Final answer:
An antihistamine is a drug that prevents histamines from causing allergic symptoms by targeting histamine receptors in the body. Different types of antihistamines act on different histamine receptors. H₁-antihistamines treat nasal allergy symptoms, while H₂-antihistamines treat gastric acid conditions.
Step-by-step explanation:
An antihistamine is a drug that helps treat allergies by preventing histamines from stimulating itching and swelling of body tissues. Histamines are released by mast cells in response to allergens, and antihistamines target the receptors that histamines bind to. There are different types of antihistamines that act on different histamine receptors, such as H₁-antihistamines and H₂-antihistamines. H₁-antihistamines are used to treat allergic reactions in the nose, while H₂-antihistamines are used to treat gastric acid conditions.