Understanding Asthma Pathophysiology

Asthma is best described as a chronic disease that involves inflammation of the pulmonary airways and bronchial hyperresponsiveness that results in the clinical expression of a lower airway obstruction that usually is reversible. Physiologically, bronchial hyperresponsiveness is documented by decreased bronchial airflow after bronchoprovocation with methacholine or histamine. Other triggers that provoke airway obstruction include cold air, exercise, viral upper respiratory infection, cigarette smoke, and respiratory allergens. Bronchial provocation with allergen induces a prompt early phase immunoglobulin E (IgE)- mediated decrease in bronchial airflow (forced expiratory volume in 1 second) followed in many patients by a late-phase IgEmediated reaction with a decrease in bronchial airflow for 4-8 hours. The gross pathology of asthmatic airways displays lung hyperinflation, smooth muscle hypertrophy, lamina reticularis thickening, mucosal edema, epithelial cell sloughing, cilia cell disruption, and mucus gland hypersecretion. Microscopically, asthma is characterized by the presence of increased numbers of eosinophils, neutrophils, lymphocytes, and plasma cells in the bronchial tissues, bronchial secretions, and mucus. Initially, there is recruitment of leukocytes from the bloodstream to the airway by activated CD₄ T-lymphocytes. The activated T-lymphocytes also direct the release of inflammatory mediators from eosinophils, mast cells, and lymphocytes. In addition, the subclass 2 helper T-lymphocytes subset of activated T-lymphocytes produces interleukin (IL)-4, IL-5, and IL-13. IL-4 in conjunction with IL-13 signals the switch from IgM to IgE antibodies. The cross-linkage of two IgE molecules by allergen causes mast cells to degranulate, releasing histamine, leukotrienes, and other mediators that perpetuate the airway inflammation. IL-5 activates the recruitment and activation of eosinophils. The activated mast cells and eosinophils also generate their cytokines that help to perpetuate the inflammation. Regardless of the triggers of asthma, the repeated cycles of inflammation in the lungs with injury to the pulmonary tissues followed by repair may produce long-term structural changes ("remodeling") of the airways. This review will discuss in greater detail the relationships of inflammation and airway hyperresponsiveness to the pathophysiology of asthma.