Practice question

Radioimmunoassay principle involves fixed limited antibody, fixed trace amount of radiolabeled antigen, and variable unlabeled antigen from sample competing for binding. At equilibrium, higher concentration of unlabeled antigen occupies more antibody sites, displacing labeled antigen into free fraction that is washed away. Radioactivity measured in bound pellet therefore decreases as sample antigen increases. This inverse relationship allows construction of standard curve. No change or direct proportionality would violate competition kinetics, equal distribution would indicate no affinity. Bound counts fall with rising antigen levels.