1. An antigen is any substance that causes the immune system to produce antibodies against it.
2. Antigens are typically proteins, polysaccharides, lipids, or nucleic acids that are foreign to the host organism (e.g., parts of bacteria, viruses, pollen, or transplanted tissues).
3. When the immune system recognizes a substance as foreign (an antigen), it mounts an immune response, a key part of which is the formation of antibodies specifically targeted against that antigen.
4. Option (A) is incorrect; antibodies and immune cells destroy harmful bacteria. Option (B) refers to antidotes. Option (C) refers to antipyretics.
5. Therefore, an antigen is a substance which stimulates formation of antibody.