Question

what are the detection antibodies binding to in the wells?
○ they bind to the plastic walls of the wells.
● they bind to the indicator dye and cause a color change.
○ they bind directly to the antigen.
○ they bind to antibodies.
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Explanation:

In immunoassays (like ELISA), detection antibodies bind to antibodies (e.g., primary antibodies that have bound to the antigen). The option "They bind to the indicator dye and cause a color change" is incorrect as detection antibodies don't bind to the dye directly; the dye's color change is a result of an enzyme - substrate reaction after antibody - antigen - detection antibody complex formation. "They bind to the plastic walls" is incorrect as capture antibodies (not detection) might bind to the well. "They bind directly to the antigen" is incorrect as detection antibodies bind to the antibody (or antigen - antibody complex), not directly to the antigen in typical setups. So the correct option is the one stating they bind to antibodies.

Answer:

They bind to antibodies. (The option with the unselected circle next to "They bind to antibodies.")