Non-specific responsiveness occurs in all subjects. However, subjects with asthma and chronic obstructive pulmonary disease (COPD) not only need a much smaller dose of the agonist to elicit airway narrowing, but the maximal degree of airway narrowing often greatly exceeds that in ‘normals’. The airway response to an agonist can be assessed from the relationship between the dose of the agonist and the degree of airway narrowing (dose-response curve). The dose-response curve can be quantitated by the sensitivity, reactivity and maximal response to the agonist. Increased sensitivity (hypersensitivity) and reactivity (hyperreactivity) usually occur together. The non-specific term 'airway (bronchial) hyperresponsiveness' is often used to designate any difference in the dose-response curve related to sensitivity, reactivity or the maximal response. Non-specific stimuli eliciting airway narrowing can be chemical (e.g. SO2, ozone), physical (e.g. cold air, dry air, hyper- or hypotonic solutions), pharmacological (e.g. methacholine, histamine) or physiological (e.g exercise). These nonspecific stimuli almost always exert an effect through multiple cell types within the airways. Airway responsiveness is most frequently assessed by inhalation of histamine or methacholine aerosol. In recent years inhaled mannitol is increasingly used; it increases the osmolarity at the airway surface and thus liberates mediators.
About histamine