In asthma, there is a continuous state of hyperreactivity of the bronchi, during which exposure to a wide variety of bronchial irritants will precipitate an asthmatic attack.

In normal subjects, inhalation of histamine or carbachol may cause a little increase in airway resistance. By contrast, the bronchi of the patients with asthma are highly reactive and show marked bronchoconstriction in response to these
substances. This phenomenon is termed bronchial hyperreactivity. In addition to these pharmacologic agents, bronchial hyperreactivity has also been shown to occur with a variety of nonspecific irritants such as dusts and cold air. In subjects with seasonal grasspollen asthma, hyperreactivity increases during the pollen season whether attacks are occurring or not. In subjects with hyperreactivity associated with nonallergic states, chronic bronchitis, clinical observation of the effects of removal of inhaled irritants such as smoking or moving to a less polluted environment suggests that some degree of reversibility is possible.

A number of factors may cause hyperreactive bronchi to constrict and for convenience they are all termed irritants. Although the end-result of their action is similar, it is important clinically to decide whether the irritant acts by inducing an allergic reaction or by some other mechanism because specific therapy is available only for the allergic type.

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