CD200 Receptor 1 (CD200R1) is a cell surface glycoprotein belonging to the immunoglobulin superfamily, primarily expressed on myeloid cells such as macrophages, monocytes, and dendritic cells, as well as on some T and B lymphocytes. It functions as a key inhibitory receptor that modulates immune responses by delivering negative signals upon ligand binding. The activation of CD200R1 is crucial for maintaining immune homeostasis and preventing excessive inflammation, as it counteracts the pro-inflammatory signals triggered by activating receptors. Dysregulation of the CD200R1 pathway has been implicated in various autoimmune diseases, inflammatory disorders, and cancers, highlighting its importance as a potential therapeutic target for immune modulation. CD200R1 binds specifically to its ligand CD200, a membrane protein expressed on a variety of cell types including neurons, endothelial cells, and lymphocytes, forming a heterophilic interaction that inhibits myeloid cell activation.Briefly, rat CD200 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to CD200R1-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-CD200 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µL stop solution to the wells and read at 450/630nm immediately. Measured by its binding ability in a functional ELISA. When Recombinant human CD200R1 is lmmobilized at 2 µg/mL(100 uLwell), the concentration of rat CD200 that produces 50% optimal bindingresponse is found to be approximately 0.594 µg/mL.