## Pathophysiology and Management of Bullous Pemphigoid **Key Point:** Bullous pemphigoid is an **IgG-mediated** autoimmune disorder, NOT IgE-mediated. The disease is driven by **IgG autoantibodies against hemidesmosomal proteins** (BP180 and BP230), leading to complement activation and subepidermal blister formation. ### Immunological Mechanism **High-Yield:** The correct immune mechanism in BP: 1. **IgG autoantibodies** bind to BP180 and BP230 at the basement membrane zone 2. **Complement activation** (C3 deposition) follows 3. **Neutrophil and eosinophil recruitment** via complement chemotaxis 4. **Proteolytic enzyme release** causes dermal-epidermal separation ### Why NOT IgE? **Warning:** IgE-mediated mechanisms are characteristic of: - **Atopic dermatitis** (IgE to environmental allergens) - **Urticaria** (IgE to food, drugs, insect venom) - **Anaphylaxis** (IgE cross-linking on mast cells) Bullous pemphigoid is **NOT** an IgE-mediated disease. Direct and indirect immunofluorescence show **IgG and C3**, never IgE. ### Treatment Algorithm for Bullous Pemphigoid ```mermaid flowchart TD A[Bullous Pemphigoid Diagnosis]:::outcome --> B{Disease Extent?}:::decision B -->|Localized/Mild| C[Topical Corticosteroids ± Topical Calcineurin Inhibitors]:::action B -->|Generalized/Moderate-Severe| D[Systemic Corticosteroids: 0.5-1 mg/kg/day Prednisolone]:::action D --> E{Response at 2-4 weeks?}:::decision E -->|Good| F[Gradual taper over 3-6 months]:::action E -->|Inadequate/Steroid-dependent| G[Add Steroid-sparing Agent]:::action G --> H[Azathioprine, Mycophenolate Mofetil, or Methotrexate]:::action C --> I[Reassess in 2-4 weeks]:::decision I -->|Controlled| J[Maintenance topical therapy]:::action I -->|Uncontrolled| D ``` ### Management Principles | Scenario | First-Line | Adjunctive/Steroid-sparing | |----------|-----------|---------------------------| | **Localized disease** | Topical corticosteroids (Class III–IV) | Topical tacrolimus if face involved | | **Generalized mild-moderate** | Systemic corticosteroids (0.5–1 mg/kg/day) | Consider early addition of steroid-sparer | | **Generalized severe** | Systemic corticosteroids + steroid-sparer | Azathioprine, MMF, or methotrexate | | **Steroid-dependent/resistant** | Continue systemic steroids | Azathioprine, MMF, rituximab (refractory) | **Clinical Pearl:** Most patients with generalized BP respond well to systemic corticosteroids alone; however, steroid-sparing agents should be introduced early in steroid-dependent cases to minimize long-term corticosteroid toxicity. [cite:Harrison 21e Ch 297]
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