Environment and infant immunity

Pregnancy is associated with a skewing towards T-helper (Th)2-like cell pop ulations. During the first years of life, the neonatal immune responses tow ards allergens deviate towards a balanced Th1 and Th2-like immunity. The di fference between atopic and nonatopic individuals may be how readily the im mune deviation takes place. The high prevalence of allergies in industriali zed countries is in contrast with the low prevalence of allergies in Easter n 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 in cidence of asthma and other allergies has been limited mostly to the possib le 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 differ ences in the prevalence of allergic diseases. The concept of 'life style' s hould therefore be expanded considerably. The mother is a significant 'envi ronmental factor' in early infancy. Human milk contains components that enh ance the maturation of the immune system of the newborn infant. However, th ere are considerable individual variations in the composition of human milk . Recent studies indicate an imbalance in the gut flora of allergic, compar ed with non allergic infants, and in Swedish children compared with Estonia n children. As the microbial flora drives the maturation of the immune syst em, 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 intestin al microflora is one of these factors that deserve a closer analysis.