What reaction occurs in ELISA testing if no antigen is present in the sample?

In ELISA testing, when no antigen is present in the sample, the correct event is that the enzyme-labeled antibody remains in the solution. ELISA, or Enzyme-Linked Immunosorbent Assay, operates based on the principle of antigen-antibody interactions.

In the presence of an antigen, the enzyme-labeled antibody binds to it, forming a complex. After this reaction, the unbound antibodies are washed away. Thus, if the antigen is not present in the sample, there will be nothing for the enzyme-labeled antibody to bind to, and subsequent washing steps will result in the removal of any unbound enzyme-labeled antibody, leaving it in the solution.

The other outcomes, such as a color change or the sample turning purple, specifically indicate a positive reaction where the antigen is present, leading to the formation of an enzyme-substrate complex that produces color. Therefore, without the antigen, no color change occurs, and no complex is formed, affirming the conclusion that the enzyme-labeled antibody, if no binding occurs, will be washed away from the reaction wells.