In the annals of medical science, the fluid-filled sac encasing the lungs has long been relegated to a role of passive protection, shielding the delicate pulmonary structures from external trauma. However, a groundbreaking revelation has emerged from the depths of research, illuminating a previously overlooked aspect of this seemingly mundane anatomical feature: it serves as a reservoir for formidable virus-eating cells known as macrophages, which spring into action during influenza infections, orchestrating a defense mechanism of remarkable potency and precision.