ELISA is based on the principle of

The correct answer is Option (3) → Antigen - Antibody interaction

ELISA (Enzyme Linked Immuno-sorbent Assay) is a method for early diagnosis of infection, used for example in testing for HIV.

Option (1) Antigen - Antigen interaction — Incorrect. Two antigens do not bind each other in the test. An antibody is needed to recognise an antigen.

Option (2) B-cells and T-cells interaction — Incorrect. B-cells and T-cells are the lymphocytes of the body’s acquired immunity. They work together inside the body, but ELISA does not test them.

Option (3) Antigen - Antibody interaction — Correct. An antibody binds only its own antigen, and an enzyme-linked reaction shows that the binding has taken place.

Option (4) T-cell - Antibody interaction — Incorrect. Antibodies are made by B-cells. They bind antigens, not T-cells.

Note: A similar question was also asked in CUET (UG) 2026 on 25-05-2026 (Slot 1) and on 14-05-2026 (Slot 2), and in CUET (UG) 2025 on 20-05-2025 (Slot 1) and on 13-05-2025 (Slot 1).

NCERT Extract:
Molecular Diagnosis – …