Next you come to the true Lichenoid pattern with a much denser infiltrate of lymphohistiocytic cells hugging the undersurface of the basement membrane. The degree of damage to the basal layer epidermal cells is often less in this reaction pattern than in the Interphase pattern but there can still be extensive damage to the basement membrane itself. Damage to basal layer melanocytes in this reaction pattern leads to pigment incontinence where melanin pigment is found in macrophages in the dermis. Hence clinically, diseases showing a lichenoid reaction pattern histologically, will show post inflammatory pigmentation clinically. The classic examples are Lichen planus and lupus erythematosus (discoid type). Although the cellular infiltrate in the band is usually lymphocytic you can see a plasma cell infiltrate in Syphillis and some eosinophils in a lichenoid drug reaction. Again note that most of the action is in the dermis so these will be mainly red nonscaly diseases but lichen planus does have Wickham's striae on the surface of the papules and pityriasis lichenoides as it's name suggests also has a scaly component. In T cell lymphoma, if the atypical lymphocytes invade the epidermis, you will also get a bit of scale.
Lupus erythematosus can also show a lichenoid pattern but it is usually not as extensive as lichen planus and it is more likely to involve the walls of the hair follicles. Unlike lichen planus it also causes a thickening of the basement membrane despite cells crossing this membrane and causing some gamage to the basal layer of cells. It can also cause melanin incontinence in macrophages in the dermis contributing to the post inflammatory hyperpigmentation seen in discoid lupus.
