The development of allergy, or allergic sensitization, and asthma is determined by genetic and environmental factors. Genetic factors include cytokine dysregulation with a tendency toward Th2 responses, or a deficiency of Th1 activity, either of which would promote allergy sensitization. Environmental factors include infections, allergen exposure, and pollutants. Furthermore, the possibility that these interactions will lead to allergy and asthma is dependent upon the age at which these interactions occur.