The primary antibody (in purple) binds to an antigen (in red). A labeled secondary antibody (in green), then binds to the primary antibody. The label is then used to indirectly detect the antigen.

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The primary antibody (in purple) binds to an antigen (in red). A labeled secondary antibody (in green), then binds to the primary antibody. The label is then used to indirectly detect the antigen.