Patients fulfilling the inclusion criteria of the study will be treated according to the recommendations of the Reference Centre for the management of thrombotic microangiopathies. If patients present refractory TTP, infusions of rituximab (375 mg/m2) will be added to this treatment at day 1, 4 and 15, immediately after plasma exchange sessions.On diagnosis, during treatment and after remission achievement, the following values will be explored: ADAMTS13 activity, ADAMTS13 inhibitors and anti-ADAMTS13 antibodies, B-cell lymphocytes quantification by immunophenotyping, and serum gammaglobulin level by serum protein electrophoresis. Rituximab will also be quantified.