The gastrointestinal tract is a major organ of the immune system. Gut associated lymphatic tissue (GALT) contains approximately 40% of mature lymphocytes and represents a significant part of gut mass. GALT consists of a complex of immune cells found in discrete organs e.g. Peyer's patches and mesenteric lymph nodes, as well as dispersed within the lamina propria and the epithelium. Both B cells and the two main subsets of T cells, CD4(helper/inducer) and CD8 (suppressor/cytotoxic) are found. This significant involvement of the immune system in the gut should not be a surprise, given that the gut represents a major site of interaction with the ‘outside world’ and therefore represents a major challenge to homeostasis, specifically from infectious disease.

The gut also has a fundamental dilemma, it must respond vigorously with a protective response against any pathogens presented at its mucosal surface, yet sustain digestive and absorptive function of dietary non-harmful antigens. On occasion it gets it wrong, resulting in damaging hypersensitivity reactions e.g. Coeliac Disease, milk intolerance in children, food allergies and diarrhoea in the postweaned pig and the pre-ruminant calf. It is these inappropriate immunological responses that can prove to be a constraint on animal production, specifically on the use of legumous protein sources for animal feeds.