antigen

This term is strictly technical and resides within the realm of immunology and medicine. It describes a molecular trigger that the body identifies as non-self, sparking a defensive biological reaction. It carries a neutral scientific connotation, though in a clinical setting, it identifies the target of a disease or the active component of a vaccine.

While antibodies are the soldiers of the immune system, antigens are the signals or markers that tell those soldiers who to attack. The term is distinct from "pathogen," as not all antigens cause disease; some are harmless proteins or synthetic molecules used in medical diagnostics.