How do first-generation antihistamines, such as diphenhydramine, exert their therapeutic effects in allergic reactions?
1). By blocking the release of histamine from mast cells
2). By inhibiting prostaglandin synthesis
3). By competitively blocking histamine H1 receptors in both the central and peripheral nervous systems
4). By increasing the degradation of histamine in the liver
📝 Explanation:
First-generation antihistamines such as diphenhydramine act as competitive antagonists (inverse agonists) at histamine H1 receptors. They block the effects of histamine in peripheral tissues, reducing itching, vasodilation, increased vascular permeability, and edema. Because they readily cross the blood-brain barrier, they also block central H1 receptors, producing sedation and drowsiness.
MCQ