The MONIRITUX study aimed to evaluate whether monitoring (i) circulating B-cell reconstitution or (ii) serum rituximab levels could help identify relapse of autoimmune diseases in patients treated with rituximab. Retrospective data suggest that B-cell reconstitution or the appearance of anti-drug antibodies are associated with rituximab's failure to prevent relapses (i.e. rheumatoid arthritis, systemic lupus erythematosus, autoimmune cytopenia...). According to the routine care provided by our institution, patients undergoing rituximab therapy are monitored every three months during the first year after treatment induction and every six months thereafter. At each clinical visit, a blood test is performed to quantify total gammaglobulins, IgG and CD19+ cells (along with other tests depending on the disease). This study will use the remaining blood in the tubes from routine care to quantify CD27+ and CD38+ B cells, as well as serum rituximab and anti-rituximab antibodies, during the first year of follow-up. The primary outcome will be to identify risk factors for clinical relapse according to circulating B-cell or rituximab status.