allergist vs Dr.

allergist and Dr. both are nouns.

Word	Noun	Adjective	Verb	Adverb
allergist	Yes	No	No	No
Dr.	Yes	No	No	No

As nouns, Dr. is a hypernym of allergist; that is, Dr. is a word with a broader meaning than allergist:

Other hypernyms of allergist include doc, doctor, MD, medico, physician.

allergist (noun)	Dr. (noun)
a physician skilled in the diagnosis and treatment of allergies	a licensed medical practitioner
	a person who holds Ph.D. degree (or the equivalent) from an academic institution