The purpose of this study is to determine how children with a history of severe, chronic Idiopathic Thrombocytopenic Purpura (ITP) who were treated with rituximab might respond to vaccines. Eligible patients are previously or currently enrolled in a study entitled "Open Label, Phase I/II Trial of Rituximab for Chronic, Severe Idiopathic Thrombocytopenic Purpura in Children and Adolescents" and have decided to obtain an inactivated influenza vaccination. These patients will be invited to provide one blood sample prior to vaccination and a second sample following vaccination to quantify immune response to vaccination.