Antihistamines are effective for which of the following purposes?

Antihistamines are primarily designed to block the action of histamine, a substance in the body that is released during allergic reactions. By doing so, they alleviate symptoms associated with allergies, such as itching, runny nose, sneezing, and hives. Their effectiveness in providing symptom relief for allergic responses makes this the correct answer.

While antihistamines can be helpful in managing symptoms of conditions fueled by histamine release, they do not have a role in treating blood clots, depression, or hypertension. Understanding the specific use of antihistamines further emphasizes their importance in the management of allergic conditions.