ELISA Assay. Detects soluble human PD-1 (sPD-1) in serum, plasma and cell culture supernatant.. Assay Type: Sandwich. Sample Type: Cell Culture Supernatant,Plasma,Serum. Range: 3.125 to 200pg/ml. Sensitivity: 1.6pg/ml. PD-1 (Programmed Cell Death Protein 1, CD279) is a type I transmembrane protein belonging to the CD28/CTLA-4 family of immunoreceptors that mediate signals for regulating immune responses. Members of the CD28/CTLA-4 family have been shown to either promote T cell activation (CD28 and ICOS) or downregulate T cell activation (CTLA-4 and PD-1). PD-1 is expressed on activated T cells, B cells, myeloid cells and on a subset of thymocytes. Ligation of PD-1 by PD-L1 (B7-H1, CD274) or PD-L2 (B7-DC, CD273) inhibits TCR-mediated T cell proliferation and production of IL-1, IL-4, IL-10 and IFN-gamma. In addition, PD-1 ligation also inhibits BCR mediated signaling. PD-1 deficient mice have a defect in peripheral tolerance and spontaneously develop autoimmune diseases. Soluble form of PD-1 (sPD-1) is expressed upon immune cells activation and can be secreted in the serum/plasma. Soluble checkpoints, such as sPD-1 or sPD-L1, are involved in positive or negative immune regulation and changes in their serum/plasma levels affect the development, prognosis and treatment of cancer. Increase of sPD-1 or sPD-L1 levels have been detected in patients with different types of cancer. This PD-1 (human) ELISA kit is developed to detect specifically sPD-1 (human) in different biological fluids.