The intrauterine and postnatal environments

Pregnancy is associated with a strong skewing toward T(H)2 cytokine pattern , which enables the survival of the fetus, including fetal allergen-specifi c immune responses, The postnatal maturation of the immune system which is characterized by the development of a balanced T(H)1/T(H)2 immunity is gene tically determined and modified by the environment. The process seems to pr oceed at a slower rate in atopic than in nonatopic infants. There is a clos e immunologic interaction between the mother and her offspring through the breast milk. Individual variations in the composition of human milk may el; plain the controversy with regard to the possible allergy-preventive effect s of breast-feeding. Recurrent respiratory infections have been suggested t o enhance immune deviation. The microbial flora are a more likely source, h owever, because they are a major driving force in the maturation of the imm une system. Changes in its composition, as a consequence of an altered life style and diet, may play a role in the higher prevalence of allergy. So far , primary prevention of allergy has failed. Future studies should therefore focus on factors enhancing immune deviation (ie, "success" factors) rather than on "risk" factors. The intestinal microflora is one of these factors that deserves closer analysis.