(1) An agent that antagonizes actions of histamine. Physiologic manifestations of histamine release are determined by the subclass of histamine receptors (H1 or H2) found on tissue. Antistaminics may selectively block the effects of histamine for one receptor subclass or the other. (2) An agent that relieves symptoms of allergy (H1 antagonist) or gastric hyperacidity (H2 antagonist).
