Largely results from an immune reaction (7)
I believe the answer is:
allergy
'an immune reaction' is the definition.
(I've seen this before)
'largely results' is the wordplay.
'results' indicates anagramming the letters (I've seen 'result of' mean this).
'largely' anagrammed gives 'ALLERGY'.
'from' is the link.
(Other definitions for allergy that I've seen before include "Cause of ill-feeling" , "Strong aversion" , "Hypersensitive reaction of the body to a foreign substance" , "Hypersensitivity to a substance" , "Antipathy (colloq.)" .)