With regard to the role that the immune system plays in health maintenance and improvement, the traditional viewpoint has been one of immunity as a defence system against intrinsic (neoplasms and tumours) and extrinsic disease-causing agents (pathogens). However, this definition forms only part of the pic ture. Through control and orchestration of immune responses, the immune system is also able to regulate inflammatory events and control or limit the development of pathologies. This occurs mainly via the production of modulatory hormones (cytokines) that are able to shape and modify the character of a developing immune or inflammatory reaction (see Devereux, Chapter 1, this volume). In this context, it should be realized that gut-dwelling microbes are far from passive inhabitants of the intestinal-tract mucosa in an inert immunologi-cal sense. Paradoxically, it is the very signals generated by gastrointestinal (GI)-tract microbial interactions with the immune system that probably constitute the beneficial impact of probiotics on health. Clinical case studies have indicated that children raised in environments rich in early-life bacterial exposure (including lactobacilli-containing foods) develop fewer immune dysfunctional diseases than those experiencing more sterile environments (Alm et al., 1999). In this case, it has been suggested that early stimulation by 'appropriate' bacterial signals may regulate the development of the immune system, such that immunopathologies (e.g. atopic reactions and mucosal allergies) are limited (Matricardi et al., 1999). Indeed, a recent study has shown that supplementing the diets of newborn babies with the probiotic Lactobacillus rhamnosus (strain GG) can effectively reduce the incidence of atopic eczema during infancy and early childhood (Kalliomaki et al., 2001), suggesting that augmentation of the neonatal intestinal microflora with exogenous bacteria can provide the bacterial signals necessary to combat allergic sensitization.
There is also more direct evidence that orally delivered probiotic organisms can interact with the immune system to limit pathologies. Further studies on L. rhamnosus GG have indicated that this probiotic can alleviate immune-mediated atopy following oral delivery to infants or to nursing mothers (Majamaa and Isolauri, 1997; Isolauri et al., 2000), can partially control immune-mediated inflammatory responses in adults (via regulation of leucocyte inflammatory receptor expression) (Pelto et al., 1998) and can reduce the incidence and severity of infant diarrhoea concomitant with an increase in circulating antibody responses (Kaila et al., 1992; Majamaa et al., 1995; Table 13.1). Clearly, there is scope to exploit the beneficial effects of probiotics on the immune system, with a view to the development of safe, dietary adjuncts/food-borne alternatives to pharmaceutical intervention for the control of a wide range of human pathologies (Elmer et al., 1996).
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