Wight saliva gland is a quest item in the Blood and Wine expansion.
It is one of two alternative ingredients for the "Resonance" potion, which Geralt and Regis are trying to brew during the main quest La Cage au Fou. It can be looted off the so-called Spoon Collector if Geralt kills the spotted wight of Trastamara Estate Ruins, rather than lifting the curse.