Is Allergie der, die or das?

The correct German article for Allergie is die. The noun Allergie is Feminine. So you always say: die Allergie. Learn the article together with the noun — German nouns don't follow predictable gender rules most of the time.

The plural of Allergie is: die Allergien. In the plural, every German noun takes the article "die". So: die Allergien. Example: ein Allergie → zwei Allergien.

Correct: die Allergien. The form "die Allergies" is wrong. The plural of die Allergie is die Allergien. In the nominative plural, you always use the article "die".

die Allergie follows the four German cases. Nominative: die Allergie, Accusative: die Allergie, Dative: der Allergie, Genitive: der Allergie. In plural: die Allergien / den Allergien. Learn these forms together with the noun.

The genitive singular of die Allergie is: der Allergie. The genitive plural is der Allergien. German uses the genitive to express possession or belonging. Example: die Farbe der Allergie.

The dative plural of Allergie is: den Allergien. In German, dative plural nouns usually end in "-n". So you say: "mit den Allergien". Use this case after prepositions like mit, von, bei, zu.

Allergie ends in -e. German nouns with this ending are almost always feminine. Nouns ending in -e are often feminine (die), especially two-syllable words. That's why the article for Allergie is die.

Allergie is a noun and must be capitalized in German. You'll find example sentences above for each case. The article die stays the same in nominative singular but changes in other cases. Practice declension in your own sentences.

Yes, Allergie is a noun (also called a substantive). German nouns are always capitalized. The article for Allergie is die. Every German noun has a fixed article you must memorize.