RESUMEN
IgG4-related disease belongs to the group of immune-mediated diseases. It is a relatively new condition, classified in 2003. It is characterized by involvement of multiple organs, over time causing organ failure. The observed radiological changes are slow growing. As a rule, the course of the disease is subclinical, which means that the diagnosis is often made incidentally, retrospectively and many years after the onset of symptoms. In the initial stage of diagnosis, it is often confused with Sjögren's syndrome, systemic vasculitis, or neoplastic disorders. A CASE REPORT: We report the case of a female patient who was diagnosed with glomerulonephritis at the age of 34. The patient was qualified for renal replacement therapy with chronic hemodialyses in 2020 due to end-stage renal disease. In the same year, the patient was hospitalized for pneumonia of unknown etiology. After discharge from the hospital, the patient developed fever up to 39oC, chills, and pleural fluid. A suspicion of systemic disease was raised. Computed tomography of the chest showed enlargement of the anterior and middle mediastinum with increased densitization of adipose tissue and enlargement of mediastinal lymph nodes. Antibiotic therapy was introduced, followed by systemic corticosteroids, resulting in clinical improvement and remission of infiltrative lesions in radiological studies. The differential diagnosis excluded neoplastic lesions of the left lung, systemic connective tissue diseases, e.g. systemic lupus erythematosus and systemic vasculitis. Taking into account the whole clinical picture the suspicion of IgG4-related disease was raised. After the treatment, serum IgG4 level was found to be normal at 36.6 mg/ dl, anti-nuclear antibodies (ANA) were negative, rheumatoid factor was not elevated, complement component 4 (C4) was normal, complement component 3 (C3) decreased to 71 mg/dl (n: 90-180 mg/dl). Treatment with prednisone 10 mg/day was maintained. Histopathologic reanalysis of the renal biopsy (from a biopsy performed in 2012) was performed, which showed lymphocytic infiltration in the renal interstitium with segmental vascular loop sclerosis in the glomeruli and the presence of abundant granular IgG4 deposits in the glomerular mesangium by immunofluorescence. CONCLUSIONS: IgG4-related disease poses great diagnostic difficulty due to its non-specific and long-term course. Patients usually seek help with various specialists many years before the full picture of the disease is presented. It is poorly understood as it is a recently described condition (two decades ago). The classification criteria of IgG4 disease developed in 2019, the growing number of publications and recommendations being developed give hope for better understanding and more efficient diagnosis of the disease.