Antigen

Etymology

A blend of "antibody" and the Greek-derived "-gen" (producer, generator) — literally an "antibody generator." The word encodes its own job: a substance defined by its power to make the body produce antibodies against it.

noun · /AN-tih-jen/
Any molecule the immune system recognizes as foreign and responds to.
What it names
Reach for it when you mean the thing that provokes an immune reaction — the trigger, not the response.
Example
A vaccine works by showing the immune system a harmless antigen so it learns the target in advance.
Related idea
An antigen is the provoker and an antibody is the answer, so they’re named as a cause-and-effect pair. A rapid "antigen test" looks for the invader’s marker itself, whereas an "antibody test" looks for your body’s past response to it.