What is the expected effect of antihistamines?

Antihistamines are primarily designed to block the action of histamine, a substance in the body that plays a central role in allergic reactions and the physiological response to allergens. By inhibiting histamine receptors, particularly the H1 receptors, antihistamines effectively reduce or alleviate symptoms associated with allergies, such as itching, sneezing, and runny nose. They work by preventing histamine from binding to its receptors, thereby stopping the cascade of allergic responses that lead to these symptoms.

While sedation is a common side effect of many first-generation antihistamines, the primary therapeutic action they perform is the blockade of histamine, making individuals less reactive to allergens. This is counter to the other options, which either describe unrelated effects—such as mucous secretion or immune stimulation—or highlight potential side effects rather than intended therapeutic actions. The core function of antihistamines is thus best captured by their role in blocking histamine activity in the body.