J. Comp. Path. 1999 Vol. 120, 129–145 Analysis of Leucocyte Subsets in the Canine Intestine A. J. German*†, E. J. Hall* and M. J. Day† *Department of Clinical Veterinary Science and †Department of Pathology and Microbiology, University of Bristol, Langford House, Langford, Bristol BS40 5DU Summary Immunohistochemistry and computer-aided morphometric analysis were used to define populations of leucocyte subsets in the intestinal tract of an outbred population of dogs with no evidence of gastrointestinal disease. In the small intestinal lamina propria, B cells and plasma cells (IgA + , IgM + and IgG + ) were prominent in peri-crypt regions, with a significant trend for a reduction in the number of cells towards the villous tip ( P<0·0001). By contrast, lamina propria T cells (CD3 + ) and T-cell subsets (CD4 + and CD8 + ) were present in greatest numbers at the tip of the villus, with significantly decreasing numbers towards the crypt regions ( P<0·0001). In the lamina propria, CD4 + cells outnumbered CD8 + cells (P=0·05), but the opposite was true of the epithelial compartment (P<0·001). The distribution of CD5 + lymphocytes was similar to that of CD3 + cells, in both the lamina propria and epithelial compartments. The numbers and distribution of cells expressing MHC class II, L1 and CD45 were recorded. Numerous eosinophils were present in the lamina propria, and an intra-epithelial population was also noted, especially in the crypt epithelium. Mast cells, which were mainly found in the subepithelial lamina propria, were also present within muscle layers, and cells expressing IgE had a similar distribution. Similar populations of cells were recorded in the colonic lamina propria and epithelium. The quantitative and qualitative data from this study will enable comparisons to be made with dogs suffering from inflammatory bowel diseases.  1999 W.B. Saunders Company Limited Introduction The gastrointestinal tract contains the largest number of immune cells in the body, comprising a heterogeneous population of T and B lymphocytes, plasma cells, macrophages, neutrophils, eosinophils, mast cells, dendritic cells and a variety of non-professional antigen-presenting cells. Appropriate interactions between these different cell types are essential in generating either immune responsiveness or tolerance to the large array of environmental antigens. In man, the cells that comprise this population have been well characterized ( James, 1993), but there have been limited studies of gastrointestinal immune cells in the domestic animals, including dogs. Investigations of the canine intestinal tract have identified immunoglobulin (Ig)-positive cells (Vaerman and Heremans, 1969; Hart, 1979; Willard and Leid, 1981), CD3 + T lymphocytes ( Jergens et al., 1996) and CD4 + and CD8 + T-cell subsets (Elwood et al ., 1997). Because the populations of dogs, immunohistochemical markers, intestinal 0021–9975/99/020129+17 $12.00/0  1999 W.B. Saunders Company Limited