 The investigators know from the literature and data collected as part of the SEE TTP study group, the epidemiology of TTP and mortality rate. With the improvement in laboratory techniques, there is improved detection of antibodies against ADAMTS 13. As antibodies to ADAMTS 13 can be detected in up to 90 of cases, having excluded specific precipitating causes, monitoring of ADAMTS 13 levels and the ADAMTS 13 antibody can be made regularly in our research facility. As per the patients local treatment protocol for TTP i.e. PEX and steroid treatment, all patients will have a daily PEX until their platelet level count is 150 X 10 to the power of 9L. Patients will receive pre-Rituximab medication Hydrocortison, Piriton, and Paracetamol, as per local protocol, 30 minutes prior to Rituximab infusion to reduce any potential side effects. Rituximab will be administered intravenously, immediately following PEX. PEX should be withheld for 24 hours following Rituximab therapy, if appropriate to the patients clinical condition. Rituximab will be given once a week for 4 weeks in the first instance. The first treatment will be given within the first 3 days of admission, as there is a need to administer Rituximab in working office hours and allows participating sites to obtain the test results to ascertain whether the patient meets the studys inclusion criteria. If patients are well enough to be discharged before their 4th treatment, treatment will be continued in their local haematology day unit. If there is still evidence of antibodies to ADAMTS 13 and the ADAMTS 13 level has not been correctednormalised, up to a maximum 4 additional Rituximab infusion will be given weekly. Nurses will monitor the patients blood pressure, pulse, breathing rate, temperature and oxygen saturation level every 15 min for the first hour of Rituximab infusion and then hourly until infusion is complete. The patient will continue to be monitored as per the local protocol. 