Question
The competitive indirect ELISA differs from direct ELISA, as in competitive indirect ELISA:
  1. Unlabeled antibody is incubated in the presence of its antigen.
  2. Labelled antibody is incubated in the presence of its antigen.
  3. Labelled antigen is incubated in the presence of its antibody.
  4. None of the above.

Answer

  1. Unlabeled antibody is incubated in the presence of its antigen.

Explanation:

In direct ELISA the presence of an antigen is analyzed, and only a labelled primary antibody is used while in competitive ELISA antigen concentration is measured by noting the extent of the signal reduction. Thus, if the detection antibody is labelled, then this would be a direct competition ELISA and if unlabelled, then this would be an indirect competition ELISA.

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