphocytes, which produce antibodies specific to the invading pathogen. All aspects of the immune system function together, although one aspect will often dominate, depending on the type and severity of the infection. This paper reviews the basics of avian immune function in general and discusses the immune system in the digestive tract in particular in birds. The consequences of activation of the immune system are presented. Currently, growth-promoting antibiotics are not used in poultry in many countries; the North American industry may be moving in that direction as well, either through legislation or consumer pressure. Several nonantibiotic means of manipulating the immune system to prevent the health- and performance-suppressing effects of immune system activation are presented here.