TLR9 and lichen planus: The Koebner phenomenon is relatively common in psoriasis, lichen planus and vitiligo, with different mechanisms: In psoriasis, keratinocytes release antimicrobial peptides to activate TLR9, thereby triggering an immune response along the IL-23/IL-17 axis (19); Lichen planus involves MHC-I expression and CD8+ T cell-mediated epidermal damage (20).