What is defined as a foreign substance recognized as an invader by the immune system?

An antigen is a foreign substance that prompts the immune system to mount a response. When the body identifies these substances as invaders, it activates various immune mechanisms to neutralize or eliminate them. Antigens can be found on the surface of pathogens like bacteria and viruses, as well as on other substances such as pollen or even certain foods, indicating their importance in both pathogen defense and allergies.

In contrast, antibodies are proteins produced by the immune system in response to antigens. They specifically bind to antigens to help neutralize them, but they are not foreign substances themselves. Pathogens refer to organisms that can cause disease, such as bacteria, viruses, fungi, and parasites, and while they contain antigens, the term itself indicates the organism rather than the immune reaction component. Pathophysiology describes the functional changes that occur in the body due to a disease process, but it does not define a foreign substance recognized by the immune system.