Study of the Role of Regulator T Cells in the Pathophysiology of Childhood Henoch Schönlein Purpura

General inclusion criteria for all sub-populations included in the study * The child (with age- and comprehension-skill-appropriate information) and parents (or persons exercising parental authority) have been informed about the implementation of the study, its objectives, constraints and patient rights * The child (depending on age) and parents (or persons exercising parental authority) have given their free and informed consent and signed the consent * The patient must be insured or beneficiary of a health insurance plan Inclusion criteria for population A: Henoch Schönlein purpura, acute episodes * The diagnosis of Henoch Schönlein purpura was made by a physician according to the EULAR / PRES / PRINTO 2010 criteria (purpura predominantly on the lower limbs associated with one of the following criteria: abdominal pain, arthralgia or arthritis, kidney damage or suggestive histology (immune deposits dominated by Immunoglobulin A (IgA)) * The patient is not treated via immunosuppression (steroids or other immunosuppressive / biotherapy) and has not been treated like this for at least 15 days Inclusion criteria for population B: Henoch Schönlein purpura in remission * The diagnosis of Henoch Schönlein purpura was made by a physician according to the EULAR / PRES / PRINTO 2010 criteria * The patient has has a Henoch Schönlein purpura episode in the past, and no longer has any symptoms of the disease * The patient is not treated via immunosuppression (steroids or other immunosuppressive / biotherapy) and has not been treated like this for at least 15 days Inclusion criteria for population C: controls * Subjects free from infectious, inflammatory or autoimmune diseases * Candidates for elective surgery (circumcision, urological surgery, removal of tonsils and adenoids) Exclusion Criteria: * The patient is participating in another interventional study or is in an exclusion period determined by a previous study * The child refuses to participate in the study * Parents (or persons with parental responsibility if any) refuse to sign the consent * It is impossible to correctly inform the patient or his/her parents (or persons with parental authority if any) * The patient has another inflammatory or autoimmune disease * Patient on immunosuppressive / biotherapy treatments