Complement fragment 4d staining as a potential diagnostic tool for bullous disorders with esophageal involvement

Mucous membrane pemphigoid (MMP) is an autoimmune blistering disorder that can involve the esophagus, potentially leading to complications such as stricture or stenosis. Diagnosis of MMP is challenging owing to overlapping clinical features with other subepithelial blistering diseases and limitations of current diagnostic techniques. Although direct immunofluorescence (DIF) remains the gold standard, it requires fresh tissue samples, which are not always available, particularly in cases without active mucosal or cutaneous lesions. Complement fragment 4d (C4d) immunohistochemistry (IHC) on formalin-fixed tissue has demonstrated diagnostic utility in bullous pemphigoid but has been less explored in MMP. This case highlights that C4d IHC may serve as a useful ancillary test to support MMP diagnosis, especially for mucosal or esophageal biopsies received in formalin, offering a potential diagnostic pathway when fresh biopsy samples are unavailable.