It provokes an immune response - gentian (anag.) (7)

I believe the answer is:

antigen

(Other definitions for antigen that I've seen before include "toxin" , "Immunity-booster" , "Something inducing an immune response, neat gin (anag.)" , "Substance inducing an immune response" , "It stimulates immune responses - tag nine (anag)" .)

I've seen this clue in The Guardian.

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