What action does an antihistamine primarily stop within the body?

An antihistamine primarily inhibits the inflammatory process by blocking the action of histamine, a chemical released during allergic responses and inflammation. When histamine binds to its receptors, it leads to various symptoms such as swelling, redness, and increased mucus production. Antihistamines work by blocking these receptors, thereby reducing the symptoms associated with allergic reactions and inflammation. This mechanism is especially important in managing conditions such as hay fever, allergic rhinitis, and other allergic responses.

While the other options mention processes that could be affected by histamine, they do not directly relate to the primary action of antihistamines. Antihistamines do not increase or decrease heart rate on their own, they do not prevent histamine production, and their direct effect on fluid retention is not their primary role. Their main function is focused specifically on mitigating the effects of histamine in the body.