What is an antigen?

An antigen is defined as a foreign substance that is capable of provoking an immune response in the body. This means that when the immune system encounters an antigen, it recognizes it as a non-self entity. Antigens can be proteins, polysaccharides, or other molecular structures found on the surface of pathogens like bacteria and viruses, as well as on allergens and even some cancerous cells. The presence of an antigen prompts the immune system to produce specific antibodies or activate immune cells that bind to and neutralize the pathogen.

In contrast, other options do not accurately capture the definition of an antigen. While a substance boosting immune function is essential for overall health, it doesn't directly define an antigen. Antibodies are the immune proteins that respond to antigens, but they are not antigens themselves. Finally, while viruses can indeed contain antigens and be pathogenic, not all antigens are viruses, and defining an antigen solely as a virus would be too narrow, overlooking the broader roles antigens play in immunity. Thus, the correct interpretation of an antigen in relation to the immune system is that it is a foreign substance that triggers that response.