Antigen is a substance which

An antigen is any substance that the body recognizes as foreign and that provokes an immune response; the terms immunogen and antigen are often used interchangeably and such substances can come from viruses, bacteria, or be synthetically produced. Exposure to an antigen stimulates B lymphocytes to produce specific antibodies and establishes immunologic memory, enabling faster responses on re‑exposure [1]. Vaccines work by presenting harmless parts of a germ (antigen) to train the immune system and develop acquired immunity, illustrating the antigen’s role in triggering immune defence [1]. Thus option 3 is correct. Options 1, 2 and 4 describe antipyretic, bactericidal and antidote functions respectively, which are not definitions of an antigen.

Sources

1. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 3: Health: The Ultimate Treasure > Ability of the body to fight diseases > p. 37