The small intestine is a major immunological organ with aggregated lymphoid tissue of Peyer’s diffuse lymphoid tissue of lamina propria and intra-epithelial lymphocytes as 3 key components. Immunopat ho genesis is of central importance for coeliac disease, Crohn’s disease and gastrointestinal food allergy. Small intestinal enteropathy of variable severity, related to T-cell activation is common to all.