## Lichen Planus vs Lichenoid Drug Eruption: Histological Discrimination ### Histopathological Comparison | Feature | Lichen Planus | Lichenoid Drug Eruption | |---------|---------------|------------------------| | **Acanthosis Pattern** | Saw-tooth (regular) | Irregular, variable | | **Basal Cell Damage** | Prominent, lichenoid | Prominent, lichenoid | | **Melanin Incontinence** | Present | Present | | **Follicular Involvement** | **Absent** | **Often present** | | **Dermal Infiltrate Depth** | Band-like, superficial-mid dermis | Superficial, may be patchy | | **Eosinophils** | Rare/absent | May be present | **Key Point:** The most reliable histological discriminator is **follicular involvement**. Lichen planus characteristically spares hair follicles, whereas lichenoid drug eruptions frequently involve follicular epithelium. ### Mechanism of Follicular Sparing in Lichen Planus **High-Yield:** Lichen planus shows a distinctive pattern of: 1. Dense lymphocytic infiltrate around dermal-epidermal junction 2. Saw-tooth acanthosis of interfollicular epidermis 3. **Follicles remain uninvolved** — the infiltrate respects follicular structures This is thought to reflect the specific T-cell mediated autoimmune response targeting the basal keratinocytes of the interfollicular epidermis. ### Why Other Features Are Unreliable Discriminators **Clinical Pearl:** Both conditions show: - Melanin incontinence (present in both) - Basal cell damage and lichenoid interface dermatitis (hallmark of both) - Dense lymphocytic infiltrate (present in both) Eosinophils may be present in drug eruptions but are not a sensitive discriminator because they can occasionally be seen in lichen planus. ### Mnemonic **"LP Follows the Follicle Rule"** — Lichen Planus respects follicles (follicles are spared), while Lichenoid drug reactions may involve them. 
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