Induction of oral tolerance with effects on numbers of IgE‐carrying mast cells and on bystander suppression in young rats

The presence of IgE+ mast cells in the small intestine, bystander suppression of DTH and antibody responses to human serum albumin (HSA) were studied in young rats, made tolerant to ovalbumin (OA) by feeding an OA‐containing diet for 1–4 weeks starting from weaning, and in sensitized control rats. One week after finishing the OA diet, both groups of rats were immunized with a mixture of OA and HSA in Freund’s complete adjuvant (FCA) at one site on the back. The animals were then colonized for 5 days with a genetically manipulated Escherichia coli producing OA. Immunohistochemical staining of the small intestine of the rats fed the OA diet for 4 weeks showed significantly fewer IgE+ mast cells in the lamina propria, a lower level of MHC class antigen was found in the epithelial cells and in the lamina propria, and the villus crypt depth was also significantly less in tolerant compared with sensitized rats (P