Bronchi, when challenged with various stimuli, may respond with airway narrowing. The stimuli are classified as either specific or nonspecific. Those stimuli which elicit an immunological response are specific, the predominant immunological pathway in the airways is the type I, IgE-mediated response, which is driven by allergens (atopy). In contrast to specific immunologic stimuli, nonspecific stimuli such as histamine, methacholine and cold air are non-allergic in nature.
Specific responsiveness to substances with antigenic properties is a hallmark of atopic asthma. Many substances capable of inducing airway narrowing have been identified. For example house dust mites feed on skin which scales off from humans or animals. The excreta of the house dust mite contain antigens which accumulate in house dust and bedding. Inhalation of this antigenic material can result in an inflammatory process of the lungs of atopic individuals. Other antigenic substances include animal dander (e.g. cat, dog, horse), pollen from plants (e.g. grasses and trees) and spores from molds. The bronchi of non-asthmatic subjects do not respond to these specific stimuli. However, the airways of non-asthmatic subjects do respond to non-specific stimuli, but the response occurs at a greater dose and with a smaller magnitude when compared to the airways of asthmatic subjects.