[Viruses and asthma].

More than one-third of infants with respiratory syncytial virus (RSV) bronchiolitis subsequently experience recurrent episodes of wheezing. In asthmatic children, viral infections frequently trigger asthmatic attacks. Indeed, viruses induce immunological and respiratory functional disorders. As compared with non-wheezing RSV infected infants, wheezing infants with bronchiolitis or asthma due to RSV have: a) increased T-lymphocyte responsiveness to RSV antigen; b) more persistent IgE bound to nasopharyngeal epithelial cells; c) higher titers of RSV-specific IgE and higher histamine concentrations in nasopharyngeal secretions. In children with an atopic constitution, virus infections may trigger allergic sensitization. Furthermore, some viruses enhance the immunologic and non-immunologic human basophil histamine release. Non-specific bronchial hyperreactivity, which is a fundamental feature of asthma, can also be observed to a variable degree in viral respiratory infections. Pulmonary function testing allows in vivo determination of bronchial sensitivity and bronchial reactivity (respectively threshold dose and dose-response curves). Four factors may be involved in bronchoconstriction: a pre-existing diminution of bronchial diameter (which is lacking in many studies); hyperplasia or hypertrophy of bronchial muscles (which is to be excluded in recent viral infections); a non-specific decrease in the threshold for stimulation of irritant receptors due to airway epithelial injury; a partial beta blockade. Further studies are needed to ascertain whether this bronchial hyperreactivity is congenital or acquired.