Gene by environment interactions in the development of asthma

Present knowledge suggests that asthma is a heterogeneous condition. Differ ent genetic backgrounds may or may not express themselves as asthma-like sy mptoms, depending on both the nature and the timing of exposures in the ind ividual. Although particular attention has been paid to environmental facto rs that may increase the risk of asthma, it is equally important to underst and 'protective' exposures that may have decreased during the last decades and may thus explain the reported increases in asthma prevalence. Among the se 'protective' factors are certain infections and the role they may have i n the inception of allergies and asthma, particularly when they occur at cr itical times in the development of the immune system.