Asthma Mechanisms
Learning Objectives
- Describe the immunological mechanisms underlying allergic asthma
- Explain how air pollutants trigger and exacerbate asthma
- Distinguish between immediate and late-phase asthmatic responses
- Analyze the role of inflammatory mediators in bronchoconstriction
- Interpret spirometry data for obstructive lung disease
The Phenomenon
"Why do children living near high-traffic areas have 1.5x higher rates of asthma, and why do their symptoms worsen on high-ozone days?"
Asthma Pathophysiology
Immediate Response (0-30 min)
- Allergen/pollutant crosses epithelium
- IgE-sensitized mast cells degranulate
- Histamine and leukotrienes released
- Smooth muscle contraction
- Acute bronchoconstriction
Late-Phase Response (4-24 hr)
- Eosinophils and T-cells recruited
- Cytokine cascade (IL-4, IL-5, IL-13)
- Epithelial damage and edema
- Mucus hypersecretion
- Airway hyperresponsiveness
Air Pollutants and Asthma
| Pollutant | Mechanism | Effect |
|---|---|---|
| Ozone (O3) | Oxidizes membrane lipids, activates epithelial TLRs | Inflammation, hyperreactivity |
| PM2.5 | Induces oxidative stress, carries allergens deep into lungs | Enhanced sensitization, exacerbation |
| NO2 | Increases airway permeability, enhances allergen penetration | Reduced threshold for triggers |
| SO2 | Direct bronchoconstrictor effect on smooth muscle | Immediate symptoms in asthmatics |
| Formaldehyde | Irritant, enhances Th2 immune response | Sensitization, exacerbation |
Cellular and Molecular Players
Key Inflammatory Mediators
- Histamine: Immediate bronchoconstriction, vasodilation, increased permeability
- Leukotrienes (LTC4, LTD4): Prolonged smooth muscle contraction, mucus secretion
- Prostaglandins (PGD2): Bronchoconstriction, eosinophil recruitment
- IL-4, IL-13: IgE class switching, goblet cell metaplasia
- IL-5: Eosinophil survival and activation
Therapeutic targets: Antihistamines, leukotriene receptor antagonists, corticosteroids (suppress cytokine production), anti-IgE biologics
Activity: Interpreting Spirometry
Spirometry measures lung function using forced expiratory volumes:
- FEV1: Forced expiratory volume in 1 second
- FVC: Forced vital capacity (total exhaled volume)
- FEV1/FVC ratio: Normally >0.70 in healthy adults
Case Analysis: A 16-year-old student presents with:
- Pre-bronchodilator: FEV1 = 2.8 L, FVC = 4.2 L
- Post-bronchodilator: FEV1 = 3.5 L, FVC = 4.3 L
- Calculate FEV1/FVC ratio pre- and post-bronchodilator
- Is this obstructive, restrictive, or mixed pattern?
- Calculate percent improvement in FEV1 (>12% indicates reversibility)
- What does reversibility indicate about the diagnosis?
Key Takeaway
Asthma involves complex immunological cascades where air pollutants act as both triggers and sensitizers. The immediate bronchoconstriction and late-phase inflammation explain why asthma management requires both rescue bronchodilators and preventive anti-inflammatory therapy. Understanding these mechanisms reveals why indoor air quality is critical for asthma management.