A 6-year-old child with atopic dermatitis presents with severe pruritus and lichenified plaques over the flexural surfaces. Regarding the pathophysiology and clinical features of atopic dermatitis, all of the following are TRUE EXCEPT:

Explanation

## Pathophysiology of Atopic Dermatitis **Key Point:** Atopic dermatitis is fundamentally a Th2-mediated inflammatory disease, NOT a Th1-mediated condition. The immune dysregulation in AD is characterized by upregulation of Th2 cytokines, not downregulation. ### Correct Mechanisms in Atopic Dermatitis | Feature | Mechanism | Clinical Relevance | |---------|-----------|--------------------| | **Barrier Defect** | Filaggrin mutations → reduced NMF → TEWL ↑ | Xerosis, increased irritant penetration | | **Immune Response** | Th2 > Th1 (IL-4, IL-5, IL-13 ↑) | IgE elevation, eosinophilia, allergic sensitization | | **Secondary Infection** | S. aureus colonization (30–90%) | Exacerbation via superantigens, impaired AMPs | | **Itch-Scratch Cycle** | Neurogenic inflammation + barrier breakdown | Lichenification, excoriation | **High-Yield:** The **Th2 predominance** in AD is well-established: - IL-4 and IL-13 promote IgE synthesis - IL-5 recruits eosinophils - This is why antihistamines and topical corticosteroids (Th2 suppressors) are effective **Warning:** ~~Th1-mediated response~~ — this is **incorrect** and is the trap in this question. Early AD is Th2-dominant; chronic AD may show mixed Th1/Th2, but Th2 remains predominant. ### Why Each Option Is Correct (Except One) 1. **Filaggrin mutations** → impaired barrier is a cornerstone of AD pathophysiology [cite:Robbins 10e Ch 25] 2. **Th2-mediated with elevated IgE** → classic immune signature of AD 3. **Th1-mediated with decreased IL-4/IL-5** → **FALSE** — this describes the opposite of AD 4. **S. aureus colonization** → well-documented secondary phenomenon due to loss of antimicrobial peptides and barrier dysfunction