A research scientist is comparing surface antigens X and Y that come from 2 different viruses. He obtains antibodies against antigen X from an animal previously exposed to the virus expressing this antigen. These antibodies are then attached to assay plates. Next, a fixed quantity of radiolabeled X antigen is added to the plates. Unlabeled Y antigens are then added in increasing concentrations to each plate, and the plates are washed to remove unbound antigens. Radioactivity is plotted as a function of Y antigen concentration, as shown in the graph below. Which of the following best describes the results of this experiment?

Question

A research scientist is comparing surface antigens X and Y that come from 2 different viruses.  He obtains antibodies against antigen X from an animal previously exposed to the virus expressing this antigen.  These antibodies are then attached to assay plates.  Next, a fixed quantity of radiolabeled X antigen is added to the plates.  Unlabeled Y antigens are then added in increasing concentrations to each plate, and the plates are washed to remove unbound antigens.  Radioactivity is plotted as a function of Y antigen concentration, as shown in the graph below.  Which of the following best describes the results of this experiment?

Quizplus · Accepted Answer

11ec160b_37b2_0ebb_9249_130d6ef49c7f_MD0001_00 A radioimmunoassay is a laboratory technique that uses specific antibodies and a known quantity of radiolabeled antigen to determine the amount of antigen present in an unknown sample. In this technique, specific antibodies against a known antigen are attached to an assay plate. Next, a fixed quantity of radiolabeled antigen and varying quantities of unlabeled antigen are added to the plate. The system is subsequently washed to remove unbound antigens, and radioactivity is measured. The researcher in the above experiment observes no change in radioactivity with increasing Y antigen concentrations. This indicates that antigen Y cannot bind the available antibodies and therefore shares no epitopes with antigen X. (Choice B) If the radiolabeled and unlabeled antigens are identical (eg, have the same epitopes), they should compete equally for antibody binding sites. The amount of radiolabeled antigen displaced from the antibodies will be proportionate to the amount of unlabeled antigen added. Consequently, increasing concentrations of antigen Y would result in a proportionate decrease in measured radioactivity. (Choices C and D) If antigen Y shared most or some of the epitopes of antigen X, then antigen Y would bind at a rate proportionate to both the amount of antibodies able to recognize the common epitopes and the amount of antigen Y added to the system. Educational objective: A radioimmunoassay uses specific antibodies and a fixed quantity of radiolabeled antigen to determine the amount of antigen present in an unknown sample. This is done by measuring the amount of radiolabeled antigen displaced from the antibodies in the system. __________ References: Radioimmunoassay. (http://www.ncbi.nlm.nih.gov/pubmed/4119239) Radioimmunoassay: review of basic principles. (http://www.ncbi.nlm.nih.gov/pubmed/164695)

A Research Scientist Is Comparing Surface Antigens X and Y