What defines an antigen?

An antigen is defined as a molecule that can trigger an immune response. This means that when an antigen enters the body, it can be recognized by the immune system, specifically by antibodies or immune cells, prompting a defensive reaction. Antigens are typically found on the surface of pathogens like bacteria and viruses, as well as on other foreign substances.

The ability of antigens to elicit an immune response is crucial to the functioning of the immune system, allowing it to recognize and attack invaders. This is an essential aspect of how vaccinations work, as they introduce a harmless form of an antigen to stimulate an immune response without causing disease, thus preparing the body to fight off actual infections in the future.

In contrast, the other options describe different concepts within the immune response. The second option refers to white blood cells that play a role in fighting infections but does not define what an antigen is. The third option mentions a signal involved in the activation of the immune system, which could refer to various molecules but not specifically to antigens. The fourth option describes byproducts of pathogen destruction, which are not the same as antigens. Thus, understanding the specific role of antigens in initiating immune responses is vital for grasping broader topics in immunology.