Chī hǎixiān hòu tā chūxiàn le yánzhòng de guòmǐn fǎnyìng.
After eating seafood he had a severe allergic reaction.
Tips
usage
反应 here is pronounced fǎnyìng (yì 4th tone), not fǎnyīng. The 应 yìng reading is used when it means 'respond / react'; the 应 yīng reading is for 'should / ought'.