Used in immunological research to quantify soluble programmed death-1 (sPD-1) levels in biological samples such as serum, plasma, or cell culture supernatants. Helps researchers study immune checkpoint regulation, particularly in the context of cancer, autoimmune diseases, and chronic infections. Enables monitoring of sPD-1 as a potential biomarker for disease progression or response to immunotherapy. Widely applied in preclinical studies and clinical research to understand immune evasion mechanisms and evaluate therapeutic interventions targeting the PD-1 pathway.