Origin: Greek prefix anti-
Antigen has 2 different meanings across 1 category:
any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies)
"The virus contains proteins that act as antigens to trigger the body's production of protective antibodies."
A substance that induces an immune response, usually foreign.
"The vaccine contains a harmless antigen designed to trigger the body's immune system without causing disease."
In plain English: An antigen is any substance that triggers your immune system to attack it because it recognizes something foreign, like a virus or bacteria.
"The vaccine works by training your immune system to recognize specific antigens on viruses before they can infect you."
Usage: An antigen is any external substance, such as bacteria or viruses, that triggers your body's immune system to produce antibodies. Use this term specifically when discussing how the body identifies and fights off invaders rather than describing general symptoms of illness.
The word antigen comes from the French term antigène, which combines the prefix anti- with a root related to genes or birth. It entered English to describe substances that trigger an immune response in the body.