Identify the correct statement.

The correct answer is Option (1) → Administration of preformed antibodies is necessary to treat a viper snake bite.

Immunity can be active or passive. In passive immunity, ready-made antibodies are given to the body; this is needed when a quick response is required. Antibodies are made by B-lymphocytes, while T-lymphocytes mediate cell-mediated immunity (CMI).

Option (1) Administration of preformed antibodies is necessary to treat a viper snake bite. — Correct. Snake venom acts fast, so there is no time for the body to make its own antibodies. The injection given for a snakebite contains preformed antibodies against the venom. This is passive immunisation.

Option (2) T-lymphocytes are responsible for producing antibodies against the smallpox pathogen. — Incorrect. Antibodies are produced by B-lymphocytes. T-cells do not secrete antibodies; they only help B-cells to produce them.

Option (3) Antibodies are protein molecules that consist of four light chains each. — Incorrect. Each antibody has four peptide chains: two light and two heavy. So it is written as H₂L₂, not as four light chains.

Option (4) B-lymphocytes play a role in kidney graft rejection. — Incorrect. Graft rejection is due to the cell-mediated immune response, and CMI is mediated by T-lymphocytes, not B-lymphocytes.

NCERT Extract:
Acquired Immunity – “… The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies. The T-cells themselves do not secrete antibodies but help B cells to produce them. Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. … The second type is called cell-mediated immune response or cell-mediated immunity (CMI). The T-lymphocytes mediate CMI. … The body is able to differentiate ‘self’ and ‘nonself’ and the cell-mediated immune response is responsible for the graft rejection.”
Active and Passive Immunity – “… Even in cases of snakebites, the injection which is given to the patients, contain preformed antibodies against the snake venom. This type of immunisation is called passive immunisation.”