Immediate allergic and nonallergic reactivity of the airways in Ascaris skin-sensitive sheep

Immediate airway reactivity to Ascaris suum allergen was compared with the responses to nonallergic airway challenge using histamine and acetylcholine in sheep with positive skin reactions to the allergen. In a control group of 6 sheep with negative skin test responses, allergen aerosol inhalation and allergen extract infusion only caused minor changes in mechanics of breathing as well as in arterial plasma histamine concentration. Nearly 40% of the animals with a positive skin test responded to allergen inhalation with an acute airway obstruction, whereas all animals bronchoconstricted after allergen infusion. Bronchoconstriction was associated with an increase in arterial plasma histamine concentrations. The difference between the two groups of skin test-positive sheep seems to be due to the amount of histamine liberation after allergen exposure. Allergen infusion caused a more severe increase of airway resistance and plasma histamine levels in animals responding to allergen inhalation than in animals responding to allergen infusion only. Similar responses were obtained in both groups after inhalation and infusion of histamine and acetylcholine. Since the slope of the responses (dEdyn/dHi.Pl.) was similar in both groups of animals we conclude that the sensitivity of receptors is similar in responders to allergen inhalation and in animals responding to allergen infusion only.