Phosphatidylserine (PS), an anionic phospholipid. In healthy, viable cells, PS is actively maintained on the inner (cytoplasmic) leaflet of the plasma membrane by enzymes called flippases. However, during apoptosis (programmed cell death), this asymmetry is lost, and PS is translocated to the outer leaflet, serving as a universal "eat-me" signal for phagocytes. Importantly, tumor cells, tumor vascular endothelial cells, and the envelopes of many enveloped viruses also aberrantly expose PS on their outer surfaces. Exposed PS is not merely a marker; it actively contributes to an immunosuppressive microenvironment