Qualitative and Quantitative Differences between Species in the Gut Immune System: Controversial and Unsolved Aspects

Many parameters of the gut immune system, such as the development of tolerance or protective immune responses, can only be studied in detail in experimental animals. Species differences have to be considered when such results are extrapolated to humans with their mixed genetic background, non-standardised eating conditions and variable exposure to microbial stimulation. In this review some of the major species differences in the gut immune system are discussed: the development, structure and function of Peyer ́s patches show enormous differences, and the M-cells differ in number and reagents which label them. Recently discovered lymphoid structures in the gut wall, i.e. cryptopatches and lymphocyte-filled villi, have so far only been described in mice and humans & rats, respectively. Why are there huge species differences in the number of γδ T-cell receptor intraepithelial lymphocytes? What is the function of the different antigen presenting cells in the gut wall, e.g. dendritic cells? The functional relevance of the unique cellular composition of the gut wall in early childhood for tolerance development or vaccination protocols has to be studied in more detail with respect to the role of cytokines, chemokines and the enteric nervous system. Despite the great advances in recent studies on the gut immune system there still seem to be many unsolved questions for future research in laboratory animals and even more in humans.