This prospective cohort study aims to establish reliable histological reference values for normal small-bowel mucosa, improve histological diagnostic quality in celiac disease, and develop an advanced molecular profile for disease diagnosis and treatment response evaluation. The study will collect duodenal biopsies from three groups: healthy controls undergoing clinically indicated gastroscopy, patients referred for primary celiac disease diagnostics, and patients with small-bowel mucosal injury unresponsive to a gluten-free diet. Patients will undergo routine clinical assessment via standard pathology review of diagnostic biopsies. Biopsies will be analyzed using digital morphometry, AI-based image analysis, RNA sequencing (transcriptomics), and intestinal organoid cultures.