BackgroundThe pleural cavity represents a unique immunological compartment that can mount inflammatory reactions during infections, after surgery and in chronic immunological diseases. The connection between systemic immune reactions in the blood and local immune reactions in pleural effusions remains unclear. This study provides the first comprehensive immunological characterization of paired blood and pleural effusion samples, utilizing combined cell and cytokine analyses in pediatric patients undergoing cardiac surgery.MethodsIn 30 pediatric patients (median age: 22 months) with pleural effusion after cardiac surgery for congenital heart defects, corresponding peripheral blood and pleural effusion samples were analyzed for their immune response. We used flow cytometry and multiplex immunoassays to quantify 14 T cell subpopulations and 12 T cell associated cytokines in each biosample.ResultsIL-6, IL-8, IL-10, TNF (p