Immune response trigger (7)
I believe the answer is:
antigen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">immune response trigger</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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)" .)