In an immunodiffusion test, a strip of filter paper containing diphtheria antitoxin is placed on a solid culture medium. Then bacteria are streaked perpendicular to the filter paper. If the bacteria are toxigenic,

a. the filter paper will turn red.

b. a line of antigen–antibody precipitate will form.

c. the cells will lyse.

d. the cells will fluoresce.

e. none of the above

Video transcript

Hi, everybody. Let's take a look at the next problem. It says the immunodiffusion test is useful for a detecting antigen antibody complexes. B measuring antibody concentration C both A and B or D, none of the above. So the immunodiffusion test we need to recall is a form of precipitation reaction and we can call that precipitation reactions or when you have a soluble antigens that begin to form these large complexes with uh antibody molecules. So IgG or IgM antibodies and they form these very large complexes that actually precipitate out of the solution. But this sort of reaction only occurs at very specific concentrations of the antigen and antibody. And immunodiffusion represents a way of ensuring that correct ratio. And in immunodiffusion, you actually run your antibody and your antigen as samples on an agar gel and you place them next to each other. So therefore, when we look at what this test makes it possible to check for, we have the choices of detecting antigen antibody complexes or measuring antibody concentration. Well, this is sort of a yes or no test. Is there precipitate, is there not precipitate? It doesn't give you measurable quantities. So, choice B measuring antibody concentration doesn't apply. In this case is there's no way of quantifying something like that from these test results. You either have reciprocate or you don't. So that leaves us with choice a detecting antigen antibody complexes. Although we do have that none of the above an option. But yes, choice A is our answer. When precipitate forms, you do you know that you have these antigen antibody complexes present. So choice D now the above out and choice C both A and B wrong since B is incorrect. So again, the immunodiffusion test is useful for choice A detecting antigen antibody complexes. See you in the next video.