 The primary outcome will be the overall response rate 1 year after inclusion defined by a platelet count 30 x 109L with at least a two-fold increase of the initial pre-treatment count. For the patients treated with rituximab as a standard of care, based on the overall expected response-rate 40-50 and based on preliminary data on FcgammaRIIIA VF158 distribution, the inclusion of 85 patients should be sufficient to show an association of the V158 allele and the response b risk 20 and a 5. Responders and non responders will be compared by non parametrical tests, a multivariate analysis will be than performed using a logistic regression model. The immunological data B and T cells subsets obtained in both the responders and the non responders will be compared over time To, M3, M6 and M12 by non parametrical matched tests. 