A 68-year-old Indian man presents with a 3-week history of tense, fluid-filled blisters on the flexural aspects of his forearms, lower abdomen, and inner thighs. The blisters do not rupture easily and the surrounding skin appears erythematous but not erosive. He denies oral involvement. Examination reveals intact bullae that do not blanch with pressure. Serum testing shows IgG antibodies against the basement membrane zone. A skin biopsy demonstrates subepidermal blistering with intact basal layer and eosinophil-rich infiltrate. What is the most likely diagnosis?

Explanation

## Diagnosis: Bullous Pemphigoid ### Clinical Presentation **Key Point:** Bullous pemphigoid (BP) classically presents with **tense, intact bullae** on flexural and intertriginous areas in elderly patients (typically >60 years). ### Pathognomonic Features in This Case - **Tense bullae** that do not rupture easily — hallmark distinguishing feature from pemphigus - **Flexural distribution** (forearms, lower abdomen, inner thighs) — typical sites - **Absence of oral involvement** — BP rarely affects mucous membranes - **Intact basal layer on histology** — subepidermal blistering with basal cells remaining attached - **IgG antibodies against basement membrane zone** — serologic hallmark ### Histopathology Correlation | Feature | Bullous Pemphigoid | Pemphigus Vulgaris | |---------|-------------------|--------------------| | **Blister level** | Subepidermal | Intraepidermal (suprabasal) | | **Basal layer** | Intact ("tombstone" appearance) | Acantholysis; basal cells lost | | **Infiltrate** | Eosinophil-rich | Lymphocyte-rich | | **Bullae character** | Tense, intact | Flaccid, rupture easily | | **Oral involvement** | Rare | Common | ### Immunofluorescence Pattern **High-Yield:** Direct immunofluorescence (DIF) shows **linear IgG and C3 deposition along the basement membrane zone** — this is the diagnostic gold standard for BP. ### Autoantigen IgG antibodies target **BP180 (collagen XVII) and BP230** — transmembrane and intracellular components of hemidesmosomes. ### Clinical Pearl The **Nikolsky sign is negative** in BP (unlike pemphigus where it is positive) because the blister is subepidermal; the epidermis remains cohesive. [cite:Robbins 10e Ch 25]