Environment and infant immunity.

Pregnancy is associated with a skewing towards T-helper (Th)2-like cell populations. During the first years of life, the neonatal immune responses towards allergens deviate towards a balanced Th1 and Th2-like immunity. The difference between atopic and nonatopic individuals may be how readily the immune deviation takes place. The high prevalence of allergies in industrialized countries is in contrast with the low prevalence of allergies in Eastern Europe, with a life style similar to that prevailing in Western Europe 40 years ago. The discussion on the impact of environmental changes on the incidence of asthma and other allergies has been limited mostly to the possible effects of a deteriorating air quality, poorly ventilated houses and an increased exposure to certain allergens, notably house dust mites. None of these factors can more than marginally explain the observed regional differences in the prevalence of allergic diseases. The concept of 'life style' should therefore be expanded considerably. The mother is a significant 'environmental factor' in early infancy. Human milk contains components that enhance the maturation of the immune system of the newborn infant. However, there are considerable individual variations in the composition of human milk. Recent studies indicate an imbalance in the gut flora of allergic, compared with non allergic infants, and in Swedish children compared with Estonian children. As the microbial flora drives the maturation of the immune system, changes in its composition may play a role for the higher prevalence of allergy. The future search for significant environmental factors should be directed towards other areas that have not yet been explored. The intestinal microflora is one of these factors that deserve a closer analysis.