A 28-year-old man presents with a 6-month history of intensely pruritic vesicles and papules on the extensor surfaces of his elbows, knees, and buttocks. The lesions are grouped and recurrent. He reports that scratching provides temporary relief but lesions reappear within days. On examination, excoriations and crusting are prominent; intact vesicles are rare. He mentions a lifelong tendency to loose stools and abdominal bloating after wheat-containing meals. Serum tissue transglutaminase (tTG) IgA is elevated. Skin biopsy shows granular IgA deposits at the dermal–epidermal junction. What is the most likely diagnosis?

Explanation

## Diagnosis: Dermatitis Herpetiformis ### Clinical Presentation **Key Point:** Dermatitis herpetiformis (DH) is a blistering disorder characterized by intensely pruritic grouped vesicles and papules on extensor surfaces (elbows, knees, buttocks, scalp). ### Pathognomonic Features in This Case 1. **Extensor surface distribution** — elbows, knees, buttocks are classic sites 2. **Intense pruritus** — often described as burning or stinging; scratching is irresistible 3. **Grouped lesions** — "herpetiform" arrangement (resembling herpes, though not viral) 4. **Gastrointestinal symptoms** — loose stools and bloating after wheat suggest underlying celiac disease 5. **Elevated tTG IgA** — strong association with celiac disease; ~75% of DH patients have villous atrophy 6. **Granular IgA at DEJ** — pathognomonic on direct immunofluorescence; deposits are at the basement membrane zone ### Immunopathology **High-Yield:** DH is an IgA-mediated autoimmune response to gluten in genetically predisposed individuals (HLA-DQ2/DQ8 positive). Tissue transglutaminase 3 (epidermal tTG) is the primary autoantigen. ### Management Implications - **First-line:** Gluten-free diet (definitive, but takes weeks–months to clear skin) - **Symptomatic:** Dapsone (rapid response within 24–72 hours); sulfapyridine or sulfasalazine are alternatives - **Screening:** All DH patients require celiac serology and small-bowel biopsy **Clinical Pearl:** Dapsone works rapidly on skin symptoms but does NOT treat the underlying celiac disease; gluten avoidance is essential for long-term management and prevention of GI complications. ### Why Granular IgA at DEJ Is Diagnostic | Feature | DH | Linear IgA Disease | Bullous Pemphigoid | | --- | --- | --- | --- | | **IF Pattern** | Granular IgA at DEJ | Linear IgA along basement membrane | Linear IgG at basement membrane | | **Clinical Sites** | Extensor surfaces | Variable; often lower abdomen, genitals | Flexural, lower abdomen | | **Pruritus** | Intense | Mild to moderate | Mild | | **Celiac Association** | ~75% | Rare | None | **Mnemonic:** DH = **D**apsone-responsive, **H**erpetiform (grouped), **H**LA-DQ2/8, **H**igh IgA at DEJ [cite:Robbins 10e Ch 25]