Final answer:
Antihistamines prevent the release of histamines and block the activity of histamine receptor sites in the body. They are commonly used to treat allergic reactions.
Step-by-step explanation:
Antihistamines prevent mast cells from releasing histamines and causing allergy symptoms. They work by blocking the activity of histamine receptor sites in the body. Antihistamines are typically used to treat allergic reactions, such as itching, runny nose, and sneezing.