ANTIGEN

n. foreign substance which stimulates the production of antibodies when introduced into a living organism

An antigen or immunogen is a molecule that stimulates an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not only consist of antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system.
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Noun
1. any substance (as a toxin or enzyme) that stimulates the production of antibodies
(hypernym) substance, matter
(hyponym) immunogen, immunizing agent

n. antigen, foreign substance which stimulates the production of antibodies when introduced into a living organism

Something potentially capable of inducing an immune response. Antibodies are elicited by antigens.