Allergen-induced airway responses

Environmental allergens are an important cause of asthma and can contribute to loss of asthma control and exacerbations. Allergen inhalation challenge has been a useful clinical model to examine the mechanisms of allergen-induced airway responses and inflammation. Allergen bronchoconstrictor responses are the early response, which reaches a maximum within 30 min and resolves by 1–3 h, and late responses, when bronchoconstriction recurs after 3–4 h and reaches a maximum over 6–12 h. Late responses are followed by an increase in airway hyperresponsiveness. These responses occur when IgE on mast cells is cross-linked by an allergen, causing degranulation and the release of histamine, neutral proteases and chemotactic factors, and the production of newly formed mediators, such as cysteinyl leukotrienes and prostaglandin D2. Allergen-induced airway inflammation consists of an increase in airway eosinophils, basophils and, less consistently, neutrophils. These responses are mediated by the trafficking and activation of myeloid dendritic cells into the airways, probably as a result of the release of epithelial cell-derived thymic stromal lymphopoietin, and the release of pro-inflammatory cytokines from type 2 helper T-cells. Allergen inhalation challenge has also been a widely used model to study potential new therapies for asthma and has an excellent negative predictive value for this purpose.

Allergen-Induced Asthma

It was only in the late 19th century that specific allergens, pollen, animal antigens and, later, house dust mite, were identified to cause upper and lower airway disease. Early allergen challenge studies, crudely monitored before measurement of forced expiratory volume in 1 s became widespread in the 1950s, focused on the immediate effects but noted in passing prolonged and/or recurrent asthma symptoms. The late asthmatic response, recurrent bronchoconstriction after spontaneous resolution of the early responses occurring 3 h to 8 h or more postchallenge, has been identified and well characterized over the past 50 years. The associated allergen-induced airway hyper-responsiveness (1977) and allergen-induced airway inflammation (1985) indicate that these late sequelae are important in the mechanism of allergen-induced asthma. Allergens are now recognized to be the most important cause of asthma. A standardized allergen inhalation challenge model has been developed and is proving to be a valuable research tool in the investigation of asthma pathophysiology and of potential new pharmacological agents for the treatment of asthma.