antigen (n.)

"substance developed in blood as an antitoxin," 1901, a hybrid formed from anti- "against" + body. Probably a translation of German Antikörper, condensed from a phrase such as anti-toxischer Körper "anti-toxic body" (1891).

word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," which is from the same source as genos "birth," genea "race, family," from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. First used in late 18th century French chemistry (see oxygen), it probably involves a misunderstanding of -genes, as though it meant "that which produces."

"substance causing allergy," 1912, from allergy on model of antigen.