the term  isotopic response  connotes the occurrence of a new skin disorder at the site of another , unrelated , and already healed skin disease . since the introduction of this phenomenon in dermatology , a variety of dermatoses we hereby describe a young girl who developed lesions of lichen planus pemphigoides ( lpp ) over preexisting atrophic scars , an example of  wolf 's isotopic response .  to the best of our knowledge , such a clinical presentation of lichen planus ( lp ) pemphigoides is not documented yet in the english literature . an 18-year - old girl presented with multiple itchy hyperpigmented papules and plaques , along with intermittent blisters over the lower limbs and buttocks for last 3 months [ figures 1 and 2 ] . past history was significant for painful ulcers over the lower limbs and buttocks 6 years back . the diagnosis of pyoderma gangrenosum was made ( as evident from documents available with patient ) , and she was treated for a year , resulting in complete resolution of ulcers . the bulla was noted over both lichenified plaques and otherwise normal appearing skin of scars . interestingly , most of the lichenified plaques and bullae were found over preexisting scars only [ figure 3 ] . skin biopsy was done from multiple sites , for both histopathological examination and direct immunofluorescence ( dif ) . the histological features of lichenified plaque were orthokeratotic hyperkeratosis , acanthosis , wedge shaped hypergranulosis , basal layer degeneration and band - like of infiltrate of lymphocytes in the papillary dermis ; thus corroborating with a diagnosis of lp [ figure 4a  c ] . histopathology of bulla showed a subepidermal bulla with eosinophilic infiltrate ; which were suggestive of bullous pemphigoid ( bp ) [ figure 5a  c ] . the scar tissue was characterized histologically by the presence of abundant collagen fibers without any features of inflammation [ figure 6 ] . dif from plaque showed a linear pattern of deposition of igg and c3 along the basement membrane zone [ figure 7 ] . dif from the perilesional skin surrounding a bulla showed linear igg and c3 along basement membrane zone ; a feature suggestive of bp [ figure 8 ] . thus , in conjunction with the clinical , histopathological and dif findings , the final diagnosis for our case was lp pemhigoides developing over preexisting scars , as an isotopic phenomenon . atrophic scars with overlying lichenoid papules and plaques ; and tense bulla on lower limbs atrophic scars with overlying lichenoid papules and plaques on buttocks close - up of lesions on the leg . lesions are restricted to area of scar only histopathology from plaque showing hyperkeratosis , hypergranulosis , saw toothing of rete ridges , band - like infiltration in upper dermis ( a ) ( h and e , 40 ) and ( b ) ( h and e , 100 ) with basal layer degeneration ( c ) ( h and e , 400 ) histopathology from bulla showing partial loss of epidermis including basal layer ( a ) ( h and e , 40 ) and ( b ) ( h and e , 100 ) and plenty of eosinophils in split ( c ) ( h and e , 400 ) histopathology from scar showing unremarkable epidermis , increased collagen in dermis and no inflammatory cells ( h and e , 40 ) direct immunofluorescence from plaque showing linear deposition of ig g and c 3 along basement membrane zone direct immunofluorescence from perilesional skin of bulla showing linear deposition of ig g and c3 along basement membrane zone lpp is an uncommon variant of bp , clinically manifested with bullous lesions over papules and plaques of lp , as well as on the uninvolved skin . it was first described by kaposi in 1892 as a classical case of lp complicated by extensive bullous dermatosis ; which was termed as  lpp . it can be precipitated by certain medications like cinnarizine , captopril , ramipril , psoralen and ultraviolet a ( puva ) , simvastatin , and antituberculous medications . igg autoantibodies against both bpag2 and bpag1 are thought to play a pivotal role in the development of the dermatosis . the widely accepted mechanism proposed is that lp or sometimes , puva causes damage to basal cells leading to unmasking of hidden antigens ; thus paving the pathway for the formation of antibodies and development of lesions of bp . a report described the correlation of levels of antibodies against bpag2 with the disease activity in lpp . the salient points of difference between lpp and bullous lp have been summarized in table 1 . dif testing show a smooth linear distribution of immunoreactants ( c3 , igg , igm , and iga ) along the basement membrane zone of dermoepidermal junction . however , a unique 200 kd antigen has been recognized on the epidermal side of salt split skin of patients with lpp and thus , lpp may represent a clinically distinct entity and not merely , a co - existence of lp and bp . effective treatment modalities include topical and systemic steroids , tetracycline , nicotinamide , dapsone , isotretinoin , methotrexate and other immunosuppressive drugs . this could be explained by the hypothesis that lpp has a unique antigenic target and thus , represents a distinct entity from bp . lpp has been reported in association with keratoacanthoma and colon adenocarcinoma . a unilateral presentation has also been reported . the occurrence of a different or unrelated dermatological disease at the site of the healed disease is termed as an isotopic phenomenon . the proposed etiologies of isotopic response include viral , immunologic , neural , vascular and locus minoris resistentiae ( a site of lessened resistance ) . however , development of lpp over preexisting scars is extremely unique and to the best of our knowledge , is not described in the english literature . it is a well - known fact that bulla formation in lpp is mediated by immunological damage against bp antigen along basement membrane zone . the predilection of the blisters to develop on the lower extremities could be explained by sluggish circulation in those regions . in our case , scars involving the dermis and subcutaneous tissue with largely unaffected epidermis might have altered the blood circulation in that particular area . this could explain the preferential deposition of immunoglobulins in the scar area and subsequent bulla formation . wolf 's isotopic response in lpp has probably not been described earlier in the medical literature . wolf 's isotopic response in lpp has probably not been described earlier in the medical literature . wolf 's isotopic response in lpp has probably not been described earlier in the medical literature .
an 18-year - old girl presented with multiple itchy hyperpigmented papules and plaques , along with tense blisters over the lower limbs and buttocks for last 3 months . 
 these papules , plaques , and bullae were mostly localized to preexisting scars . 
 the histopathological findings from papule and bulla were consistent with lichen planus ( lp ) and bullous pemphigoid , respectively . 
 direct immunofluorescence ( dif ) of perilesional skin around bulla showed linear deposition of igg and c3 . 
 considering clinical , histopathological and dif findings , diagnosis of lp pemphigoides ( lpp ) was made . 
 the preferential localization of lpp lesions over preexisting scars was a very interesting finding in our case an extremely rare instance of the isotopic phenomenon .