A 35-year-old man with a history of atopy presents with recurrent episodes of allergic rhinitis triggered by dust mite exposure. Which of the following is the most common cell type responsible for the chronic inflammatory phase of this Type I hypersensitivity reaction?

Explanation

## Chronic Phase of Type I Hypersensitivity **Key Point:** While mast cells and basophils mediate the acute phase of Type I hypersensitivity, **eosinophils** are the predominant cell type in the chronic inflammatory phase, typically occurring 4–12 hours after initial exposure. ### Biphasic Response in Type I Hypersensitivity ```mermaid flowchart TD A[Antigen re-exposure]:::outcome --> B[IgE-FcεRI cross-linking]:::outcome B --> C[Mast cell/basophil degranulation]:::action C --> D[Acute phase<br/>Minutes to 2 hours]:::outcome D --> E[Histamine, tryptase<br/>Urticaria, angioedema<br/>Bronchospasm]:::outcome C --> F[Newly synthesized mediators]:::action F --> G[Chronic phase<br/>4-12 hours]:::outcome G --> H[Eotaxin, RANTES<br/>IL-5 production]:::action H --> I[Eosinophil recruitment]:::action I --> J[Persistent inflammation<br/>Tissue damage]:::outcome ``` ### Role of Eosinophils in Chronic Phase 1. **Recruitment**: Mast cells and activated T cells secrete eotaxin (CCL11) and RANTES (CCL5), which are potent eosinophil chemoattractants. 2. **Activation**: IL-5 (produced by Th2 cells) is the primary cytokine promoting eosinophil differentiation, recruitment, and activation. 3. **Effector functions**: Eosinophils release: - Major basic protein (MBP) - Eosinophil peroxidase (EPO) - Eosinophil-derived neurotoxin (EDN) - These cause tissue damage and perpetuate inflammation. **High-Yield:** In allergic rhinitis, the chronic phase is characterized by eosinophilic infiltration of nasal mucosa, leading to persistent congestion, post-nasal drip, and mucosal edema. ### Comparison of Cell Roles in Type I Hypersensitivity | Cell Type | Phase | Mediators/Function | Timeline | |-----------|-------|-------------------|----------| | Mast cell | Acute | Histamine, tryptase, heparin | Minutes | | Basophil | Acute | Histamine, IL-4, IL-13 | Minutes | | **Eosinophil** | **Chronic** | **MBP, EPO, EDN, IL-5 response** | **4–72 hours** | | Neutrophil | Late chronic | Proteases, ROS | 24+ hours | | T cell (Th2) | Chronic | IL-4, IL-5, IL-13 | Ongoing | **Clinical Pearl:** Eosinophilia in nasal smears or sputum is a hallmark of allergic inflammation. Patients with allergic asthma and rhinitis typically show elevated eosinophil counts in blood and tissue. **Mnemonic: EASING** — **E**osinophils in **A**llergic **S**ensitivity **I**nflammation **N**eed **G**ranule proteins (MBP, EPO).