## Direct Immunofluorescence in Bullous Pemphigoid **Key Point:** Direct immunofluorescence (DIF) microscopy of perilesional skin in bullous pemphigoid shows a **linear pattern of IgG and C3 deposition along the basement membrane zone (BMZ)**, which is pathognomonic for this condition. ### Mechanism of Immunofluorescence Pattern The linear pattern reflects: 1. **IgG deposition** — autoantibodies bound to BP180 and BP230 antigens at the DEJ 2. **C3 deposition** — complement activation via the classical pathway (IgG + C1q → C3 fixation) 3. **Continuous linear pattern** — the hemidesmosomal antigens are distributed continuously along the entire BMZ ### DIF vs. Indirect Immunofluorescence (IIF) | Test | Specimen | Pattern | Sensitivity | |------|----------|---------|-------------| | **DIF** | Perilesional skin (affected) | Linear IgG/C3 at BMZ | ~95% | | **IIF** | Patient serum | Circulating IgG antibodies | ~70–80% | | **ELISA** | Serum | Anti-BP180 ± anti-BP230 | ~90% (most specific) | **High-Yield:** The **linear pattern at the BMZ is the gold standard** for diagnosing BP. C3 alone (without IgG) can occur but is rare; IgG is nearly always present. ### Comparison with Other Bullous Disorders | Disorder | DIF Pattern | |----------|-------------| | **Bullous Pemphigoid** | Linear IgG + C3 at BMZ | | **Pemphigus Vulgaris** | Intercellular IgG ("tombstone") | | **Linear IgA Disease** | Linear IgA at BMZ | | **Dermatitis Herpetiformis** | Granular IgA at dermal–epidermal junction | **Clinical Pearl:** DIF should be performed on **perilesional (not active blister) skin** because the active blister site may show loss of the linear pattern due to blister formation and antigen stripping. 
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