Allergens, viruses, and asthma exacerbations.

In adults and children with asthma, viral infections (rhinovirus [RV] infection being the most prevalent) will often trigger an increase in symptomatology. The mechanisms responsible for viral-induced exacerbations remain uncertain. Proposed mechanisms include direct infection of the lower respiratory tract, the inflammatory response to viruses, increases in bronchial responsiveness and up-regulation of intercellular adhesion molecule-1 expression in bronchial epithelium. In addition, exposure to allergens, especially seasonal allergens, in sensitized asthmatic individuals have been implicated in asthma attacks. Increased levels of exposure in sensitized asthmatics have been related to increases in hospital admissions and emergency room visits, increased bronchial hyperresponsiveness, increased levels of exhaled nitric oxide, lower levels of lung function, increased treatment requirements, and even increased mortality. In recent years studies have suggested that viruses and allergens may have a synergistic effect on individuals with asthma, thus having a greater influence on exacerbation rate together than either factor alone. Models of experimentally induced RV infection in both allergic and nonallergic individuals using bronchoalveolar lavage and segmental allergen challenge have helped researchers to investigate the possibility of an interaction between allergen sensitization, exposure, and virus infection and their role in the induction of an asthma exacerbation. This review aims to summarize the evidence supporting the role of viruses (in particular RV) as well as the role of, and interaction with, allergen sensitization and exposure on exacerbations of asthma.