knee jerk vs reflex

knee jerk and reflex both are nouns.

Word	Noun	Adjective	Verb	Adverb
knee jerk	Yes	No	No	No
reflex	Yes	Yes	No	No

As nouns, reflex is a hypernym of knee jerk; that is, reflex is a word with a broader meaning than knee jerk:

knee jerk: a reflex extension of the leg resulting from a sharp tap on the patellar tendon
reflex: an automatic instinctive unlearned reaction to a stimulus

knee jerk (noun)	reflex (noun)
a reflex extension of the leg resulting from a sharp tap on the patellar tendon	an automatic instinctive unlearned reaction to a stimulus

knee jerk (adjective)	reflex (adjective)
	without volition or conscious control