Atopic allergy and Allergic reaction

Atopic allergy noun - An allergic reaction that becomes apparent in a sensitized person only minutes after contact.

Allergic reaction and atopic allergy are semantically related. In some cases you can use "Allergic reaction" instead a noun phrase "Atopic allergy".

Allergic reaction noun - Hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity.

Atopic allergy and allergic reaction are semantically related. Sometimes you can replace phrase "Atopic allergy" with "Allergic reaction", this phrases are similar.