Histamine H1 receptor
Also known as: H1 receptor, HRH1, histamine receptor
The receptor through which histamine, released from mast cells in an allergic reaction, causes itching, sneezing, a runny nose, swelling and hives. Antihistamines such as cetirizine block it; older antihistamines also reach H1 receptors in the brain and cause drowsiness.
- Kind
- receptor
- Also called
- H1 receptor, HRH1, histamine receptor
- Where it is found
- Endothelium and smooth muscle of small blood vessels, sensory nerve endings in the skin and mucosa, bronchial and gut smooth muscle, and neurons in the brain.
What it does
When an allergen cross-links IgE on mast cells in the skin, nose, airways or gut, the cells release histamine within minutes. Acting on H1 receptors, histamine widens small vessels and makes them leak (redness and swelling, the wheal of a hive), stimulates itch and pain nerve endings, contracts bronchial and gut smooth muscle and increases mucus secretion. H1 antagonists compete for the receptor (technically as inverse agonists) and relieve the itch, sneezing and rhinorrhoea of hay fever and urticaria, but they do little for the airway narrowing of asthma, which involves many other mediators. First-generation antihistamines (chlorphenamine, promethazine) cross the blood–brain barrier, where H1 receptors help maintain wakefulness, so they sedate and impair driving; second-generation drugs (cetirizine, loratadine, fexofenadine) are kept out of the brain by transporters and cause far less drowsiness. Histamine's other receptors do different jobs: H2 drives stomach acid, H3 and H4 modulate nerves and immune cells.
Role in the body
Mediates the acute allergic response (vasodilation, vascular leak, itch, bronchoconstriction, mucus) and contributes to wakefulness in the brain.