Common triggers for asthma exacerbations are mediated by different pathways, frequently involving steroid-unresponsive neutrophils and CD8 cells
| Allergen exposure | Viral infection | Ozone/diesel exhaust | Cigarette smoke | Bacterial infection | |
| Predominant pathway | Th2 cells, IgE | Th1 cells, leukotrienes | Th1 cells, leukotrienes | Th1 cells, leukotrienes | Th17 cells, IL-1β, IL-6, TNF |
| Predominant effector cells | Eosinophils, Mast-cells, CD4 T-cells | Neutrophils, CD8 T-cells | Neutrophils, CD8 T-cells | Neutrophils, CD8 T-cells | Neutrophils |
| Steroid responsiveness | High | Low | Low | Low | Low |
Th1/2 cells: type 1/2 CD4+ T-cells; IL: interleukin; TNF: tumour necrosis factor.