FACS protocols allow for some receptor quantification although best used in a relative manner. Next to the determination whether a cell is positive for the specific receptor, the amount of fluorescence (MFI) allows for quantification of receptor expression by these cells. However for absolute quantification you probably need specialized protocols and techniques and the FACS might not be your best equipment.
the same principle, add the selected ligand for the specific receptor, then the ligamd- receptor complex is labeled with radioactive isotope or enzyme substrate color reaction.quantify by reading the radioactivity or color ...