atopy vs allergic reaction

atopy and allergic reaction both are nouns.

Word	Noun	Adjective	Verb	Adverb
atopy	Yes	No	No	No
allergic reaction	Yes	No	No	No

As nouns, allergic reaction is a hypernym of atopy; that is, allergic reaction is a word with a broader meaning than atopy:

atopy: an allergic reaction that becomes apparent in a sensitized person only minutes after contact
allergic reaction: hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity

Other hypernyms of atopy include allergy.

atopy (noun)	allergic reaction (noun)
an allergic reaction that becomes apparent in a sensitized person only minutes after contact	hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity