Immunohistochemistry detects differences between lichen planus-like keratosis, lichen planus, and lichenoid actinic keratosis.

Lichen planus-like keratosis (LPLK) (benign lichenoid keratosis) is a common skin lesion that shows some morphologic features of lichen planus (LP) and lichenoid actinic keratosis (LAK). To try to detect differences among these three entities, immunohistochemical staining for S100 protein and HLA-DR (with the antibody LN3) was performed in 31 cases of LPLK, 26 of LAK, and 25 of LP. Langerhans cells (LC) were counted per linear mm of epidermis in the S100 and LN3 slides. With anti-S100 staining, LP cases showed higher numbers of LC (mean = 25.3, SE = 2.84, median = 21.2) than did LPLK (mean = 17.3, SE = 2.26, median = 14.5) and LAK (mean = 9.7, SE = 1.5, median = 5.4). With LN3 stains, LP cases also showed higher numbers of LC than did LPLK and LAK. These results suggest that the involvement of LC in the production of lesions may be different in these three entities. However, due to the overlap in distribution of values observed, the use of these stains does not allow a definite diagnosis to be made exclusively based on the number of LC.