Bullous pemphigoid is an autoimmune subepidermal blistering disease characterized by eosinophilic inflammation and variable disease severity. This retrospective observational study aims to evaluate the relationship between disease severity and histopathological features, serum IgE levels, peripheral eosinophil counts, and the expression of eosinophilic cationic protein (ECP) and periostin in skin biopsy specimens of patients with bullous pemphigoid.

Immunohistochemical staining for ECP and periostin will be assessed semi-quantitatively and correlated with clinical and laboratory parameters, including BPDAI scores, eosinophil counts, serum IgE levels, hospitalization history, and disease relapse. The study seeks to identify potential tissue biomarkers associated with disease activity and severity in bullous pemphigoid.

Bullous pemphigoid (BP) is the most common autoimmune subepidermal blistering disease and is characterized by autoantibodies directed against hemidesmosomal proteins. Eosinophils and eosinophil-associated mediators are believed to play a central role in disease pathogenesis. Periostin and eosinophilic cationic protein (ECP) have been implicated in eosinophilic inflammation; however, their relationship with clinical disease severity in BP remains incompletely understood.

This single-center retrospective observational study will include patients diagnosed with bullous pemphigoid at the University of Health Sciences, Istanbul Training and Research Hospital. Clinical, laboratory, and histopathological data will be collected from medical records.

The study will evaluate demographic characteristics, disease severity measured by BPDAI, serum IgE levels, peripheral eosinophil counts, hospitalization history, relapse frequency, and other clinicopathological parameters. Archived skin biopsy specimens will undergo immunohistochemical staining for periostin and ECP. Staining intensity will be assessed using semi-quantitative scoring methods and correlated with clinical and laboratory findings.

The primary objective is to determine whether periostin and ECP expression levels are associated with disease severity. Secondary objectives include evaluating correlations between tissue biomarker expression and serum IgE levels, eosinophil counts, hospitalization, and disease recurrence.

Inclusion Criteria:

• Age ≥18 years.
• Diagnosis of bullous pemphigoid confirmed by compatible clinical, histopathological, and/or direct immunofluorescence findings.
• Availability of archived skin biopsy specimens suitable for immunohistochemical evaluation of periostin and eosinophilic cationic protein (ECP).
• Availability of clinical and laboratory data, including BPDAI score, serum IgE level, and peripheral eosinophil count.

Exclusion Criteria:

• Age <18 years.
• Diagnosis other than bullous pemphigoid.
• Inadequate or unavailable biopsy specimens for immunohistochemical analysis.
• Missing clinical, histopathological, or laboratory data required for study analyses.

Adult patients diagnosed with bullous pemphigoid and followed at the University of Health Sciences, Istanbul Training and Research Hospital. Participants will be identified retrospectively from hospital records and pathology archives. Clinical, laboratory, and histopathological data, including BPDAI scores, serum IgE levels, peripheral eosinophil counts, and immunohistochemical expression of periostin and eosinophilic cationic protein, will be evaluated.

Individual participant data (IPD) will not be shared. This retrospective, single-center study uses patient data collected as part of routine clinical care. Data sharing is restricted due to institutional policies, ethical considerations, and patient confidentiality requirements.