A 6-year-old boy presents with severe pruritus, lichenified plaques on flexural surfaces (antecubital and popliteal fossae), and a history of asthma and allergic rhinitis. Regarding the pathophysiology and clinical features of atopic dermatitis, all of the following are true EXCEPT:

Explanation

## Pathophysiology of Atopic Dermatitis ### Barrier Dysfunction **Key Point:** Atopic dermatitis is fundamentally a disease of impaired epidermal barrier function combined with immune dysregulation. - **Filaggrin mutations** (FLG gene) are present in 20–30% of AD patients and lead to: - Reduced filaggrin protein → defective corneodesmosomes - Increased transepidermal water loss (TEWL) - Reduced natural moisturizing factor (NMF) - Enhanced allergen penetration - **Lipid abnormalities:** Decreased ceramides, cholesterol, and free fatty acids in the stratum corneum → compromised barrier integrity ### Immune Dysregulation **High-Yield:** The immune response in AD is **Th2-predominant**, NOT Th1-predominant. | Phase | Immune Profile | Key Cytokines | Cells Involved | | --- | --- | --- | --- | | Acute/Early | Th2-predominant | IL-4, IL-5, IL-13 | Eosinophils, mast cells | | Chronic/Late | Mixed Th1/Th2 | IFN-γ, IL-2 (Th1); IL-4, IL-13 (Th2) | T cells, macrophages | **Clinical Pearl:** The acute phase is characterized by: - Elevated serum IgE (in ~80% of patients) - Eosinophilia - Th2 cytokine predominance (IL-4, IL-5, IL-13) - Mast cell and eosinophil infiltration The chronic phase shows a shift toward Th1 response, but Th2 remains important throughout. ### Cardinal Symptom: Pruritus **Key Point:** Pruritus is the defining symptom of AD — "the itch that rashes" rather than "the rash that itches." - Often precedes visible lesions - Driven by: - Increased sensory nerve fiber density - Elevated tryptase and histamine from mast cells - Th2 cytokines (IL-31 is a major pruritogen) - Barrier dysfunction → irritant penetration ### Lipid Abnormalities **Key Point:** Stratum corneum lipids are quantitatively and qualitatively abnormal: - Reduced total lipid content - Altered ceramide composition (especially ceramide 1 deficiency) - Impaired lipid lamellae organization --- ## Why the Correct Answer is Wrong Option 1 states that a **Th1-mediated response predominates in the acute phase**. This is **FALSE**. The acute phase of AD is **Th2-predominant**, characterized by IL-4, IL-5, and IL-13 production, elevated IgE, and eosinophilia. While Th1 responses emerge in chronic disease, they do not predominate in the acute phase. --- ## Summary Table: AD Pathophysiology | Feature | Status | Evidence | | --- | --- | --- | | Filaggrin mutations | True | 20–30% of patients; impairs barrier | | Ceramide/cholesterol deficiency | True | Documented in stratum corneum | | Th2-predominant (acute phase) | True | IL-4, IL-5, IL-13; elevated IgE | | Th1-predominant (acute phase) | **FALSE** | Th1 emerges in chronic phase | | Pruritus precedes rash | True | Cardinal symptom; itch-scratch cycle |