How Drugs Work: Antihistamines

 
Hello, my molecular massive! You may have noticed a slight absence of posts last week; my exciting new job has been taking all my brain power, all that remembering new names and learning what to do, and loving it. Regular posting will resume as I settle into a rhythm of blogging in the evenings, in amongst training for the triathlon I rashly entered in July, and obviously habut in the meantime here’s one I wrote earlier. As the sun has finally made an appearance this year, this seems a timely subject….
 
If you’re anything like me then you might just love spring; love the colour, the flowers, the trees, and the lush effervescent greenery of it all. But if you’re even more like me, your enjoyment of this festival of life and growth may be occasionally curtailed by the inevitable contact with flecks of pollen spinning gaily through the breeze, transforming of your head into a leaky, red-eyed, sneezing ball of misery. Yes, I’m talking about hay fever. This weekend, for the first time this year, it really felt like spring. Almost like summer. It makes me want to go and lie in the grass in a summer dress, with a daisy chain in my hair. And I want to do so without puffy eyes and a streaming nose ruining my picturesquely romantic image. To do this, I’m going to need my faithful antihistamine.
 
A meadow of spring flowers just waiting to make me sneeze.

A meadow of spring flowers just waiting to make me sneeze.

 
Antihistamines have a fairly self-explanatory name; they alleviate the effects of histamine on the body. And anyone who’s ever experienced hay fever, or any other similar allergic reaction, will know the effects of histamine well. A runny nose and red puffy watery eyes. Perhaps some itching, some pain, some redness of the skin and some swelling. 
 
An allergic reaction is your body assuming that something harmless (pollen, for example, or cat hair) is a dangerous invader. And once it senses the invader, it sets into action an inflammatory response. The job of sensing invaders is down to antibodies, and the antibody that senses and binds to allergens is immunoglobulin E (IgE). IgE antibodies live on the surface of cells that are specifically designed to be involved in the inflammatory response, mast cells and basophils. Once the IgE has bound to an allergen, it sets off a series of events within the cells that lead to degranulation, the process of releasing molecules that mediate the immune response. Amongst these molecules is histamine.
 
Histamine, once released, causes effects on the body by binding to histamine receptors. There are four types of histamine receptors, H1–H4, but the key receptor involved in hay fever and allergic reactions is the H1 receptor. Histamine binding to the H1 receptor has a number of effects. First, histamine causes an increase in vascular permeability; this means that the walls of blood vessels are altered to allow more water and small molecules to pass through them. Water leaving the bloodstream can pass into the surrounding areas, often causing swelling, for example in response to an insect bite. This may also be responsible for watery eyes. 
 
Second, histamine causes increased mucus production, leading to a runny nose and, after a few hours of exposure, congested nasal passages. Histamine also causes an itching sensation in the immediate area of its release, in hay fever this tends to be in the back of the throat, the nasal passages and the eyes. The eyes also water in an attempt to remove the cause of irritation, and may become red and swollen. Finally, histamine can cause smooth muscles to contract, particularly the muscles in the airways, causing shortness of breath and an asthmatic-type wheeze. 
 
Taking an antihistamine then, prevents these effects. The drug competes with histamine to bind the H1 receptor, and wherever the drug wins and binds the receptor, histamine cannot, and therefore it also can’t cause any of the irritating effects above. Since they bind competitively, antihistamines are more effective if you take them before you come into contact with your allergen so they have the best possible chance of out-competing the allergen for the receptor. It’s like getting a good head-start in a molecular gladiator battle. 
 
Side effects: staying awake at the wheel
 
Antihistamines are relatively free of side-effects, but the most well-known is drowsiness. Drowsiness is caused by the drug crossing the blood–brain barrier (BBB), which is the barrier that exists between the circulatory system and the nervous system to stop just any old molecule getting into the brain and causing untold havoc. The first generation of antihistamine drugs, which include diphenhydramine, tended to cross the BBB relatively easily, and once there were able to act to cause sedation and drowsiness. Second-generation antihistamines, however, have been modified to prevent them crossing the BBB so much. Most common over-the-counter antihistamines (including cetirizine and loratadine) belong to the second generation and therefore are much less likely to cause drowsiness. However, some drug may still be able to cross the BBB and for that reason, some people (including myself!) may still find drowsiness a side effect of the drugs. If this is the case for you, you know the rules…. don’t operate heavy machinery. 
 
So, that’s how histamine causes the effects of hay fever, and how antihistamines stop them. Now, if you’ll excuse me, I have a date with an antihistamine tablet, a patch of grass and a daisy chain. But no heavy machinery.
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2 thoughts on “How Drugs Work: Antihistamines

  1. I LOVE antihistamines. They helped with my hayfever, help when around cats and dogs and helped the year my body was allergic to the SUN (hives and pains and general awfulness relieved by factor 50 sunscreen, hates, sleeves and a combination of anti histamine tablets (cetrizine hydrochloride FTW) and anti histamine lotion. I have been grateful for them ever since – so glad to read the science in a way that I understand – thank you!

  2. Pingback: How Drugs Work: Antihistamines | Science Communication Blog Network

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