What are antigens?

Antigens are substances that elicit an immune response when recognized by the immune system. This typically occurs when the body identifies these substances as foreign or potentially harmful, such as bacteria, viruses, or abnormal cells. When antigens enter the body, the immune system responds by activating various immune cells and producing antibodies, which specifically recognize and target the antigens for destruction or neutralization.

In contrast, the other choices refer to different biological functions or components. Proteins that transport oxygen, like hemoglobin, play a critical role in gas exchange but are not involved in immune responses. Cells that fight infection, such as white blood cells, do respond to antigens, but they are part of the immune response itself rather than the triggers for it. Lastly, hormones are chemical messengers in the body that regulate physiological processes but do not activate the immune response like antigens do.

Thus, the definition of antigens as substances that trigger an immune response is foundational to understanding how the body defends itself against infections and diseases.