The Intestinal Immune System is obliged to recognise not only antigens associated with pathogenic organisms but also harmless dietary protein. Since chronic immune responses to these ubiquitous antigens present in food are likely to be harmful to normal intestinal function, regulatory mechanisms have evolved which reduce immune responses to such antigens to levels acceptable to the individual. It is essential to the effective working of these mechanisms that the introduction to new antigens should be gradual and not be associated with the sudden presentation of large amounts of a new dietary material. Modern intensive weaning methods pay scant attention to the needs of these immunoregulatory systems, and the resultant immune response may be the cause of the villous atrophy and crypt hyperplasia seen immediately after weaning and which are thought to precipitate post weaning E. coli diarrhoea.