Metabolic and Functional Study of γδ T Cells in Critically Ill Patients

Inclusion Criteria: 1. Healthy Control Group (NHC): * Age ≥ 18 years. * No acute or chronic major diseases. * Provide written informed consent. 2. Non-septic Critical Illness Group (CI-NS): * Age ≥ 18 years. * Admitted to the ICU and meeting the definition of critical illness. * Excluded from sepsis according to the Sepsis-3 criteria (infection + ΔSOFA ≥ 2 points). * Written informed consent provided by the participant or legally authorized representative. 3. Septic Critical Illness Group (CI-Sep): * Age ≥ 18 years. * Admitted to the ICU and meeting the Sepsis-3 criteria (infection + ΔSOFA ≥ 2 points). * Written informed consent provided by the participant or legally authorized representative. Exclusion Criteria: * Age \< 18 years. * Known immunodeficiency, HIV infection, active hematologic malignancy, or history of hematopoietic stem cell or solid organ transplantation within the past 3 months. * Receipt of T-cell-targeted immunosuppressive therapy (e.g., antithymocyte globulin, calcineurin inhibitors, mycophenolate mofetil, methotrexate, or high-dose corticosteroids \>1 mg/kg/day prednisone equivalent) before ICU admission or within 24 hours after ICU admission. * Use of immune checkpoint inhibitors (e.g., anti-PD-1/PD-L1/CTLA-4 antibodies) within the past 6 weeks. * Expected ICU stay \< 24 hours or imminent risk of death (moribund state). * Pregnancy or breastfeeding. * Active major bleeding. * Inability to obtain informed consent.