How to Say "reaction" in Spanish
The most common Spanish word for “reaction” is “reacción” — use 'reacción' when describing a response to an event, stimulus, or a chemical/physical process.
reacción
Examples
Su primera reacción al ver el regalo fue sonreír.
Her first reaction upon seeing the gift was to smile.
reacción
Examples
El profesor explicó la reacción exotérmica del compuesto.
The professor explained the exothermic reaction of the compound.
alergia
ah-LEHR-hee-ahaˈleɾxja
Examples
Tengo alergia al polen.
I have an allergy to pollen.
Mi hermana tiene una alergia muy fuerte a las nueces.
My sister has a very strong allergy to nuts.
El médico me hizo una prueba de alergia.
The doctor gave me an allergy test.
Using 'a' for the trigger
In Spanish, you always use the word 'a' (to) to name what you are allergic to. For example: 'alergia a los gatos' (allergy to cats).
Alergia is feminine
This word is feminine, so you should use 'la' or 'una' with it. If you describe it, use feminine endings: 'una alergia extraña' (a strange allergy).
Don't say 'en'
Mistake: “Tengo alergia en gatos.”
Correction: Tengo alergia a los gatos. (English uses 'to', but Spanish specifically requires 'a' followed by an article like 'los' or 'las'.)
Confusing 'reacción' and 'alergia'
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