What are antigens?

Antigens are substances that are recognized by the immune system as potential threats. They can be found on the surface of pathogens, such as bacteria and viruses, or can be present in other foreign substances, such as pollen or transplanted tissues. When antigens are detected, they stimulate a specific immune response, leading to the production of antibodies or activation of T-cells that help to neutralize or eliminate the threat.

This characteristic of antigens is crucial for the function of the immune system, as it helps the body differentiate between its own cells and foreign invaders. The role of antigens is central to the body's defense mechanisms, making them essential for the development of immunity, whether from natural exposure or vaccinations.

While other options mention various biological functions, none accurately define antigens or their role in the immune system. The function of proteins attacking pathogens, cells filtering blood, or enzymes aiding in digestion do not align with the fundamental role of antigens as triggers of immune responses. Therefore, identifying antigens as substances that trigger an immune response is the most accurate and informative response.