## Distinguishing Feature of Bullous Pemphigoid **Key Point:** Bullous pemphigoid is characterized by **subepidermal blistering** with **minimal mucosal involvement**, unlike pemphigus vulgaris where oral mucosa is almost always affected early. ### Clinical Presentation **High-Yield:** Bullous pemphigoid typically presents with: - **Tense, firm blisters** that do not rupture easily (due to subepidermal location) - **Flexural surfaces**: inner arms, thighs, abdomen, lower legs - **Urticarial or eczematous lesions** may precede blisters - **Mucosal involvement is RARE** (occurs in <10% of cases) ### Immunological Hallmarks | Feature | Bullous Pemphigoid | Pemphigus Vulgaris | |---------|-------------------|--------------------| | **Blister location** | Subepidermal | Intraepidermal | | **DIF pattern** | Linear IgG + C3 at BMZ | Intercellular IgG | | **Circulating antibodies** | Anti-BP180, Anti-BP230 | Anti-desmoglein 3 (±1) | | **Mucosal involvement** | Rare (<10%) | Common (>90%) | | **Blister fragility** | Tense, firm | Flaccid, ruptures easily | ### Why Mucosal Involvement is Rare in BP Bullous pemphigoid autoantibodies (BP180, BP230) target hemidesmosomal antigens in the basement membrane zone. The oral mucosa has a different basement membrane composition and hemidesmosomal density compared to skin, making it less susceptible to autoimmune attack in BP. **Clinical Pearl:** When a patient with bullous disease presents with **extensive oral involvement**, think **pemphigus vulgaris** first, not bullous pemphigoid. Oral involvement in BP is an exception, not the rule. **Warning:** Do not confuse bullous pemphigoid with pemphigus vulgaris based on immunofluorescence pattern alone — always correlate with clinical presentation and histology location (subepidermal vs. intraepidermal). [cite:Robbins 10e Ch 25]
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