Immune response trigger (7)
I believe the answer is:
antigen
'immune response trigger' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for antigen that I've seen before include "Immunity-booster" , "Substance inducing an immune response" , "Something inducing an immune response, neat gin (anag.)" , "Biological substance" , "It stimulates immune responses - tag nine (anag)" .)