The purpose of this ancillary, pilot-phase study is to determine how children with a history of severe, chronic 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" (CHB 02-12-160) and have decided to obtain the trivalent, inactivated influenza vaccination. These patients will be invited to provide one blood sample prior to vaccination and a second sample 4-8 weeks after vaccination to quantify immune response to vaccination. Additionally, if patients are scheduled to receive a tetanus booster vaccination within one month before or after the influenza vaccination, response to tetanus will also be quantified. This sample will be collected during the same phlebotomy as the influenza sample. In some cases, a blood sample was stored prior to rituximab treatment and will be used for baseline assessment. The primary and secondary objectives for this study are as follows: