Structure and Function of the Gut Mucosal Immune System
Reinhard Pabst and Hermann J. Rothkötter
For many decades immune reactions have been classified as humoral and cellular, innate
and acquired, and the essential cells for protective and tolerogenic reactions were at first
subdivided into TH1 and TH2. Later TH3 and TH0 cells were added and the antigen-presenting cell family is continuously growing. Many molecular pathways initiated by cytokine–receptor interactions have been clarified in vitro. However, in vivo all the players in this interacting orchestra of cells have to be at the right location to create the appropriate microenvironment as a basic requirement for meaningful immune reactions.1 Therefore, the different compartments of the gut immune system will be described, focusing on the effects of age, species differences and influence of the nutritional and microbial content of the gut.
The gut immune system is integrated in the mucosal immune system in general. However, there is growing evidence for preferential routes from one organ to the other and immunization at one site does not result in protective effects in all other mucosal organs. Thus, the term “common mucosal immune system” should be replaced by “integrated mucosal immune system”.2 Examples will be mentioned to stress the heterogeneity of structures with at first sight similar appearance. The functional anatomy of the gut immune system is the basis for understanding the detailed description of the location of individual cells, which will be dealt with in the following chapters.