His initial research focused on a newly discovered cytokine, BAFF (B-cell Activating Factor of the TNF Family). This protein, secreted by immune system cells, plays a crucial role in the activation of autoimmune B lymphocytes, involved in a wide variety of autoimmune diseases. "In my field, rheumatoid arthritis is the most common. But in neurology, there's multiple sclerosis, in gastroenterology, Crohn's disease and in endocrinology, type 1 diabetes in children." Professor Mariette focused on the rare Sjögren's syndrome, which was characterised by dry eyes and mouth due to lymphocyte infiltration around the secretory glands, joint pain, complications in many organs and an increased risk of a lymph node cancer called lymphoma. Upon his arrival, Xavier Mariette initiated the development of a clinical research sector within his department, where he introduced the use of biotherapies for inflammatory rheumatism and autoimmune diseases. At the same time, he expanded the translational research activities of the Immune Response Regulation, HIV and Autoimmunity laboratory. In 2003, with his team, he was the first to demonstrate the role of the BAFF cytokine in the development of Sjögren's syndrome. "This marked a decisive turning point in our approach to autoimmune diseases, paving the way for new targeted therapies." Indeed, Belimumab, an anti-BAFF antibody used for the past ten years to treat lupus - an autoimmune disease characterised by joint, skin and kidney symptoms - and Ianalumab, an anti-BARR receptor antibody currently in development, are now used in the treatment of lupus and Sjögren's syndrome.