Per Brandtzaeg

Laboratory for Immunohistochemistry and Immunopathology (LIIPAT), Institute of Pathology, University of Oslo, Rikshospitalet, N-0027 Oslo, Norway

The mammalian host defence has successfully handled environmental confrontations for millions of years. To this end, numerous genes involved in innate and acquired (adaptive) immune protection have been subjected to evolutionary modifications, thus being shaped according to the microbial pressure and environmental (including dietary) impact. In humans, this modulation has been influenced by various ways of living, such as hunting, fishing, gathering, agriculture and animal husbandry.

In the process of evolution, the mucosal immune system has generated two arms of adaptive defence: (i) antigen exclusion, performed by secretory antibodies of the immunoglobulin (Ig)A and IgM classes, to modulate or inhibit surface colonization of microorganisms and dampen penetration of potentially dangerous soluble agents; and (ii) suppressive mechanisms to avoid local and peripheral overreaction (hypersensitivity) against innocuous substances bombarding the mucosal surfaces (Fig. 14.1). The latter arm is referred to as 'oral tolerance' when induced via the gut against dietary antigens (Brandtzaeg, 1996a); it probably explains why overt and persistent hypersensitivity to food proteins is relatively rare (Bischoff et al., 2000). Similar down-regulatory mechanisms apparently operate against antigens from the commensal microbial flora (Duchmann et al., 1997; Karlsson et al., 1999; Helgeland and Brandtzaeg,

Oral tolerance generally seems to be a rather robust adaptive immune function, in view of the fact that more than a ton of food may pass through the

© CAB International 2002. Nutrition and Immune Function

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