What is an antigen?

An antigen is a substance that triggers the immune response in the body, specifically leading to the production of antibodies. This is primarily associated with proteins found on the surface of pathogens such as bacteria, viruses, and even some fungi. When an antigen enters the body, it is recognized as foreign by the immune system, prompting the production of specific antibodies that can bind to and neutralize or eliminate the antigen.

The correct answer highlights this crucial role of antigens in immunology and their significance in the body's defense mechanisms against infections. They can vary widely in structure, often being proteins or polysaccharides, and their presence is key for the immune system to identify and combat pathogens effectively. This understanding is vital in fields such as vaccine development, where attenuated or inactivated forms of these antigens are used to prompt a protective immune response without causing disease.