Abstract: To observe the number and distribution of langerhans cells(LCs) and the expression of HLADR antigen in oral licen planus(OLP) and normal control buccal mucosa, and therefore to explore the pathogenesis of OLP. Methods： Polyclonal antibody to S100 protein and monoclonal antibody to HLADR antigens were applied to paraffinembedded sections by SABC immunohistochemical method in 25 cases of OLP and 8 cases of normal control. Results：LCs were identified with anitS100 and antiHLADR. In normal buccal mucosa, LCs mainly located in the basal cell layer, occasionally in the upper basal layer; whereas in OLP, they mainly located in the middle and lower regions of the prickle cell layer and the basal cell layer, but throughout the prickle cell layer in some. S100+ LC and HLADR+ LC significantly increased in the epithelia of OLP (P<0.001), and 56% of the OLP specimens（14/25）showed HLADR+ keratinocytes. There was correlation between the severity of the lymphatic infiltration in lamina propria and the basal cell degeneration in OLP, and more severe the lymophatic infiltration was, more severe the basal cell degeneration was. Conclusions： Presenting specified antigen to cytotoxic T cell, LCs induce the degeneration of keratinocytes. Thecellular immunity mediated by cytotoxic T cells may play a major role in the pathogenesis of OLP, and therefore LC may play an important role for OLP.

Key words: Licen planus，oral, Langerhans cells, Immunohistochemistry, HLADR antigen