1. An antigen is defined in immunology as any molecule, often a protein or polysaccharide found on the surface of pathogens like bacteria or viruses, or other foreign substances.
2. When an antigen enters the body, it is recognized by cells of the immune system (like lymphocytes).
3. This recognition triggers the immune system to mount a response, which can include producing specific antibodies against the antigen or activating T-cells to eliminate the source of the antigen.
4. Therefore, the primary characteristic of an antigen is its ability to provoke an immune response.
5. Options A, B, and D describe other types of substances: antipyretics, antimicrobials/antibiotics, and antidotes, respectively.