What are antigens?

Antigens are substances that trigger an immune response when recognized by the immune system. They are often found on the surface of pathogens, such as bacteria and viruses, but can also be present on other foreign substances, including pollen, toxins, and even certain foods. The presence of an antigen stimulates the body to produce specific antibodies or activate immune cells to target and eliminate the foreign substance, making them vital in the body’s defense mechanism against infections and diseases.

The other options do not represent the definition of antigens. For example, while some proteins produced by white blood cells (such as antibodies) are integral to the immune response, they are not themselves antigens. Similarly, pathogens can indeed cause disease, but they are usually recognized by the immune system due to their antigens. Lastly, immune cells that combat infections are not classified as antigens; instead, they respond to antigens as part of the immune response. Thus, the essential role of antigens is their ability to provoke an immune response, which aligns with the correct choice.