Late-phase reaction in asthma: basic mechanisms.

Allergens are recognized by mast cells and sensitized T cells. In the early-phase reaction, mast cells produce mediators that cause changes in the airways. The mediators also induce chemotactic recruitment and activation of eosinophils and neutrophils during the late-phase reaction (LPR). The numbers of eosinophils and neutrophils are increased 4 h after challenge with allergen in the broncho-alveolar lavage fluid (BAL) of asthmatics who show an LPR. This is associated with an increased sensitivity to methacholine or histamine. The increase in the levels of eosinophils and their cationic proteins in the BAL is related to the magnitude of the LPR. The cationic proteins are toxic to human epithelium. Basophils, macrophages and lymphocytes are also involved in the inflammatory reaction. Some authors believe that basophils are more important than mast cells in the LPR to antigen. T lymphocytes release lymphokines, such as interleukin 5, which enhances the degranulation of eosinophils. All of the inflammatory cells release a range of mediators, in particular leukotrienes and platelet activating factor.