A Rare Case of COPA Syndrome: Multisystem Relapse and Fatal Septic Complication.

COPA syndrome is a rare autosomal dominant autoimmune disorder caused by COPA gene mutations, leading to immune dysregulation and multisystem involvement. We report a 45-year-old man with recurrent respiratory failure, haemoptysis, renal dysfunction and arthritis. Symptoms began at Age 27 with diffuse alveolar damage requiring mechanical ventilation. Imaging showed progressive interstitial lung disease and biopsy revealed follicular bronchiolitis. Immunosuppression provided temporary stabilisation. In 2024, next-generation sequencing confirmed a likely pathogenic COPA variant. The patient later discontinued therapy and remained untreated for 1 year, returning with worsening renal dysfunction and significant proteinuria, while pulmonary findings remained stable. Cyclophosphamide was initiated for suspected renal relapse. During treatment, he developed acute diverticulitis complicated by sepsis, progressing to septic shock and multiorgan failure despite surgery and antibiotics, resulting in death. Early recognition and multidisciplinary management are essential to improve outcomes of the disease.