Respiratory Viruses: Do They Protect From or Induce
Asthma is a chronic, inﬂammatory disease of the airways in which both genetic and environmental factors contribute to disease development and expression. Sensitization to aeroallergens with the subsequent formation of allergen-speciﬁc IgE antibodies in individuals with a genetic predisposition to atopy is believed to be central to the pathogenesis of the disease. This abnormal immune response to inhaled allergens leads to chronic airway inﬂammation with a characteristic eosinophilic inﬁltrate. There is also increasing evidence that T lymphocytes expressing a predominantly type 2 pattern of cytokine production orchestrate this inﬂammatory response. Despite the central role of allergic mechanisms in asthma pathogenesis, it is recognized that other nonallergic factors inﬂuence the asthmatic phenotype, including exercise, airborne pollutants, psychological factors, and respiratory tract infections.