A 52-year-old woman presents with painful oral erosions and flaccid blisters on the flexural surfaces of her arms. Histopathology shows intraepidermal acantholysis with suprabasal cleft formation. Direct immunofluorescence reveals IgG and C3 deposits in an intercellular pattern. All of the following are true regarding pemphigus vulgaris EXCEPT:

Explanation

## Immunological Basis of Pemphigus Vulgaris **Key Point:** Pemphigus vulgaris is an autoimmune blistering disorder caused by IgG (not IgM) autoantibodies directed against desmogleins, leading to loss of cell-to-cell adhesion (acantholysis). ### Correct Antibody Profile | Feature | Details | |---------|----------| | **Primary antibody** | IgG (not IgM) | | **Target antigens** | Desmoglein 3 (mucosal type) and/or Desmoglein 1 (mucocutaneous type) | | **Detection method** | Indirect immunofluorescence on patient serum | | **Pattern** | Intercellular (IgG and C3 in a "chicken wire" pattern on DIF) | **High-Yield:** The distinction between IgG and IgM is critical: - **IgG** = pathogenic antibodies in pemphigus vulgaris (detected by indirect IF on serum) - **IgM** = NOT the primary antibody in pemphigus; this is a classic distractor ### Desmogleins and Clinical Phenotype **Mnemonic:** **Dsg-3 = Mucosal; Dsg-1 = Mucocutaneous** - **Desmoglein 3 antibodies alone** → mucosal-dominant disease (oral erosions, pharyngeal involvement) - **Desmoglein 1 + Desmoglein 3 antibodies** → mucocutaneous disease (widespread blisters + oral involvement) ### Why Nikolsky Sign is Positive The loss of desmoglemal cell adhesion molecules (desmogleins) causes: 1. Breakdown of desmosomes 2. Acantholysis (loss of intercellular connections) 3. Mechanical separation of epidermal cells with minimal pressure → positive Nikolsky sign **Clinical Pearl:** A positive Nikolsky sign in the context of intraepidermal acantholysis on histology is pathognomonic for pemphigus. ### Why Option 2 is Incorrect Indirect immunofluorescence in pemphigus vulgaris detects **IgG** circulating antibodies against desmogleins, NOT IgM. IgM antibodies are not the pathogenic mechanism in pemphigus vulgaris and are not typically elevated in this condition. This is a common exam trap that conflates pemphigus (IgG-mediated) with other autoimmune conditions. **Warning:** Do not confuse: - **Indirect IF (serum)** in pemphigus = IgG antibodies - **Direct IF (skin biopsy)** in pemphigus = IgG and C3 in intercellular pattern