Immunological development of the avian gut
Beal RK., Powers C., Davison TF., Smith AL.
The gut is a highly specialized site for absorption of nutrients but also represents an important barrier between the external and internal environments. It is the site of replication and/or portal of entry for several important pathogens and requires an array of different immune effector cells that are organized in specialized tissues such as the bursa of Fabricius, caecal tonsils, Meckel's diverticulum and Peyers' patches, or more broadly distributed in the gut epithelium or the sub-epithelial lamina propria. Both the innate and adaptive immune effector mechanisms play important roles in protection from gut pathogens. The period after hatching is especially crucial for the development of the gut immune system. With the onset of food intake and microbial colonization the immune system must differentiate between innocuous food components and potentially harmful pathogens. At this stage the immune system is immature and the chick must rely on innate effector mechanisms and maternal antibodies, mainly IgY transmitted from the hen via the yolk. This makes the chick especially vulnerable to a number of pathogens, until the adaptive immune system has developed sufficiently to produce effective immune responses. Development of the adaptive immune system depends on the arrival and replication of specialized leukocytes at key locations in the gut, especially the B and T lymphocytes. Investigations on the influence of age at primary infection with Salmonella enterica have shown that clearance from the gut is associated with the ability to produce strong adaptive immune responses. Salmonella persists in the gut beyond 8-9 weeks of age, irrespective of the age at first exposure, indicating the importance of a fully mature gut immune system to control infection. © CAB International 2006.