Study of the Immunological Pathophysiological Mechanisms Associated With Acute Respiratory Distress Syndrome

Inclusion Criteria: * ARDS risk factors: bacterial or viral pneumonia, extrapulmonary infection, major trauma, transfusion, inhalation injury, or shock. * Pulmonary edema not explained by a cardiogenic cause or volume overload. * Onset of respiratory symptoms within \<7 days. * Bilateral pulmonary involvement on chest X-ray, CT scan, or ultrasound. * PaO₂/FiO₂ ≤ 300 assessed with PEEP ≥ 5 cmH₂O. Exclusion Criteria: * ARDS with intubation for more than 48 hours. * Contraindications to bronchoscopy: effective anticoagulation, dual antiplatelet therapy, thrombocytopenia \<50 G/L. * Pre-existing immunodeficiency: active solid tumor or remission \<5 years, active hematologic malignancy or remission \<5 years, systemic disease (even without specific treatment), solid organ or bone marrow transplant, HIV infection with CD4 \<200/mm³. * Cardiac arrest with a poor prognosis (NSE \>60, malignant EEG, diffuse ischemia on imaging, loss of trunk reflexes). * Patients \<18 year-old * Patients under legal guardianship, curatorship, or deprived of liberty. * Ongoing pregnancy. * Patients without social security coverage.