## Th2 Cytokine Profile in Atopic Dermatitis ### IL-4: The Master Th2 Cytokine **Key Point:** Interleukin-4 (IL-4) is the primary driver of Th2 cell differentiation and is markedly elevated in atopic dermatitis, particularly in the acute inflammatory phase. **High-Yield:** IL-4 functions: - Promotes naive CD4+ T cell → Th2 cell differentiation - Induces IgE class switching in B cells - Upregulates VCAM-1 on endothelial cells (recruits eosinophils) - Enhances expression of IL-4 receptors (positive feedback loop) ### Th2 Cytokine Cascade in AD ```mermaid flowchart TD A[Barrier dysfunction + allergen penetration]:::outcome --> B[Dendritic cell activation]:::action B --> C[IL-4 production]:::action C --> D[Th2 cell differentiation]:::outcome D --> E[IL-4, IL-5, IL-13 production]:::action E --> F[IgE elevation + eosinophil recruitment]:::outcome E --> G[Pruritus and inflammation]:::urgent ``` ### Comparison of Key Cytokines in AD | Cytokine | Source | Role in AD | Phase | |----------|--------|-----------|-------| | IL-4 | Th2, mast cells, basophils | Master Th2 driver | Acute & chronic | | IL-5 | Th2 cells | Eosinophil recruitment | Chronic | | IL-13 | Th2 cells | IgE switching, barrier dysfunction | Chronic | | IFN-γ | Th1 cells | Minimal role (Th1 is suppressed) | Minimal | | TNF-α | Macrophages, keratinocytes | Proinflammatory (secondary) | Acute | **Clinical Pearl:** Dupilumab, a monoclonal antibody against IL-4 receptor alpha (shared by IL-4 and IL-13), is highly effective in moderate-to-severe AD because it blocks the central Th2 signaling axis. **Mnemonic:** **IL-4 in AD** = **I**mmune **L**ymphocyte **4** drives **A**topic **D**ermatitis via Th2 polarization [cite:Harrison 21e Ch 318] 
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