What is an antigen?

An antigen is defined as a substance that stimulates the immune system to produce antibodies. This typically occurs when a foreign substance enters the body, such as bacteria, viruses, or other pathogenic agents. Antigens are usually large molecules, often proteins or polysaccharides, present on the surface of these pathogens. When an antigen is detected by the immune system, it triggers an immune response, which includes the activation of B-lymphocytes that produce antibodies specific to that antigen.

The production of antibodies is a critical process in the body's defense mechanism, as antibodies bind specifically to antigens, marking them for destruction by other immune cells. This is a fundamental concept in immunology and helps to explain how vaccinations work to provide immunity against specific diseases by exposing the immune system to harmless forms of the pathogen's antigen.

The other options provided do not accurately describe an antigen. Antibodies are produced in response to antigens, and while some molecules may inhibit immune responses, they do not define what an antigen is. Additionally, while proteins can be involved in pathogen destruction, this description does not capture the role of an antigen in stimulating an immune response.