## Histopathology of Lichen Planus **Key Point:** The pathognomonic histological feature of lichen planus is the **saw-tooth (or serrated) appearance of the rete ridges** combined with **basal layer vacuolization** and a dense band-like lymphocytic infiltrate in the superficial dermis. ### Characteristic Features | Feature | Description | |---------|-------------| | Rete ridge pattern | Saw-tooth or serrated appearance (hyperkeratosis) | | Basal layer | Vacuolization (liquefactive degeneration) | | Inflammatory infiltrate | Dense, band-like, lymphocytic (lichenoid) in superficial dermis | | Colloid bodies | Civatte bodies (apoptotic keratinocytes) in basal layer | | Parakeratosis | Absent (unlike psoriasis) | **High-Yield:** The combination of **saw-tooth rete ridges + basal vacuolization + band-like lymphocytic infiltrate** is virtually diagnostic of lichen planus and distinguishes it from other lichenoid dermatoses. **Clinical Pearl:** The histological finding of "lichenoid inflammation" (dense superficial dermal lymphocytic infiltrate) is shared by several conditions, but the saw-tooth rete pattern is unique to lichen planus. **Mnemonic:** **SALT** — **S**aw-tooth rete, **A**poptotic bodies (Civatte), **L**ichenoid infiltrate, **T**orturous vacuolization of basal layer.
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