What is the definition of an antigen in the context of public health?

An antigen is defined as a substance that induces an immune response, specifically the production of antibodies. In the context of public health, antigens can be proteins or polysaccharides found on the surface of pathogens such as bacteria, viruses, and other microorganisms. When these antigens enter the body, they are recognized as foreign, triggering the immune system to respond. This response is crucial because it helps the body to identify and combat infections by creating specific defenses tailored to neutralize the pathogen.

Understanding the role of antigens is essential in public health for the development of vaccines, which work by introducing a harmless form of an antigen to the body, thereby training the immune system to recognize and combat the actual pathogen in the future. Other choices listed do not accurately capture the definition of an antigen, as they refer to harmful substances or entities but do not emphasize the immune response aspect that is central to understanding what an antigen is in public health.