Regarding the pathogenesis and immunological mechanisms of Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN), all of the following are established mechanisms EXCEPT:

Explanation

## Pathogenesis of SJS/TEN: T Cell-Mediated Keratinocyte Apoptosis ### Established Mechanisms **Key Point:** SJS and TEN are primarily T cell-mediated cytotoxic reactions, NOT immune complex-mediated diseases. The hallmark is massive keratinocyte apoptosis triggered by drug-specific CD8+ T cells. ### Correct Pathogenic Pathways 1. **CD8+ T Cell Recognition** - Drug acts as hapten, binds to HLA molecules (especially HLA-B*1502, HLA-A*3101) - CD8+ T cells recognize drug-HLA-peptide complex on keratinocyte surface - Leads to T cell activation and infiltration into epidermis 2. **Granulysin-Mediated Cytotoxicity** - Granulysin is a 9 kDa cytotoxic protein released by CD8+ T cells and NK cells - **High-Yield:** Granulysin is the most potent cytotoxic mediator in SJS/TEN - Causes direct keratinocyte apoptosis and full-thickness epidermal necrosis 3. **Death Receptor Pathways** - FasL (CD95L) and TNF-α upregulated on infiltrating T cells - Binding to Fas (CD95) and TNFR1 on keratinocytes triggers apoptosis - Caspase-3 activation leads to programmed cell death ### Why Immune Complex Deposition is NOT the Primary Mechanism **Warning:** Immune complex deposition (Type III hypersensitivity) is the mechanism in **Serum Sickness-like Reaction**, NOT in SJS/TEN. SJS/TEN is a **Type IV hypersensitivity** (delayed-type, T cell-mediated). - Histology shows **full-thickness epidermal necrosis** with minimal vasculitis - Direct keratinocyte apoptosis is the hallmark, not vasculitis - Immune complexes may be present but are NOT the primary driver ### Comparison Table: SJS/TEN vs. Serum Sickness-like Reaction | Feature | SJS/TEN | Serum Sickness-like | | --- | --- | --- | | Hypersensitivity Type | Type IV (T cell-mediated) | Type III (Immune complex) | | Primary Pathology | Keratinocyte apoptosis | Vasculitis, immune complex deposition | | Histology | Full-thickness epidermal necrosis | Vasculitis in dermis | | Mucosal Involvement | Severe (SJS) | Minimal | | Onset | 1–8 weeks | 7–21 days after drug | **Clinical Pearl:** The massive epidermal necrosis in SJS/TEN results in "skin sloughing like a scalded patient" — this is the consequence of widespread keratinocyte apoptosis, not immune complex-mediated vasculitis. [cite:Harrison 21e Ch 56]