In a competitive ELISA, which statement describes the correct competition mechanism?

• A. Sample antigen competes with labeled antigen for antibody binding
• B. Labeled antibody competes with sample antibody for antigen binding
• C. Antigen-coated wells bind to immobilized antibodies
• D. Antibody binding is detected directly by labeled antigen

Answer: (A)

In a competitive ELISA, two forms of antigen vie for the same antibody binding sites. A fixed amount of labeled antigen and the antigen from the sample both try to bind to the limited antibody sites. If the sample has more antigen, it occupies more sites, leaving fewer sites for the labeled antigen. Since the signal comes from the labeled antigen that remains bound, more sample antigen means a weaker signal. This inverse relationship is the hallmark of the competition mechanism.

The statement that best describes this is that the sample antigen competes with the labeled antigen for antibody binding, which directly captures the competition for those limited sites. The other options describe different setups: competition isn’t between antibodies for antigen, antigen-coated wells with immobilized antibodies reflect a capture scheme rather than competition, and detection isn’t achieved by direct binding of labeled antigen to antibody in the way described.