A 58-year-old Indian man with a 6-month history of recurrent oral erosions and painful blisters on the trunk and flexural areas is referred to dermatology. He reports that the lesions started in his mouth and gradually spread to the skin. Oral examination reveals extensive erosions on the buccal mucosa and soft palate. Skin biopsy from an active blister shows acantholysis with suprabasal clefting. Serum testing reveals high-titre circulating IgG antibodies against desmoglein 3. Which of the following statements about the pathogenesis of this condition is correct?

Explanation

## Pathogenesis of Pemphigus Vulgaris ### Mechanism of Acantholysis **Key Point:** Pemphigus vulgaris is an autoimmune disorder in which IgG autoantibodies bind to **desmoglein 3** (and/or desmoglein 1 in mucocutaneous disease), which are transmembrane adhesion molecules in desmosomes. This binding disrupts cell-to-cell adhesion (desmosomal integrity), leading to **intraepidermal acantholysis**. ### The Desmogleins **High-Yield:** Desmogleins are calcium-dependent adhesion molecules that form the core of desmosomes: - **Desmoglein 3 (Dsg3):** Predominantly expressed in lower epidermis and mucosa → mucosal-dominant pemphigus vulgaris - **Desmoglein 1 (Dsg1):** Predominantly expressed in superficial epidermis → superficial acantholysis in pemphigus foliaceus - **Desmoglein 3 + Desmoglein 1 antibodies:** Mucocutaneous pemphigus vulgaris ### Pathophysiologic Cascade ```mermaid flowchart TD A[IgG autoantibodies against Dsg3]:::action --> B[Binding to desmogleins on keratinocyte surface]:::action B --> C[Loss of desmosomal adhesion]:::action C --> D[Intraepidermal acantholysis]:::outcome D --> E[Suprabasal clefting in mucosal-dominant type]:::outcome D --> F[Subcorneal clefting in Dsg1-dominant type]:::outcome ``` **Clinical Pearl:** The antibodies do NOT activate complement or cause direct cytotoxicity; instead, they cause a functional loss of adhesion. This is why pemphigus is fundamentally different from bullous pemphigoid (which involves complement-mediated destruction of the basement membrane). ### Histopathologic Correlate **Key Point:** The location of acantholysis depends on which desmogleins are targeted: - **Dsg3 alone (mucosal-dominant):** Suprabasal acantholysis → oral and pharyngeal involvement predominates - **Dsg3 + Dsg1 (mucocutaneous):** Suprabasal acantholysis in mucosa, subcorneal in skin - **Dsg1 alone (pemphigus foliaceus):** Subcorneal acantholysis → superficial blisters, no oral involvement ### Antibody-Mediated vs. Complement-Mediated Mechanisms | Mechanism | Pemphigus Vulgaris | Bullous Pemphigoid | |-----------|-------------------|-------------------| | **Primary pathogen** | IgG against Dsg3/Dsg1 | IgG against BP180/BP230 | | **Site of action** | Desmosomal adhesion (intraepidermal) | Basement membrane zone (subepidermal) | | **Complement role** | Minimal; not required for blister formation | Essential; C5a recruits neutrophils | | **Mechanism** | Loss of adhesion (functional) | Complement-mediated lysis (destructive) | | **Blister type** | Flaccid, fragile | Tense, firm | ### Why Other Options Are Incorrect **Mnemonic:** **PEMPHIGUS = Protein (Dsg) Dysfunction, not Complement or Cells** **Clinical Pearl:** The fact that pemphigus blisters are flaccid and rupture easily reflects the intraepidermal location and the absence of a basement membrane to provide structural support. In contrast, bullous pemphigoid blisters are tense because the basement membrane remains intact beneath them.