Gr anti-, ant- < anti, against < IE *anti, facing, opposite, near (> L ante, opposite, before) < base *ant-s, front, forehead
From Webster's New World College Dictionary, 5th Edition
Greek from anti opposite ant- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition
Conversion of the prefix anti- to an adjective.
From Wiktionary
From anti–
From American Heritage Dictionary of the English Language, 5th Edition