Asthmatic symptoms that follow allergen exposure may occur immediately, within 15 - 30 minutes of exposure (immediate response), and subside within an hour. In addition, you may have a delayed response (late phase reaction) that occurs 3 - 4 hours following the immediate response. This dual reaction is most often seen in more sensitive asthmatic individuals. The late phase reaction may last many hours or days and is thought to result from an inflammatory reaction.
The late phase reaction may be associated with increased bronchial hyper-responsiveness or irritability of the airway. As a result of this process, the threshold for provoking asthmatic symptoms by irritating stimuli is lowered and may result in even more mediator release, inflammation and airway hyper-responsiveness. This can lead to a vicious cycle causing chronic symptoms, ultimately culminating in an acute attack of asthma unless the disease is effectively treated.