Inclusion Criteria:  Recurrent pericarditis (RP) cases:  Patients with RP, diagnosed by a clinician, meeting the European Society of Cardiology (ESC) diagnostic criteria. Where the aetiology of the RP is defined as idiopathic (i.e. there is no other aetiology identified, such as infective, malignant, post cardiac injury, as part of an autoimmune condition, as part of another known auto-inflammatory disease).  Relatives of RP cases:  First degree relatives of participants meeting the criteria for, and included in the study as, RP cases.  Disease Controls:  Patients with a systemic auto-inflammatory disease, diagnosed by a trained specialist.  Healthy Controls:  Sex, ethnicity and aged matched healthy individuals who do not have a personal history of pericarditis or systemic auto-inflammatory disease.  Exclusion Criteria:  Individuals under 12 years of age Adults who are unable to give written informed consent Individuals who have received a blood transfusion within 4 weeks