Asthma is a result of a dysregulated immune response to inhaled antigens. The development of an immune response dominated by T cells (Th2) that secrete interleukin 4 (IL-4) and IL-5 results in the constellation of findings associated with asthma, including elevated IgE and increased blood eosinophilia. Epidemiological studies indicate that there are several predisposing factors that may dictate whether asthma may develop in certain individuals including genetic, environmental, and age-related factors. Any one factor alone may not determine whether an inappropriate immune response develops with subsequent development of asthma; rather, asthma likely results from a combination of factors. However, once a Th2 response in the lung has occurred, the hypothesis is that it dominates and results in the development of Th2 response to inhaled neoantigens. Further, the acquired predisposition to a Th2 response may override other preexisting predisposing factors and therefore becomes the prime target for asthma therapy.