Which term refers to a piece of a germ that triggers an immune response?

The correct term that refers to a piece of a germ that triggers an immune response is antigen. Antigens are typically proteins or polysaccharides found on the surface of pathogens such as bacteria, viruses, and fungi. When the immune system detects an antigen, it initiates an immune response, which may involve the production of antibodies specifically designed to bind to that antigen and help eliminate the pathogen.

Antigens are crucial for the immune system's ability to recognize and remember specific pathogens. This recognition is fundamental for developing immunity, whether from infection or vaccination, as the body can respond more quickly and effectively upon subsequent exposure to the same pathogen.

Other terms mentioned, while related, do not define the triggering piece of a germ in the same way. Antibodies are the immune proteins produced in response to antigens. Pathogens refer broadly to any organism that can cause disease, encompassing a wide variety of germs, not just those that possess antigens. Allergens are substances that provoke an allergic reaction and are not necessarily germ-related; they can include various environmental substances that trigger immune responses in sensitized individuals.