Systemic lupus erythematosus (SLE)is an immune-mediated inflammatory disease (IMIDs) of which the cellular and molecular alterations of the immune system driving the diseases still remains largely unknown. Accordingly, it remains difficult to predict the individual patient's response to treatment. Moreover, the patient's response to treatment remains heterogeneous and difficult to predict, despite the development of a variety of novel and powerful drugs (including the so-called biologicals). Therefore, there is a clear need for the identification and validation of cellular and molecular biomarkers which can provide useful clinical information for diagnosis, classification, prognosis and treatment, as well as the development of new therapeutic strategies. Biomarkers can be found and analyzed in different body compartments, of which the peripheral blood and the intra-articular synovial fluid or tissue are most easily accessible. However, previous studies in RA and other IMIDs showed that adaptive immune responses in other tissues such as lymph nodes also play an important role. Investigating other immune compartments of the body such as the lymph nodes could result in new insights. To study the early pathogenesis of inflammatory conditions, in 2008 our department initiated core-needle inguinal lymph node biopsy sampling. Since then more than 100 lymph node biopsy procedures were performed. The procedure is well-tolerated and, other than a small hematoma which does not require therapy in most of the cases, no complications were reported. In the current study, the effects of belimumab (anti-BAFF) in SLE will be investigated by studying the immune alterations taking place in lymph nodes in comparison to peripheral blood and immune alterations taking place in the end-organ, e.g. the joint (wrist, knee or ankle) by taking synovial biopsies during a needle- or mini-arthroscopy. This procedure has been performed frequently in our department over the last 15 years. In this way immune alterations in the lymph nodes (secondary lymphoid organ), peripheral blood (systemic) and the joint (end organ for the disease) will be assessed and compared.