What is the mechanism of action for antihistamines?

Antihistamines function primarily by blocking the action of histamine at H1 receptors. Histamine is a chemical in the body that is released during allergic reactions and acts on specific receptors to produce symptoms such as itching, sneezing, and increased mucus production. Antihistamines, therefore, bind to H1 receptors and prevent histamine from exerting its effects, effectively alleviating allergy symptoms.

This mechanism is particularly beneficial in treating conditions like allergic rhinitis and urticaria, where the action of histamine leads to discomfort and inflammation. By inhibiting histamine's ability to activate these receptors, antihistamines reduce the physiological changes associated with allergies.

The other options describe mechanisms that do not align with how antihistamines operate. For example, activating H1 receptors would lead to the symptoms of allergies, while increasing histamine release would worsen allergic reactions. Reducing symptoms by boosting immune response does not accurately reflect the antihistamine's action, as these medications do not enhance immune function but rather counteract the effects of histamine.