What are antihistamines?

April 22, 2024 05:30 am | Updated 04:18 pm IST

Representative photo of a man sneezing.

Representative photo of a man sneezing. | Photo Credit: Towfiqu Barbhuiya/Unsplash

Chances are you’ve taken an antihistamine this week. Antihistamines are common drugs that can be purchased without a prescription and are used to treat short-lived allergic reactions, like a sneezing fit or an itch.

As the name suggests, an antihistamine drug acts against the activity of the histamine receptor — proteins that bind to a compound in the body called histamine. This compound consists of two attached to each other: ethylamine (CH3CH2NH2) and imidazole (C2N2H4), the latter being a ring.

Depending on its location, histamine performs broadly four types of functions, each involving different histamine receptors called H1, H2, H3, and H4. The H1 receptor is found mostly in the inner surface of blood and lymphatic vessels, neurons, and smooth muscle cells. H1 antihistamines are used to block the effects of minor allergies. Certain cells in the gastric glands release histamine, which encourage nearby parietal cells with H2 receptors to release gastric acid for digestion.

H3 receptors are located in cells in the central nervous system, where they use histamine to regulate the release of other neurotransmitters, including dopamine and serotonin. Similarly, H4 receptors use histamine to regulate the body’s inflammatory response, including allergic reactions as well.

Currently, there is a lot of research ongoing on H3 and H4 receptors, in particular to alleviate the effects of neurological and immunological disorders, respectively, using specialised drugs.

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