Transient hypersensitivity reactions of the intestinal immune system to dietary antigens result in increases in enterocyte turnover and villous atrophy. These changes occur in the intestine of the post weaned piglet and precede the proliferation of E. coli and the development of post-weaning diarrhea. We therefore postulated that a transient cell mediated immune response to dietary antigens may increase susceptibility to disease. The interaction of dietary and microbial antigens upon the gut immune system has been investigated in mice and pigs and it has been shown that both exert powerful regulatory effects upon each other.