Used in immunological research to quantify levels of Programmed Death Receptor 1 (PD-1) in human biological samples such as serum, plasma, or cell culture supernatants. Commonly applied in studies related to immune checkpoint regulation, cancer immunotherapy, autoimmune diseases, and chronic infections like HIV or hepatitis. Helps researchers evaluate T-cell activation and exhaustion, supporting the development and monitoring of PD-1-targeted therapies. Widely used in preclinical and clinical studies to assess immune response modulation.