how do antihistamines work?

Antihistamines work by blocking a chemical that your body makes called histamine. Histamine is part of your immune system, which helps your body fight off illnesses and allergies. When your body is exposed to an allergen, it releases histamine, which causes redness, itching, sneezing, and other allergy symptoms. Antihistamines stop the histamine from being released, so you don't get those allergy symptoms.