Which term refers to foreign cells that trigger an immune response?

The term that refers to foreign cells that trigger an immune response is "antigen." Antigens are typically proteins or carbohydrates found on the surface of pathogens, such as bacteria, viruses, and other foreign substances, that elicit an immune response when recognized by the immune system. When an antigen is detected, it activates the immune cells to produce specific antibodies to neutralize or eliminate the foreign invader.

Antibodies, while crucial for the immune response, are not the foreign cells themselves; instead, they are the proteins produced by the immune system in response to antigens. Allergens are specific types of antigens that cause allergic reactions, often made up of harmless substances that provoke an exaggerated immune response. Pathogens refer to microorganisms that can cause disease, such as bacteria and viruses, and they contain antigens on their surfaces that trigger the immune response. However, the more precise term directly describing the foreign substance that initiates the immune reaction is "antigen."