In radioimmunassay, a fixed concentration of labeled antigen is incubated with a constant amount of antiserum such that the concentration of antigen binding sites on the antibody is limiting. If unlabeled antigen is added to this system, there is competition between labeled antigen and unlabeled antigen for the limited and constant number of binding sites on the antibody, and thus the amount of labeled antigen bound to antibody will decrease as the concentration of unlabeled antigen increases.

Which of the following choices is correct when describing the principles of competitive radioimmunoassay procedures?