antihistamine
Meaning of antihistamine: The word usually means an allergy tablet, though the drugs that quiet stomach acid block a histamine receptor too.
Definition of antihistamine
An antihistamine is a drug that stops histamine acting on its receptors. Used without qualification the word means a blocker of the H1 receptor, taken for the sneezing, itching, streaming nose and hives of an allergic reaction. Drugs blocking the H2 receptor are antihistamines as well, but they act on the stomach and reduce acid production.
What does histamine do in the body?
Histamine is stored ready made inside mast cells scattered through skin, airway and gut lining, and it is dumped out within seconds of an allergen binding to them. Released, it widens small blood vessels and makes them leaky, which produces redness and swelling; it irritates nerve endings, which produces itch and sneezing; and it tightens the muscle around the airways. It also has jobs unrelated to allergy, acting as the signal for stomach acid production and as one of the chemicals keeping the brain awake. Those separate jobs run through different receptor types, which is what allows a drug to block one and leave the others alone. An antihistamine occupies the receptor rather than preventing histamine being released, so it blunts a reaction under way rather than stopping one from starting.
How do the two generations differ?
| Feature | First generation | Second generation |
|---|---|---|
| Entry into the brain | Crosses the blood brain barrier readily | Very little |
| Sedation | Common and often marked | Uncommon |
| Other receptors affected | Several, giving dry mouth and blurred vision | Few |
| Where they are still used | Motion sickness, nausea, sedation, night time itch | Everyday allergy control |
What else are they used for?
The sedating members of the family found second careers. They suppress motion sickness and vertigo through their effect on the balance pathways, damp nausea, and appear in remedies for coughs and colds, where the drying and sedating effects rather than any antiallergy action do most of the work. Some are sold as sleep aids for the same reason. An antihistamine is also given before a blood transfusion or certain drug infusions where allergic type reactions are expected, to reduce itching, flushing and hives. One limit is worth stating plainly: in a severe whole body allergic reaction, an antihistamine relieves the skin symptoms only. It does not reverse the airway swelling or the fall in blood pressure that make such a reaction dangerous, which is why it is never the first treatment given for one.
Used in a sentence
An antihistamine was recommended for relief of the patient's seasonal allergy symptoms.
Good to know
First-generation antihistamines cross into the brain more readily than second-generation ones, which is why they tend to cause more drowsiness even at a comparable level of allergy control.
Word origin
Where “antihistamine” comes from
Formed from anti-, Greek for against, and histamine, a substance named from histos, Greek for tissue, reflecting its wide distribution and effects throughout body tissue.