ELISA is based on the principle of -

The correct answer is Option (1) → Antigen - antibody interaction

ELISA (Enzyme Linked Immuno-sorbent Assay) is a method for early diagnosis of infection, used for example in testing for HIV. An infection can be found either by detecting the pathogen’s antigens or the antibodies the body has made against it.

Option (1) Antigen - antibody interaction — Correct. An antibody binds only its own antigen, and an enzyme-linked reaction shows that the binding has taken place.

Option (2) PCR — Incorrect. PCR is a separate diagnostic technique that amplifies the pathogen’s nucleic acid.

Option (3) Radioactive molecule — Incorrect. A radioactive probe is used in autoradiography to find a mutated gene. ELISA uses an enzyme, not radioactivity.

Option (4) Amount of DNA — Incorrect. ELISA detects proteins (antigens or antibodies), not DNA.

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NCERT Extract:
Molecular Diagnosis – …